Organic Chemistry: ICSE Class 10 Chemistry answers, notes

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Summary

Organic chemistry studies compounds containing carbon. Early ideas suggested a “vital force” was needed for organic compounds; this meant a special life energy was thought to be required to create them. This was disproved when urea, an organic compound, was lab-synthesized. Sources of organic compounds include plants, animals, coal, petroleum, and lab synthesis. Organic compounds always contain carbon, often do not dissolve in water, have lower melting points, and burn easily, unlike many inorganic compounds.

Carbon atoms are special. They show tetravalency, meaning each carbon atom can form four chemical bonds. They also show catenation, which is the ability of carbon atoms to link with each other to form long straight chains, branched chains, or even rings. Carbon can form single, double, or triple bonds with other carbon atoms or different atoms. This unique nature allows for a vast number of organic compounds and also leads to isomerism, where compounds share a molecular formula but have different structures.

Hydrocarbons are compounds made only of carbon and hydrogen. They are classified into alkanes (containing only single carbon-carbon bonds), alkenes (containing at least one carbon-carbon double bond), and alkynes (containing at least one carbon-carbon triple bond). Alkanes are called saturated because they have the maximum number of hydrogen atoms carbon can bond with. Alkenes and alkynes are unsaturated because they have double or triple bonds and can add more atoms. Compounds can also be open-chain (aliphatic) or closed-chain (cyclic). Benzene is an example of a cyclic aromatic compound.

Organic compounds are named using a system called IUPAC nomenclature. This system uses root words to indicate the number of carbon atoms in the longest chain, suffixes to show the type of bonds or functional groups, and prefixes for any attached groups or substituents. A functional group is an atom or group of atoms within a molecule that gives the compound its characteristic chemical properties. A homologous series is a group of compounds with the same functional group and similar chemical properties, where each member differs from the next by a -CH2- unit.

Alkanes, like methane and ethane, can be prepared from sodium salts of carboxylic acids or by reducing alkyl halides. They undergo combustion (burning) and substitution reactions, such as reacting with chlorine in sunlight. Methane is a significant greenhouse gas.
Alkenes, like ethene, are prepared by dehydrating alcohols or by dehydrohalogenation of alkyl halides. They characteristically undergo addition reactions where atoms add across the double bond, for example, with hydrogen (hydrogenation) or halogens. Ethene is used to make polythene and helps in ripening fruits.
Alkynes, like ethyne (commonly known as acetylene), are prepared from calcium carbide and water. They also undergo addition reactions due to their triple bond. Ethyne is widely used in welding torches.

Alcohols contain the -OH (hydroxyl) functional group. Ethanol is commonly prepared by the fermentation of sugar or by the hydration of ethene. It burns, can be oxidized to form aldehydes and then carboxylic acids, reacts with active metals like sodium to release hydrogen, and forms esters when reacting with carboxylic acids. Ethanol is used in alcoholic beverages, as a solvent, and as a fuel. Denatured alcohol has substances added to it to make it unfit for drinking, often for industrial use.
Carboxylic acids contain the -COOH (carboxyl) group. Acetic acid (IUPAC name: ethanoic acid) is the main component of vinegar, giving it a sour taste. It is a weak acid, reacts with metals and bases, and forms esters with alcohols. Acetic acid that is pure and freezes to an ice-like solid at cool temperatures is called glacial acetic acid.

Isomers are different compounds that have the same molecular formula but different structural arrangements of atoms. Chain isomerism occurs when the carbon skeleton is arranged differently, for instance, as a straight chain versus a branched chain. Position isomerism occurs when a substituent or a functional group is located at a different position on the carbon chain.

Workbook solutions (Concise/Selina)

Intext Questions and Answers I

1. (a) What are organic compounds?

Answer: Organic compounds are the compounds of carbon, and organic chemistry is the study of carbon compounds, excluding oxides of carbon, metallic carbonates and related compounds like metal cyanides and metal carbides.

(b) What is vital force theory? Why was it discarded?

Answer: The vital force theory was a belief that organic compounds obtained straight from nature were the products of some “vital force” of nature, as there was no known method of preparing them in the laboratory.
This theory was soon discarded when, in 1828, a German chemist named Friedrich Wöhler showed that it was possible to obtain an organic compound, urea, from a laboratory process.

2. (a) Name a few sources of organic compounds.

Answer: A few sources of organic compounds are:

Plants: Compounds like sugar, starch, and cellulose.
Animals: Urea, proteins, and fats.
Coal: Destructive distillation produces benzene, toluene, and naphthalene.
Petroleum: Gasoline, fuel gases, and petrol.
Fermentation: Ethyl alcohol and acetic acid.

(b) Give the various applications of organic chemistry.

Answer: Organic compounds are extremely useful in our daily life. The soaps and shampoos we use, powders, perfumes, clothes we wear, food we eat (i.e., carbohydrates, proteins, fats, vitamins, etc.), fuels we use (natural gas, petroleum products), medicines, explosives, dyes, and insecticides are all organic compounds. There is hardly any walk of life where we do not need organic compounds.

3. Organic chemistry plays a key role in all walks of life. Discuss.

Answer: Organic chemistry plays a key role in all walks of life as organic compounds are extremely useful to us in our daily life. The soaps and shampoos we use while taking bath, the powders, perfumes we apply on the body, the clothes we wear, food we eat (i.e., carbohydrates, proteins, fats, vitamins, etc.), fuels we use (natural gas, petroleum products), medicines, explosives, dyes, and insecticides, are all organic compounds. There is hardly any walk of life where we do not need organic compounds.

4. Carbon shows some unique properties, name them.

Answer: Carbon shows unique properties of tetravalency and catenation.

5. Explain the following:

(a) tetravalency,

Answer: Tetravalency of the carbon atom means that carbon has four valence electrons. It forms covalent bonds by sharing its four electrons with other atoms. This characteristic of the carbon atom, by virtue of which it forms four covalent bonds, is called the tetravalency of carbon.

(b) catenation.

Answer: Catenation is the unique property of carbon atoms to link together (self-linking) to form very long chains. The property of self-linking of atoms of an element through covalent bonds in order to form straight chains, branched chains, and cyclic chains of different sizes is known as catenation. Carbon exhibits this property to the maximum extent due to the greater strength of the carbon-carbon bond and due to the tetra-covalency of carbon.

6. Write any four properties of organic compounds that distinguish them from inorganic compounds.

Answer: Four properties of organic compounds that distinguish them from inorganic compounds are:

Presence of carbon: Carbon is a necessary element in every organic compound, whereas carbon is not an essential element in inorganic compounds.
Solubility in water: Organic compounds generally do not dissolve in water, while inorganic compounds generally dissolve in water.
Melting and boiling point: Organic compounds have low melting points (m.p.) and boiling points (b.p.) and easily decompose on heating. Inorganic compounds have high m.p. and b.p. and usually do not decompose on heating.
Combustibility: Organic compounds are inflammable, i.e., they catch fire easily, whereas inorganic compounds do not burn easily.

7. Why are organic compounds studied as a separate branch of chemistry?

Answer: Organic compounds are studied as a separate branch of chemistry because the unique nature of the carbon atom (catenation, tetravalency, capability to form isomers) gives rise to the formation of a large number of compounds. More than 5 million organic compounds are known today, and thousands of new compounds are added every year to the existing lot. All this demands a separate branch of chemistry for carbon and its compounds, known as organic chemistry.

8. What are hydrocarbons? Compare saturated and unsaturated hydrocarbons.

Answer: Hydrocarbons are compounds that are made up only of carbon and hydrogen atoms.

A comparison of saturated and unsaturated hydrocarbons is as follows:

Saturated organic compounds:

All the four valencies of each carbon atom are satisfied by forming single covalent bonds with carbon and with hydrogen atoms.
Carbon atoms are joined only by a single covalent bond (-C-C-).
They are less reactive due to the non-availability of electrons in the single covalent bond, and therefore they undergo substitution reaction.

Unsaturated organic compounds:

The valencies of at least two carbon atoms are not fully satisfied by the hydrogen atoms.
Carbon atoms are joined by double covalent bonds (>C=C<) or by triple covalent bonds (-C≡C-).
They are more reactive due to the presence of electrons in the double or the triple bond, and therefore undergo addition reaction.

9. Give reason for the existence of the large number of organic compounds.

Answer: The existence of a large number of organic compounds is due to the unique nature of the carbon atom, which includes catenation, tetravalency, and the capability to form isomers. This gives rise to the formation of a large number of compounds; more than 5 million organic compounds are known today, and thousands of new compounds are added every year.

10. Give at least one example in each case to show the structure of:

(a) single bond compound,

Answer: An example of a single bond compound is Ethane. Its structural formula is H₃C-CH₃.

(b) double bond compound,

Answer: An example of a double bond compound is Ethene. Its structural formula is H₂C=CH₂.

(c) triple bond compound.

Answer: An example of a triple bond compound is Ethyne. Its structural formula is HC≡CH.

11. Name a compound of each type and draw the figure.

(a) Cyclic compound with single bond.

Answer: An example of a cyclic compound with single bonds is Cyclopropane (C₃H₆).

(b) Cyclic compound with triple bond.

Answer:

12. Give the name of one member of each of the following:

(a) saturated hydrocarbons,

Answer: One member of saturated hydrocarbons is Methane (CH₄).

(b) unsaturated hydrocarbons.

Answer: One member of unsaturated hydrocarbons is Ethene (C₂H₄) or Ethyne (C₂H₂).

13. Define substitution and addition reactions. Give an example for each.

Answer: Substitution reaction: A substitution reaction is a reaction in which one atom of a molecule is replaced by another atom (or group of atoms). For example: CH₄ + Cl₂ → CH₃Cl + HCl.
Addition reaction: An addition reaction is a reaction involving the addition of atom(s) or molecule(s) to the double or the triple bond of an unsaturated compound so as to yield a saturated product. For example: C₂H₄ + Br₂ → C₂H₄Br₂.

Intext Questions and Answers II

1. Define a functional group and give the structural formula of the following functional groups: (a) ketone, (b) alcohols, (c) aldehydes.

Answer: The functional group is defined as an atom or group of atoms joined in a specific manner which is responsible for the characteristic chemical properties of the organic compounds.
The structural formulae are:
(a) Ketone: The functional group is Keto, >C=O.
(b) Alcohols: The functional group is Hydroxyl, -OH.
(c) Aldehydes: The functional group is Aldehyde, -CH=O

2. (a) What is a homologous series? (b) What is the difference in the molecular formula of any two adjacent homologues: (i) in terms of molecular mass, (ii) in terms of number and kind of atoms per molecule?

Answer: (a) A homologous series is a group of organic compounds represented by the same general formula and having a similar structure and similar chemical properties in which the successive compounds differ by a CH₂ group.
(b) The difference in the molecular formula of any two adjacent homologues is:
(i) In terms of molecular mass, they differ by a molecular mass of 14 a.m.u.
(ii) In terms of number and kind of atoms per molecule, they differ by one carbon atom and two hydrogen atoms (a CH₂ group).

3. Write the name and formula of fourth member of the following homologous series: (a) Alkyne (b) Alcohol

Answer: (a) Alkyne: The fourth member is Pentyne, with the molecular formula C₅H₈.
(b) Alcohol: The fourth member is Butanol, with the molecular formula C₄H₉OH.

4. Which part of an organic compound determines (i) physical properties (ii) chemical properties?

Answer: (i) In an organic compound, the alkyl group determines mainly the physical properties.
(ii) The functional group is responsible for the chemical properties or reactivity of the compounds.

5. (a) What is an alkyl group? (b) Give the names of any three alkyl groups. How are they formed?

Answer: (a) An alkyl group is obtained by removing one atom of hydrogen from an alkane (Hydrocarbon with general formula CnH2n+2) molecule.

(b) Three alkyl groups are Methyl (-CH₃), Ethyl (-C₂H₅), and Propyl (-C₃H₇).

They are formed by removing one atom of hydrogen from an alkane. An alkyl group is named by replacing the suffix ane of the alkane with the suffix -yl. For example, Methane (CH₄) forms Methyl (-CH₃) upon removal of one hydrogen atom.

6. Give the names and the structural formula of the first three members of the homologous series of alkanes.

Answer: The names and structural formulae of the first three members of the homologous series of alkanes are:

(i) Methane (CH₄):

(ii) Ethane (C₂H₆):

(iii) Propane (C₃H₈):

7. Name the alkyl radical and the functional group of the following organic compounds: (a) CH₃OH, (b) C₂H₅OH, (c) C₃H₇CHO, (d) CH₃COOH, (e) CH₃COOH, (f) C₂H₅Br.

Answer: (a) CH₃OH:
Alkyl radical: Methyl (-CH₃)
Functional group: Hydroxyl (-OH)

(b) C₂H₅OH:
Alkyl radical: Ethyl (-C₂H₅)
Functional group: Hydroxyl (-OH)

(c) C₃H₇CHO:
Alkyl radical: Propyl (-C₃H₇)
Functional group: Aldehyde (-CHO)

(d) CH₃COOH:
Alkyl radical: Methyl (-CH₃)
Functional group: Carboxyl (-COOH)

(e) CH₃COOH:
Alkyl radical: Methyl (-CH₃)
Functional group: Carboxyl (-COOH)

(f) C₂H₅Br:
Alkyl radical: Ethyl (-C₂H₅)
Functional group: Halide (-Br, specifically Bromo)

Exercise A

1. Write the IUPAC name of the following:

(a)

Answer: The IUPAC name is 2,2-dimethylpropane.

(b)

Answer: The IUPAC name is 2-methylbutane.

(c)

Answer: The IUPAC name is Propene.

(d)

Answer: The IUPAC name is 2,2-dimethylpentane.

(e)

Answer: The IUPAC name is Pent-2-yne.

(f)

Answer: The IUPAC name is 3-methylbut-1-yne.

(g)

Answer: The IUPAC name is 2,3-dichloropentane.

(h)

Answer: The IUPAC name is 4,6-dimethylnonane.

(i)

Answer: The IUPAC name is But-2-ene.

(j)

Answer: The IUPAC name is Hept-2-yne.

(k)

Answer: The IUPAC name is 5,5-dimethylhexanal.

(l)

Answer: The IUPAC name is 2-methylpentan-2-ol.

(m)

Answer: The IUPAC name is 4-methylpentanoic acid.

(n)

Answer: The IUPAC name is 2-bromo-2-methylbutane.

(o)

Answer: The IUPAC name is 1-bromo-3-methylbutane.

(p)

Answer: The IUPAC name is Propyne.

(q)

Answer: The IUPAC name is Methanal.

(r)

Answer: The IUPAC name is 2,4-dimethylheptane.

(s)

Answer: The IUPAC name is Propan-1-ol.

(t)

Answer: The IUPAC name is Ethanoic acid.

(u)

Answer: The IUPAC name is Ethanal.

(v)

Answer: The IUPAC name is 1,2-Dichloroethane.

2. Draw the structures of the following compounds:

(a) Prop-1-ene

Answer:

(b) 2, 3-dimethyl butane

Answer:

(c) 2-methyl propane

Answer:

(d) 3-hexene

Answer:

(e) Prop-1-yne

Answer:

(f) 2-methyl prop-1-ene

Answer:

(g) Alcohol with molecular formula C₄H₁₀O.

Answer:

3. Choose the correct answer:

(a) C₅H₁₁ is an

(i) alkane
(ii) alkene
(iii) alkyne
(iv) alkyl group

Answer: (iv) alkyl group

(b) A hydrocarbon of the general formula CnH₂n is

(i) C₁₅H₃₀
(ii) C₁₂H₂₆
(iii) C₈H₂₀
(iv) C₆H₁₄

Answer: (i) C₁₅H₃₀

(c) The total number of different carbon chains that four carbon atoms form in alkane is

(i) 5
(ii) 4
(iii) 3
(iv) 2

Answer: (iv) 2

(d) CH₃-CH₂-OH and CH₃-O-CH₃ are

(i) position isomers
(ii) chain isomers
(iii) homologous
(iv) functional-isomers

Answer: (iv) functional-isomers

(e) The IUPAC name of the compound is

(i) 3-trimethylhexane
(ii) 3-methyl hexane
(iii) 4-methyl hexane

Answer: (ii) 3-methyl hexane

4. Fill in the blanks.

(a) Propane and ethane are ……… (homologous, isomers)

Answer: Propane and ethane are homologous

(b) A saturated hydrocarbon does not participate in a/an ……… reaction (substitution, addition)

Answer: A saturated hydrocarbon does not participate in a/an addition reaction

(c) Succeeding members of a homologous series differ by ……… (CH, CH₂, CH₃)

Answer: Succeeding members of a homologous series differ by CH₂

(d) As the molecular masses of hydrocarbons increase, their boiling points ……… and melting points ……… (increase, decrease)

Answer: As the molecular masses of hydrocarbons increase, their boiling points increase and melting points increase

(e) C₂₅H₅₂ and C₅₀H₁₀₂ belong to ……… homologous series (the same, different)

Answer:C₂₅H₅₂ and C₅₀H₁₀₂ belong to the same homologous series

(f) CO is an ……… compound. (organic, inorganic)

Answer: CO is an inorganic compound.

(g) The chemical properties of an organic compound are largely decided by the ……… and the physical properties of an organic compound are largely decided by the ……… (functional group, number of carbon atoms)

Answer:The chemical properties of an organic compound are largely decided by the functional group and the physical properties of an organic compound are largely decided by the number of carbon atoms

(h) CHO is the functional group of an ……… (alcohol, aldehyde)

Answer: CHO is the functional group of an aldehyde

(i) The root word in the IUPAC name of an organic compound depends upon the number of carbon atoms in ……… (any chain, principal chain)

Answer: The root word in the IUPAC name of an organic compound depends upon the number of carbon atoms in principal chain

(j) But-1-ene and but-2-ene are examples of ……… isomerism. (chain, position, functional)

Answer:But-1-ene and but-2-ene are examples of position isomerism.

5. Define or explain chain isomerism and position isomerism with examples in each case.

Answer: Chain Isomerism: When two or more compounds have a similar molecular formula but are different in the arrangement of carbon atoms in straight or branched chains the compounds are referred to as chain isomers and the phenomenon is termed as chain isomerism. For example: Pentane C₅H₁₂ has chain isomers like pentane (straight chain), iso-pentane (2-methyl butane, branched chain), and neo-pentane (2, 2-dimethyl propane, branched chain).

Position isomerism: When two or more compounds with the same molecular formula differ in the position of substituent atom or functional group on the carbon atom, they are called position isomers and this phenomenon is termed as position isomerism. For example: C₄H₆ (Butyne) has two isomers:

but-1-yne

but-2-yne

6. (a) Define the term isomerism. State the main cause of isomerism.
(b) Draw the chain isomers of hexane (C₆H₁₄).
(c) Draw position isomers of butene (C₄H₈).

Answer: (a) Compounds having the same molecular formula but different structural formula are known as ISOMERS and the phenomenon is known as ISOMERISM.

The main cause of structural isomerism is difference in the mode of linking of atoms.

(b) Chain isomers of hexane (C₆H₁₄):

n-hexane

2 Methylpentane

3 Methylpentane

2,3 Dimethylbutane

2,2 Dimethylbutane

1-butene

2-butene

7. Draw structural formula for each of the following compounds :

(a) isomer of n-butane

Answer:

(b) vinegar

Answer:

(c) 2-propanol

Answer:

(d) ethanal

Answer:

(e) acetone

Answer:

(f) diethyl ether

Answer:

(g) propanoic acid

Answer:

(h) pentan-2-ol

Answer:

(i) 2, 2-dibromobutane

Answer:

What is used to describe these compounds taken together?

Answer: These compounds taken together are described by their respective IUPAC names or common names, and they represent different classes of organic compounds or isomers.

8. (a) What is the special feature of the structure of : (i) ethene, (ii) ethyne ?
(b) What type of reaction is common to both the above compounds ? Why methane does not undergo this type of reaction.
(c) What is the IUPAC, name of dimethyl ether.

Answer: (a) (i) The special feature of the structure of ethene is the presence of a carbon-carbon double bond (C=C).
(ii) The special feature of the structure of ethyne is the presence of a carbon-carbon triple bond (C≡C).

(b) The type of reaction common to both ethene and ethyne is addition reaction. Methane does not undergo this type of reaction because it is a saturated hydrocarbon, meaning all the four valencies of carbon are satisfied by single covalent bonds, and there are no double or triple bonds where atoms can be added. Methane undergoes substitution reaction.

9. Which type of reaction will (i) ethane and (ii) ethene undergo ?

Answer: (i) Ethane will undergo substitution reaction.
(ii) Ethene will undergo addition reaction.

10. Choosing only words from the following list, write down appropriate words to fill in the blanks from (a) to (e) given below.

[Addition, carbohydrates, CnH₂n-₂, CnH₂n, CnH₂n+₂, electrochemical, homologous, hydrocarbon, saturated, substitution, unsaturated]

The alkanes form an (a) ……… series with the general formula (b) ……… . The alkanes are (c) ……… (d) ……… which generally undergo (e) ……… reactions.

Answer: The alkanes form an (a) homologous series with the general formula (b) CnH₂n+₂. The alkanes are (c) saturated (d) hydrocarbon which generally undergo (e) substitution reactions.

11. Draw the structural formula of a compound with two carbon atoms in each of the following cases.

(a) An alkane with a carbon to carbon single bond

Answer:

(b) An alcohol containing two carbon atoms

Answer:

(c) An unsaturated hydrocarbon with a carbon to carbon triple bond

Answer:

12. [Ethane, Ethene, Ethanoic acid, Ethyne, Ethanol]

From the above list name
(a) The compound with -OH as the part of its structure.
(b) The compound with -COOH as the part of its structure.
(c) Homologue of Homologous series with general formula CnH₂n.

Answer: (a) The compound with -OH as the part of its structure is Ethanol.
(b) The compound with -COOH as the part of its structure is Ethanoic acid.
(c) Homologue of Homologous series with general formula CnH₂n is Ethene.

13. Give the correct IUPAC name and the functional group for each of the compounds whose structural formulae are given below:

Answer: (a) IUPAC name: Propanal
Functional group: Aldehydic (-CHO)

(b) IUPAC name: Propan-1-ol (or 1-Propanol)
Functional group: Alcoholic/Hydroxyl (-OH)

14. Copy and complete the following table which relates to three homologous series of hydrocarbons

General Formula

IUPAC name of the homologous series

Characteristic bond type

IUPAC name of the first member of the series

CₙH₂ₙ

CₙH₂ₙ₋₂

CₙH₂ₙ₊₂

Single bonds

Type of reaction with chlorine

Addition

Answer:

General Formula

IUPAC name of the homologous series

Characteristic bond type

IUPAC name of the first member of the series

CₙH₂ₙ

Alkene

Double bond

Ethene

CₙH₂ₙ₋₂

Alkyne

Triple bond

Ethyne

CₙH₂ₙ₊₂

Alkane

Single bonds

Methane

Type of reaction with chlorine

Addition

Substitution

15. Fill in the blanks with the correct words from the brackets:

(a) Alkenes are the (i) ……… (analogous/homologous) series of (ii) ……… (saturated/unsaturated) hydrocarbons. They differ from alkanes due to the presence of (iii) ……… (double/single) bonds. Alkenes mainly undergo (iv) ……… (addition/substitution) reactions.
(b) The organic compound which undergoes substitution reaction is (v) ……… (C₂H₂, C₂H₄, C₁₀H₁₈, C₂H₆) (2018)
(c) Draw the structural formulae of the two isomers of Butane. Give the correct IUPAC name of each isomer.

Answer: (a) Alkenes are the (i) homologous series of (ii) unsaturated hydrocarbons. They differ from alkanes due to the presence of (iii) double bonds. Alkenes mainly undergo (iv) addition reactions.
(b) The organic compound which undergoes substitution reaction is (v) C₂H₆.
(c) The structural formulae of the two isomers of Butane (C₄H₁₀) are:

n-Butane (IUPAC name: Butane)

Isobutane (IUPAC name: 2-Methylpropane)

16. Name:

(a) The saturated hydrocarbon containing two carbon atoms.
(b) An alcohol with three carbon atom.
(c) A triple bond hydrocarbon with two carbon atoms.

Answer: (a) The saturated hydrocarbon containing two carbon atoms is Ethane.
(b) An alcohol with three carbon atom is Propanol (or Propan-1-ol / Propan-2-ol).
(c) A triple bond hydrocarbon with two carbon atoms is Ethyne.

Exercise B

1. State the sources of Alkanes.

Answer: The principal sources of alkanes are natural gas and petroleum. Natural gas contains mainly methane, with smaller amounts of ethane, propane and butane.

2. Methane is a green house gas. Comment.

Answer: Methane is a primary constituent of natural gas. It absorbs outgoing heat radiation from the earth, and thus contributes to the green house effect and so is considered a greenhouse gas. Methane remains in the atmosphere for approximately 10 years. It is twenty times more effective in trapping heat in comparison to carbon dioxide.

3. Give the general formula of alkanes.

Answer: The general formula for alkane is CnH2n+2.

4. Draw the structures of isomers of :

(a) butane
(b) pentane

Write the IUPAC and common names of these isomers.

Answer: (a) Isomers of butane (C₄H₁₀)

n-butane. IUPAC Name: Butane.

Iso-butane. IUPAC Name: 2 Methylpropane.

(b) Isomers of pentane (C₅H₁₂)

n-pentane. IUPAC Name: Pentane.

iso-pentane. IUPAC Name: 2-methyl butane.

neo-pentane. IUPAC Name: 2, 2-dimethyl propane.

5. Write the:

(a) molecular formula
(b) electron dot formula and
(c) structural formula of methane and ethane.

Answer: (a) Molecular formula:

Methane: CH₄
Ethane: C₂H₆

(b) Electron dot formula:

Methane:

Ethane:

Methane:

Ethane:

6. How is (a) methane and (b) ethane prepared in the laboratory ?

Answer: (a) Methane is prepared in the laboratory using sodium ethanoate (sodium acetate) and soda lime.

A mixture of sodium ethanoate (sodium acetate) and soda lime is taken in a hard glass test tube and heated over a bunsen burner. The reaction is:
CH3COONa + NaOH → (CaO, 300°C) → Na2CO3 + CH4↑

The gas evolved is collected by downward displacement of water since it is slightly soluble in water and is lighter than air.

(b) Ethane is prepared in the laboratory using sodium propanoate (sodium propionate) and soda lime.

A mixture of sodium propionate and soda lime is taken in a boiling tube and heated over a bunsen burner. The reaction is: CH₃CH₂COONa + NaOH →(CaO, 300°C)→ Na₂CO₃ + C₂H₆↑

The gas evolved is collected by downward displacement of water.

7. How are methane and ethane prepared from methyl iodide and ethyl bromide ?

Answer: Methane is prepared from iodomethane (methyl iodide) when it is reduced by nascent hydrogen at ordinary room temperature.
CH₃I + 2[H] → CH₄ + HI (Iodomethane to Methane)

Ethane is prepared from bromoethane (ethyl bromide) when it is reduced by nascent hydrogen at ordinary room temperature.
C₂H₅Br + 2[H] → C₂H₆ + HBr (Bromoethane to Ethane)

Nascent hydrogen is produced by the action of Zn powder and dil. HCl or Zn/Cu couple in alcohol.

Ethane can also be prepared from methyl iodide. When methyl iodide is warmed with sodium metal in the presence of dry ether, ethane is produced.
2CH₃I + 2Na →(dry ether)→ CH₃–CH₃ + 2NaI

This reaction is referred to as Wurtz reaction. Methane cannot be prepared by this method.

8. What is a substitution reaction? Give the reaction of chlorine with ethane and name the product formed.

Answer: A substitution reaction is one in which one atom of a methane molecule is replaced by a chlorine atom.
Ethane also reacts with chlorine in the same way as methane, producing successively chloroethane, dichloroethane, trichloroethane, tetrachloroethane, pentachloroethane and hexachloroethane.

The first product formed when ethane reacts with chlorine is chloroethane (monochloroethane).
C2H6 (ethane) + Cl2 → C2H5Cl (monochloroethane) + HCl

9. Name the compounds formed when methane burns in :

(a) sufficient air,
(b) insufficient air.

Give a balanced equation.

Answer: (a) When methane burns in sufficient air (complete combustion), it forms carbon dioxide and water vapour.
CH₄ + 2O₂ → CO₂ + 2H₂O

(b) When methane burns in an insufficient supply of air (oxygen), it forms carbon monoxide and water.
2CH₄ + 3O₂ → 2CO + 4H₂O

With still less oxygen, it forms carbon (soot) and water.
CH₄ + O₂ → C + 2H₂O (soot)

10. Write the names and the formula of the products formed when:

(a) methane,
(b) ethane,

reacts with: (i) chlorine
(ii) bromine

Write the chemical equations.

Answer: (a) Methane

(i) Reaction with chlorine:
Chloromethane [CH₃Cl] and Hydrochloric acid [HCl] are formed when methane reacts with chlorine.
CH₄ + Cl₂ → (diffused sunlight or 600 K) → CH₃Cl + HCl

(ii) Reaction with bromine:
Bromomethane [CH₃Br] and Hydrogen bromide [HBr] are formed when methane reacts with bromine.
CH₄ + Br₂ → CH₃Br + HBr

(b) Ethane

(i) Reaction with chlorine:
Chloroethane [C₂H₅Cl] and Hydrochloric acid [HCl] are formed when ethane reacts with chlorine.
C₂H₆ + Cl₂ → C₂H₅Cl + HCl

(ii) Reaction with bromine:
Bromoethane [C₂H₅Br] and Hydrogen bromide [HBr] are formed when ethane reacts with bromine.
C₂H₆ + Br₂ → C₂H₅Br + HBr

11. Name the compound prepared from :

(a) sodium propionate,
(b) methyl iodide and
(c) ethyl bromide.

Write a balanced equation for the same.

Answer: (a) From sodium propionate:
Ethane is prepared.
CH₃CH₂COONa + NaOH → (CaO, 300°C) → Na₂CO₃ + C₂H₆↑ (Ethane)

(b) From methyl iodide:
Methane can be prepared by reduction.
CH₃I + 2[H] → CH₄ (Methane) + HI
Ethane can be prepared by the Wurtz reaction.
2CH₃I + 2Na → (dry ether) → CH₃–CH₃ (Ethane) + 2NaI

12. Write the equation for the complete combustion of

(i) methane
(ii) ethane.

Answer: (i) Methane:
CH₄ + 2O₂ → CO₂ + 2H₂O

(ii) Ethane:
2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O

13. Convert:

(a) Methane into chloroform,
(b) Sodium acetate into methane,
(c) Methyl iodide into ethane,
(d) Methane to methyl alcohol

Answer: (a) Methane into chloroform (Trichloromethane):
CH₄ + Cl₂ → CH₃Cl + HCl
CH₃Cl + Cl₂ → CH₂Cl₂ + HCl
CH₂Cl₂ + Cl₂ → CHCl₃ (Chloroform) + HCl

(b) Sodium acetate (Sodium ethanoate) into methane:
CH₃COONa + NaOH → (CaO, 300°C) → Na₂CO₃ + CH₄↑ (Methane)

(d) Methane to methyl alcohol:
When a mixture of methane and oxygen in the ratio 9:1 by volume is compressed to about 120 atm pressure and passed over copper tubes at 475 K, methyl alcohol is formed.
2CH₄ + O₂ → (475 K, Cu tube, 120 atm) → 2CH₃OH (Methyl alcohol)

14. Give three uses of :

(a) methane,
(b) ethane.

Answer: (a) Methane:
(i) Methane is a source of carbon monoxide and hydrogen.
(ii) It is used in the preparation of useful compounds like ethyne (acetylene), methanal (formal-dehyde), methanol, chloro-methane and tetrachloro-methane (carbon tetrachloride).
(iii) It is employed as a domestic fuel.

(b) Ethane:
(i) Ethane is used in the preparation of ethene, ethanol, ethanal (acetaldehyde) and ethanoic acid (acetic acid).
(ii) It forms ethyl chloride, which is used to make tetraethyllead. 1, 1, 1-trichloroethane is one solvent that is used a lot, in dry cleaning.
(iii) Ethane is also a good fuel.

15. Under what conditions does ethane get converted to :

(a) ethyl alcohol
(b) acetaldehyde,
(c) acetic acid.

Answer:

(a) Ethyl alcohol is obtained when ethane reacts with oxygen under a pressure of 120 atm and a temperature of 475 K, while being passed through copper tubes.
2C₂H₆ + O₂ → (Cu tube, 475 K, 120 atm) → 2C₂H₅OH (Ethyl alcohol)

(b) Acetaldehyde is formed when ethane reacts with oxygen in the presence of molybdenum oxide (MoO) as a catalyst.
C₂H₆ + O₂ → (MoO) → CH₃CHO (Acetaldehyde) + H₂O

(c) Acetic acid is produced in two steps. First, ethane reacts with oxygen under high pressure and temperature to form ethyl alcohol. Then, ethyl alcohol is oxidized in the presence of platinum at 300°C to form acetic acid.
2C₂H₆ + O₂ → (Cu tube, 475 K, 120 atm) → 2C₂H₅OH (Ethyl alcohol)
C₂H₅OH + O₂ → (Pt, 300°C) → CH₃COOH (Acetic acid) + H₂O

16. Using appropriate catalysts, ethane can be oxidized to an alcohol, an aldehyde and an acid. Name the alcohol, aldehyde and acid formed when ethane is oxidized.

Answer: When ethane is oxidized using appropriate catalysts:

The alcohol formed is ethyl alcohol (C₂H₅OH).
The aldehyde formed is ethanal (acetaldehyde, CH₃CHO).
The acid formed is ethanoic acid (acetic acid, CH₃COOH).

Exercise C

1. Write: (a) molecular formula, (b) electron dot formula and (c) structural formula of ethene (ethylene).

Answer: (a) The molecular formula of ethene (ethylene) is C₂H₄.

(b) The electron dot formula of ethene is:

2. The molecules of alkene family are represented by a general formula CnH2n.

Answer the following:

(a) What do n and 2n signify ?
(b) What is the name of alkene when n = 4?
(c) What is the molecular formula of alkene when n = 4 ?
(d) What is the molecular formula of the alkene if there are ten H atoms in it ?
(e) What is the structural formula of the third member of the alkene family ?
(f) Write the molecular formula of lower and higher homologous of an alkene which contains four carbon atoms.

Answer: (a) In the general formula CnH2n for alkenes, ‘n’ signifies the number of carbon atoms, and ‘2n’ signifies the number of hydrogen atoms.

(b) When n = 4, the alkene is Butene.

(d) If an alkene has ten H atoms, then 2n = 10, so n = 5. The molecular formula is C₅H₁₀.

(e) The first member of the alkene family is Ethene (C₂H₄, n=2). The second member is Propene (C₃H₆, n=3). The third member is Butene (C₄H₈, n=4). A structural formula for Butene is:

(f) An alkene which contains four carbon atoms is Butene (C₄H₈).
The lower homologous (with three carbon atoms) is Propene (C₃H₆).
The higher homologous (with five carbon atoms) is Pentene (C₅H₁₀).

3. (a) Distinguish between the saturated hydrocarbon ethane and the unsaturated hydrocarbon ethene by drawing their structural formulae.
(b) Draw the structures of isomers of butene and write their IUPAC names.

Answer: (a) Ethane is a saturated hydrocarbon, meaning all carbon-carbon bonds are single bonds. Ethene is an unsaturated hydrocarbon, meaning it contains at least one carbon-carbon double bond.

Structural formula of Ethane (C₂H₆):

Structural formula of Ethene (C₂H₄):

(b) Butene (C₄H₈) has three isomers:

(i) CH₃CH₂CH = CH₂

(ii) CH₃CH = CHCH₃

(iii) CH₂ = C – CH₃

4. Give a balanced equation for the lab. preparation of ethylene. How is the gas collected ?

Answer: The lab preparation of ethylene (ethene) can be done by the dehydration of ethyl alcohol.

Balanced equation:
C₂H₅OH →(Conc. H₂SO₄ (excess), 170°C)→ C₂H₄ + H₂O

The gas is collected by downward displacement of water because it is an inflammable gas and it is insoluble in water.

5. How is ethene prepared by:

(a) dehydrohalogenation reaction ?
(b) dehydration reaction ?

Give equations and name the products formed.

Answer: (a) Dehydrohalogenation is the process in which a hydrogen halide is eliminated from an alkyl halide.
C₂H₅Br + KOH (alcoholic, hot, and concentrated) → C₂H₄ + KBr + H₂O
The products obtained are ethene, potassium bromide, and water.

(b) Dehydration is the process in which elements of water are eliminated from an alcohol. Concentrated sulphuric acid acts as the dehydrating agent.
C₂H₅OH →(Conc. H₂SO₄ (excess), 170°C)→ C₂H₄ + H₂O
The products formed are ethene and water.

6. (a) Ethene when reacts with halogens (chlorine and bromine) form saturated products. Name them and write balanced equations.
(b) Give the conditions and the main product formed by hydrogenation of ethene.

Answer: (a) When ethene reacts with halogens, addition occurs across the double bond, forming saturated dihaloalkanes.
Reaction with chlorine:
CH₂ = CH₂ + Cl₂ → CH₂Cl–CH₂Cl
Product name: 1, 2-dichloroethane
Reaction with bromine:
CH₂ = CH₂ + Br₂ → CH₂Br–CH₂Br
Product name: 1, 2-dibromoethane

(b) For hydrogenation of ethene, ethene and hydrogen are passed over finely divided catalysts such as platinum or palladium at ordinary temperature or nickel at 200°C.
The main product formed is ethane (C₂H₆).
Equation: C₂H₄ + H₂ →Ni, 200°C→ C₂H₆

7. Convert ethanol into ethene using
(a) solid dehydrating agent
(b) hot conc. H₂SO₄
Give only balanced equations.

Answer: (a) Using a solid dehydrating agent like alumina (Al₂O₃) at 350°C:
C₂H₅OH→Al₂O₃, 350°C→C₂H₄ + H₂O

(b) Using hot conc. H₂SO₄ at 170°C:
C₂H₅OH→Conc. H₂SO₄, 170°C→C₂H₄ + H₂O

8. Write the following properties of ethene :

(a) Physical state,
(b) Odour,
(c) Density as compared to air,
(d) Solubility.

Answer: (a) Physical state: Ethene is a colourless and inflammable gas.
(b) Odour: Ethene has a peculiar odour (faint sweetish smell).
(c) Density as compared to air: Ethene has a vapour density of 14.
(d) Solubility: It is sparingly soluble in water but highly soluble in organic solvents like alcohol, ether and chloroform.

9. How would you convert :

(a) ethyl bromide into ethene ?
(b) ethene into 1, 2-dibromoethane ?
(c) ethene into ethane ?

Answer: (a) Ethyl bromide into ethene: By dehydrohalogenation using alcoholic hot conc. potassium hydroxide.
C₂H₅Br + KOH→(alcoholic, hot and conc.)→C₂H₄ + KBr + H₂O

(b) Ethene into 1, 2-dibromoethane: By addition of bromine to ethene.
CH₂ = CH₂ + Br₂ → CH₂Br–CH₂Br

(c) Ethene into ethane: By hydrogenation of ethene using hydrogen gas with a nickel catalyst at 200°C.
C₂H₄ + H₂→Ni, 200°C→C₂H₆

10. Give balanced equations when :

(a) ethene is burnt in excess of oxygen.
(b) ethene reacts with chlorine gas. (2019)
(c) ethene combines with hydrogen chloride.
(d) a mixture of ethene and hydrogen is passed over nickel at 200°C.

Answer: (a) When ethene is burnt in excess of oxygen:
C₂H₄ + 3O₂ → 2CO₂ + 2H₂O + Heat

(b) When ethene reacts with chlorine gas:
CH₂ = CH₂ + Cl₂ → CH₂Cl–CH₂Cl

(d) When a mixture of ethene and hydrogen is passed over nickel at 200°C:
C₂H₄ + H₂ →Ni, 200°C→C₂H₆

11. Give the formula and names of A, B, C and D in the following equations :

(a) CH₄→Cl₂,-HCl→A→Cl₂,-HCl→B→Cl₂,-HCl→C→Cl₂,-HCl→D
(b) C₂H₂→H₂→A→H₂→B→Br₂,-HBr→C→Br₂,-HBr→D
(c) C₂H₄ + B→200°C, Ni→C₂H₆

Answer: (a) This reaction sequence describes the chlorination of methane.
A is CH₃Cl (Chloromethane or Methyl chloride)
B is CH₂Cl₂ (Dichloromethane)
C is CHCl₃ (Trichloromethane or Chloroform)
D is CCl₄ (Tetrachloromethane or Carbon tetrachloride)

(b) Assuming the question intends sequential addition reactions:
C₂H₂ (Ethyne) H₂ A
A is C₂H₄ (Ethene)
A (C₂H₄) H₂ B
B is C₂H₆ (Ethane)
B (C₂H₆) HBr C (This step is substitution for an alkane)
C is C₂H₅Br (Bromoethane)
C (C₂H₅Br) Br₂,-HBr> D
D is C₂H₄Br₂

(c) C₂H₄ + B >200°C, Ni> C₂H₆
For this reaction to yield C₂H₆ (Ethane) from C₂H₄ (Ethene) by hydrogenation, B must be H₂ (Hydrogen gas).
So, B is H₂ (Hydrogen).

12. Write the name and formula of the product formed in each case below:

(a) C₂H₄ + Cl₂ →
(b) C₂H₅I + KOH (alc.) →
(c) H₂C = CH₂ →alk.KMnO₄→
(d) H₂C = CH₂ + HBr →

Answer: (a) C₂H₄ + Cl₂ →
Product name: 1, 2-dichloroethane
Formula: CH₂Cl–CH₂Cl

(b) C₂H₅I + KOH (alc.) →
Product name: Ethene (and KI, H₂O)
Formula: C₂H₄ (or H₂C = CH₂)

(d) H₂C = CH₂ + HBr →
Product name: Bromoethane (Ethyl bromide)
Formula: CH₃–CH₂Br

13. What do you observe when ethene is passed through alkaline KMnO₄ solution ?

Answer: When ethene is passed through alkaline KMnO₄ solution, the purple colour of KMnO₄ decolourises. This is because ethene is oxidised to 1, 2-Ethanediol (Ethylene glycol).

14. Name three compounds formed by ethene and give one use of each compound.

Answer: Three compounds formed by ethene are:

Polythene:
Use: Ethene is used in making polythene.
Ethanol:
Use: Ethanol is used as a starting material for other products, mainly cosmetics and toiletry preparations.
Epoxyethane:
Use: It is used in making epoxyethane (used in the manufacture of detergents).

Exercise D

1. What are the sources for alkynes? Give the general formula of alkynes.

Answer: The sources of alkynes are natural gas and petroleum. The general formula of alkynes is CnH2n-2.

2. Give an example of isomers shown by triple bond hydrocarbons (alkynes) and write their IUPAC names.

Answer: An example of isomerism shown by triple-bonded hydrocarbons (alkynes) is butyne, which exhibits position isomerism.

(i) CH₃–CH₂–C≡CH
IUPAC Name: But-1-yne

(ii) CH₃–C≡C–CH₃
IUPAC Name: But-2-yne

3. How is ethyne prepared in the laboratory?

(a) Draw a diagram.
(b) Give an equation.
(c) How is pure dry gas collected?

Answer: Ethyne is prepared in the laboratory using water and calcium carbide.

(a)

(b) The equation for the laboratory preparation of ethyne is:
CaC2 + 2H2O → Ca(OH)2 + C2H2↑
Calcium carbide + Water → Calcium hydroxide + acetylene (ethyne)

(c) The gas is collected by the downward displacement of water, since it is insoluble in water. Impurities like phosphine, hydrogen sulphide, ammonia and arsine are formed along with ethyne due to the contamination of calcium sulphide and calcium phosphide in calcium carbide. On passing through water, all impurities except phosphine are absorbed. Phosphine is absorbed by passing through acidified copper sulphate or acidified potassium dichromate solution. Flask should not contain air as ethyne forms explosive mixture with air. The air of the flask is displaced with oil or some inert gas.

4. Give the method of preparation of ethyne by: 1, 2-dibromoethane.

Answer: Ethyne is prepared from 1, 2-dibromoethane (ethylene dibromide). When 1, 2-dibromoethane (ethylene dibromide) is boiled with alcoholic potassium hydroxide, ethyne is formed.

C₂H₄Br₂ + 2KOH →(alcoholic, boiling, 200°C)→ C₂H₂ + 2KBr + 2H₂O

5. Name the organic compound which:

(a) is a tetrahedral molecule,
(b) is a planar molecule,
(c) is a linear molecule,
(d) forms a red precipitate with ammoniacal solution of copper (I) chloride,
(e) is known as paraffin,
(f) is known as olefin.
(g) will give ethyne (acetylene) gas when treated with water.

Answer: (a) Methane (CH₄) is a tetrahedral molecule.
(b) Ethene (C₂H₄) is a planar molecule.
(c) Ethyne (C₂H₂) is a linear molecule.
(d) Ethyne (acetylene) forms a red precipitate of copper acetylide with ammoniacal cuprous chloride.
(e) Alkanes are known as paraffins. For example, Methane.
(f) Alkenes are also called olefins. For example, Ethene.
(g) Calcium carbide (CaC₂) will give ethyne (acetylene) gas when treated with water.

6. Classify the following compounds as alkanes, alkenes and alkynes: C₃H₄, C₃H₈, C₅H₈, C₃H₆

Answer:

C₃H₄: Alkyne (Propyne)
C₃H₈: Alkane (Propane)
C₅H₈: Alkyne (Pentyne)
C₃H₆: Alkene (Propene)

7. Give a chemical test to distinguish between

(a) saturated and unsaturated compounds.
(b) ethane and ethene,
(c) ethene (ethylene) and ethyne (acetylene).

Answer: (a) To distinguish between saturated and unsaturated compounds: Add a few drops of bromine solution in carbon tetrachloride to the hydrocarbon. If it is an unsaturated compound (like alkenes or alkynes), the reddish brown colour of bromine solution gets decolorised. If it is a saturated compound (like alkanes), no change is observed.

Alternatively, add a few drops of alkaline potassium permanganate (purple colour) to the hydrocarbon. If it is an unsaturated compound, the purple colour fades. If it is a saturated compound, no change is observed.

(b) To distinguish between ethane and ethene: Add a few drops of bromine solution in carbon tetrachloride. With ethene (unsaturated), the reddish brown colour of bromine solution gets decolorised. With ethane (saturated), no change is observed.

Alternatively, add a few drops of alkaline potassium permanganate. With ethene, the purple colour fades. With ethane, no change is observed.

(c) To distinguish between ethene (ethylene) and ethyne (acetylene): Add a few drops of ammoniacal cuprous chloride to the hydrocarbon. With ethyne, a red precipitate of copper acetylide is formed. With ethene, no change is observed.

Alternatively, add ammoniacal silver nitrate. With ethyne, a white precipitate of silver acetylide is formed. With ethene, no change is observed.

8. Compound X is bubbled through bromine dissolved in carbon tetrachloride;

(a) Draw the structure of X.
(b) State your observation during the reaction.

Answer: (a) The compound X is ethene. The structure of X (ethene) is:

(b) When compound X (ethene) is bubbled through bromine dissolved in carbon tetrachloride, the reddish brown colour of bromine disappears due to the formation of the colourless dibromo ethane.

9. Give balanced equations for the following conversions.

(a) An alkene to an alkane.
(b) An alkene to an alcohol.
(c) An alkyne to an alkene.

Answer: (a) Conversion of an alkene to an alkane:
C₂H₄ + H₂ →(Nickel, 300°C)→ C₂H₆

(b) Conversion of an alkene to an alcohol:
C₂H₄ + H₂O →(H₃PO₄, 300°C, 60 atm)→ C₂H₅OH

10. Name the products formed and write an equation when ethyne is added to the following in an inert solvent:

(a) chlorine,
(b) bromine,
(c) iodine,
(d) hydrogen.
(e) excess of hydrochloric acid.

Answer: (a) Ethyne with chlorine:
Initially, 1,2-dichloroethene is produced, followed by the formation of 1,1,2,2-tetrachloroethane.
CH≡CH + Cl₂ (in CCl₄) → CHCl=CHCl (1,2-dichloroethene)
CHCl=CHCl + Cl₂ (in CCl₄) → CHCl₂–CHCl₂ (1,1,2,2-tetrachloroethane)

(b) Ethyne with bromine:
First, 1,2-dibromoethene is formed, and then 1,1,2,2-tetrabromoethane is obtained.
C₂H₂ + Br₂ (in CBr₄) → C₂H₂Br₂ (1,2-dibromoethene)
C₂H₂Br₂ + Br₂ (in CBr₄) → C₂H₂Br₄ (1,1,2,2-tetrabromoethane)

(d) Ethyne with hydrogen:
Ethyne first forms ethene, which on further hydrogenation gives ethane.
CH≡CH + H₂ →(Ni)→ CH₂=CH₂ (Ethene)
CH₂=CH₂ + H₂ →(Ni)→ CH₃–CH₃ (Ethane)

(e) Ethyne with excess hydrochloric acid:
Chloroethene is formed first, followed by 1,1-dichloroethane.
CH≡CH + HCl → CH₂=CHCl (Chloroethene)
CH₂=CHCl + HCl → CH₃–CHCl₂ (1,1-dichloroethane)

11. Substitution reactions are characteristic reactions of ____ (alkynes/alkenes/alkanes).

Answer: Substitution reactions are characteristic reactions of alkanes.

12. (a) Write an equation for the laboratory preparation of
(i) an unsaturated hydrocarbon from calcium carbide.
(ii) an alcohol from ethyl bromide.

(b) What would you see, when ethyne is bubbled through a solution of bromine in carbon tetrachloride?
(c) Name the addition product formed between ethene and water.

Answer: (a) (i) Equation for the laboratory preparation of an unsaturated hydrocarbon (ethyne) from calcium carbide:
CaC₂ + 2H₂O → Ca(OH)₂ + C₂H₂↑

(a) (ii) Equation for the laboratory preparation of an alcohol (ethanol) from ethyl bromide:
C₂H₅Br + KOH (aqueous) →(boiling)→ C₂H₅OH + KBr

(b) When ethyne is bubbled through a solution of bromine in carbon tetrachloride, its reddish brown colour disappears due to the formation of addition products.

(c) The addition product formed between ethene and water is ethanol (C2H5OH).

13. Give reasons:

(a) Ethyne is more reactive than ethene.
(b) Ethene is more reactive than ethane.
(c) Hydrocarbons are excellent fuels.

Answer: (a) Ethyne is more reactive than ethene due to the presence of a triple bond between its two carbon atoms, which breaks up easily for addition reactions.

(b) Ethene is more reactive than ethane because ethene is an unsaturated hydrocarbon with a double bond which can undergo addition reactions, while ethane is a saturated hydrocarbon and undergoes substitution reactions which are generally slower.

(c) Hydrocarbons are excellent fuels because they burn in air or oxygen to produce carbon dioxide and water vapour, and a large amount of heat is also given out.

14. (a) Write balanced equations
(i) when butane is burnt in oxygen
(ii) for preparation of ethylene from ethyl alcohol.

(b) (i) Convert ethyne to tetrabromoethane
(ii) Convert ethyne to ethane

Answer: (a) (i) When butane is burned in oxygen, carbon dioxide and water are formed.
2C₄H₁₀ + 13O₂ → 8CO₂ + 10H₂O

(a) (ii) Ethylene (ethene) can be prepared from ethyl alcohol by dehydration.
C₂H₅OH →(Conc. H₂SO₄ / 170°C)→ C₂H₄ + H₂O
or
C₂H₅OH →(Al₂O₃ / 350°C)→ C₂H₄ + H₂O

(b) (i) Ethyne can be converted to tetrabromoethane by addition of bromine.
C₂H₂ + 2Br₂ → C₂H₂Br₄ (1,1,2,2-tetrabromoethane)

(b) (ii) Ethyne can be hydrogenated to ethane in the presence of a nickel catalyst.
C₂H₂ + 2H₂ →(Ni)→ C₂H₆

15. (a) Write the equation for the preparation of carbon tetrachloride from methane.
(b) Draw the structural formula of ethyne.
(c) How is the structure of alkynes different from that of alkenes?

Answer: (a) CH₄ + Cl₂ →(diffused sunlight)→ CH₃Cl + HCl
CH₃Cl + Cl₂ → CH₂Cl₂ + HCl
CH₂Cl₂ + Cl₂ → CHCl₃ + HCl
CHCl₃ + Cl₂ → CCl₄ + HCl

(b) Structural formula of ethyne:

H – C ≡ C – H

(c) The structure of alkynes is different from that of alkenes in that alkynes contain a triple bond, i.e., acetylenic bond, between two carbon atoms, while alkenes contain at least one carbon-carbon double bond.

Exercise E

1. (a) What are alcohols? State their sources.

Answer: Alcohols are the hydroxyl (-OH) derivatives of alkanes. They are formed by replacing one or more hydrogen atoms of the alkane with an OH group.
Since alcohols are not found naturally in the earth’s atmosphere, they are obtained by artificial synthesis in the laboratory. For example, methanol (wood spirit) is obtained from destructive distillation of wood, while ethanol is obtained by fermentation of sugar. Cracking of petroleum is a source of ethane, which is used for preparing ethanol. Ethyl alcohol is an essential constituent of all wines and is called SPIRIT OF WINE. It is obtained from starchy grain and is also known as GRAIN ALCOHOL.

(b) Give general formulae of monohydric alcohol.

Answer: The general formula of monohydric alcohols is C_nH_2n+1OH.

2. Give the

(a) dot diagram of first member of alcohol

Answer: (a) Methyl alcohol (CH₃OH) is the first member of the alcohol series.

(b) abbreviated formula of third member of alcohol

Answer: The abbreviated formula of the third member of the alcohol group, propyl alcohol, is CH₃-CH₂-CH₂-OH.

(c) structure of second member of the alcohol group

Answer:

(d) structure of alcohol with 4 carbon atoms.

Answer:

3. State the method of preparation of ethanol :
(a) by hydrolysis of ethene,

Answer: When concentrated sulphuric acid is added to ethene (obtained from cracking of petroleum) at a temperature of 80°C and pressure of 30 atm, ethyl hydrogen sulphate is produced. Ethyl hydrogen sulphate, on hydrolysis with boiling water, gives ethanol.
C₂H₄ + H₂SO₄ →(80°C, 30 atm)→ C₂H₅HSO₄
C₂H₅HSO₄ + H₂O (boiling) → C₂H₅OH + H₂SO₄

Alternatively, ethanol is produced when ethene is heated with water at 300°C and 60 atmospheric pressure in presence of phosphoric acid (catalyst).
C₂H₄ + H₂O →(H₃PO₄, 300°C, 60 atm)→ C₂H₅OH

(b) by hydrolysis of ethyl bromide.

Answer: Ethanol can be prepared by the hydrolysis of ethyl bromide (an alkyl halide/haloalkane) on reaction with a hot dilute alkali or when an alkyl halide is boiled with aqueous alkalis. For example, with aqueous NaOH: C₂H₅Br + NaOH (aqueous) →(boil)→ C₂H₅OH + NaBr

4. Halo alkanes react with alkalies to produce alcohol. Give the equation for the preparation of second member of homologous series of alcohol. State under what condition the reaction occurs.

Answer: Haloalkanes react with alkalies to produce alcohol. The equation for the preparation of the second member of the homologous series of alcohol (ethanol) from an ethyl halide like ethyl chloride using an alkali like KOH is: C₂H₅Cl + KOH (aqueous) →(boil)→ C₂H₅OH + KCl

The reaction occurs when the alkyl halide is boiled with aqueous alkali.

5. (a) How do the boiling point and melting point change in the homologous series of alcohols ?

Answer: In the homologous series of alcohols, their boiling point increases with an increase in molecular weight.

(b) Name the product formed when ethanol reacts with acetic acid. Give an equation.

Answer: When ethanol reacts with ethanoic (acetic) acid in the presence of conc. H2SO4 at high temperature, the product formed is ethyl acetate (an ester).

The equation is: C₂H₅OH + CH₃COOH →(conc. H₂SO₄, Δ)→ CH₃COOC₂H₅ + H₂O
(Ethanol) (Acetic acid) (Ethyl acetate)

(c) What is the name given to this type of reaction ?

Answer: The reaction where alcohols react with ethanoic (acetic) acid to produce esters is known as esterification.

6. Complete and balance the following equations. State the conditions wherever necessary.

(a) CH≡CH + H₂ → _______ + H₂ → _______

Answer: CH≡CH + H₂ →(Ni) CH₂=CH₂
(Ethyne) (Ethene)
CH₂=CH₂ + H₂ →(Ni) CH₃-CH₃
(Ethene) (Ethane)
Conditions: Nickel (Ni) catalyst. This is catalytic hydrogenation.

(b) C₂H₄ + Br₂ → _______

Answer: C₂H₄ + Br₂ → CH₂Br-CH₂Br (or C₂H₄Br₂)
(Ethene) (1, 2-dibromoethane)
Condition: This reaction occurs at room temperature, often in an inert solvent like CCl₄.

(c) C₂H₄ + HCl → _______

Answer: C₂H₄ + HCl (aq) → CH₃–CH₂Cl
(Ethene) (Ethyl chloride or chloroethane)

(d) CaC₂ + H₂O → _______

Answer: CaC₂ + 2H₂O → Ca(OH)₂ + C₂H₂↑
(Calcium carbide) (Calcium hydroxide) (Ethyne/acetylene)

(e) C₂H₂ + Br₂ → _______

Answer: C₂H₂ + Br₂ (from CBr₄) → CHBr=CHBr (1,2-dibromoethene)
CHBr=CHBr + Br₂ (from CBr₄) → CHBr₂-CHBr₂ (1,1,2,2-tetrabromoethane)
(Ethyne)

(f) C₂H₅OH →([O], K₂Cr₂O₇) _______

Answer: C₂H₅OH + [O] →(K₂Cr₂O₇/H⁺) CH₃CHO + H₂O
(Ethyl alcohol) (Ethanal/Acetaldehyde)
CH₃CHO + [O] →(K₂Cr₂O₇/H⁺) CH₃COOH
(Ethanal/Acetaldehyde) (Ethanoic acid/Acetic acid)
The overall reaction can be considered as C₂H₅OH + 2[O] →(K₂Cr₂O₇/H⁺) CH₃COOH + H₂O
Condition: Acidified potassium dichromate (K₂Cr₂O₇) as an oxidising agent.

7. What is the effect of ethanol on human body ?

Answer: Ethyl alcohol affects the part of the brain which controls our muscular movements. It gives temporary relief from tiredness, but it damages the liver and kidney too in excess quantities. Spurious alcohol, which contains large proportions of methanol, is fatal for human consumption.

8. How are the following obtained :
(a) absolute alcohol,

Answer: Absolute alcohol may be obtained by distilling moist alcohol (95% pure ethanol) with benzene (an organic solvent). The mixture of water and benzene distills off and anhydrous alcohol (absolute alcohol) is left behind.

(b) spurious alcohol,

Answer: Spurious alcohol is illicit liquor made by improper distillation. It contains large proportions of methanol in a mixture of alcohols.

(b) spurious alcohol,

Answer: Spurious alcohol is illicit liquor made by improper distillation. It contains large proportions of methanol in a mixture of alcohols.

(c) methylated spirit ?

Answer: Methylated spirit or denatured alcohol is ethyl alcohol with 5% methyl alcohol, a coloured dye and some pyridine.

9. Name the products formed and give appropriate chemical equations for the following:
(a) Sodium reacting with ethyl alcohol.

Answer: When sodium reacts with ethyl alcohol at room temperature, hydrogen is evolved with the formation of sodium ethoxide.
Product formed: Sodium ethoxide and Hydrogen gas.
Chemical equation: 2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂↑

(b) Ethanol oxidised by acidified potassium dichromate.

Answer: When ethanol is oxidised by acidified potassium dichromate, it first gets converted into ethanal (acetaldehyde) and then further changes into ethanoic acid (acetic acid).
Products formed: Ethanal (acetaldehyde) and Ethanoic acid (acetic acid).

Chemical equations:
C₂H₅OH + [O] →(K₂Cr₂O₇/H⁺) CH₃CHO + H₂O
CH₃CHO + [O] →(K₂Cr₂O₇/H⁺) CH₃COOH

10. Give the trivial (common) names and the IUPAC names of the following:

(a) C₃H₆,

Answer: Trivial (common) name: Propylene
IUPAC name: Propene

(b) C₂H₄,

Answer: Trivial (common) name: Ethylene
IUPAC name: Ethene

(b) C₂H₄,

Answer: Trivial (common) name: Ethylene
IUPAC name: Ethene

(c) C₂H₂,

Answer: Trivial (common) name: Acetylene
IUPAC name: Ethyne

(d) CH₃OH,

Answer: Trivial (common) name: Methyl alcohol (also Wood spirit)
IUPAC name: Methanol

(e) C₂H₅OH.

Answer: Trivial (common) name: Ethyl alcohol (also Spirit of Wine, Grain alcohol)
IUPAC name: Ethanol

11. Ethanol can be oxidised to ethanoic acid. Write the equation and name the oxidising agent.

Answer: When ethanol is treated with an oxidizing agent such as acidified potassium dichromate under high pressure and low temperature, it gets oxidized to ethanoic acid.

C₂H₅OH —[O], K₂Cr₂O₇→ CH₃CHO —[O], K₂Cr₂O₇→ CH₃COOH

Ethanol (ethyl alcohol) first forms ethanal (acetaldehyde), which on further oxidation yields ethanoic acid (acetic acid).

12. Name an organic compound which is :

(a) used for illuminating country houses,

Answer: Ethyne (acetylene) is used as an illuminant in oxy-acetylene lamp.

(b) used for making a household plastic material,

Answer: Ethene is used in making polythene, a household plastic material.

(c) called ‘wood spirit’,

Answer: Methanol is called ‘wood spirit’.

(d) poisonous and contain OH group,

Answer: Methanol is poisonous and contains an OH group. (Ethyl alcohol in excess is also harmful, and spurious alcohol containing methanol is fatal).

(e) consumed as a drink,

Answer: Ethyl alcohol is consumed as a drink in alcoholic beverages like whisky, wine and beer.

(f) used in thermometer

Answer: Ethyl alcohol is used in thermometers.

(g) solvent for gums and resins,

Answer: Ethyl alcohol is a good solvent for gums and resins.

(h) dehydrated to produce ethene.

Answer: Ethyl alcohol is dehydrated to produce ethene.

13. Ethanol can be converted into ethene which can be changed into ethane. Choose the correct word or phrase from the brackets to complete the following sentences.

(a) The conversion of ethanol into ethene is an example of _______ (dehydration, dehydrogenation).

Answer: The conversion of ethanol into ethene is an example of dehydration.

(b) Converting ethanol into ethene requires the use of _______ (conc. HCl, conc. HNO₃, conc. H₂SO₄).

Answer: Converting ethanol into ethene requires the use of conc. H₂SO₄.

(c) The conversion of ethene into ethane is an example of _______ (hydration, hydrogenation).

Answer: The conversion of ethene into ethane is an example of hydrogenation.

(d) The catalyst used in the conversion of ethene into ethane is commonly _______ (iron, nickel, cobalt).

Answer: The catalyst used in the conversion of ethene into ethane is commonly nickel. (Platinum or palladium can also be used).

14. Write the equations for the following lab. preparations:

(a) Ethane from sodium propionate,

Answer: C₂H₅COONa + NaOH —(CaO, 300°C)→ C₂H₆ + Na₂CO₃

(b) Ethene from Iodoethane,

Answer: C₂H₅I + KOH (alcoholic) —(boil)→ H₂C=CH₂ + KI + H₂O

(c) Ethyne from calcium carbide,

Answer: CaC₂ + 2H₂O → C₂H₂ + Ca(OH)₂

(d) Methanol from Iodomethane.

Answer: CH₃I + NaOH → CH₃OH + NaI

15. Name the compound prepared by each of the following reactions:

(i) C₂H₅COONa + NaOH →

Answer: Ethane (C₂H₆)

(ii) CH₃I + 2H →

Answer: Methane (CH₄)

(iii) C₂H₅Br + KOH (alcoholic solution) →

Answer: Ethene (C₂H₄)

(iv) CO + 2H₂ (Zinc oxide catalyst) →

Answer: Methanol (CH₃OH)

(v) CaC₂ + 2H₂O →

Answer: Ethyne (C₂H₂)

16. Write the equations for the following reactions :
(a) Calcium carbide and water.

Answer: CaC₂ + 2H₂O → Ca(OH)₂ + C₂H₂↑

(b) Ethene and water (steam).

Answer: C₂H₄ + H₂O →(H₃PO₄, 300°C, 60 atm) C₂H₅OH

(c) Bromoethane and an aqueous solution of sodium hydroxide.

Answer: C₂H₅Br + NaOH (aqueous) →(boil) C₂H₅OH + NaBr

Exercise F

1. What are carboxylic acids? Give their general formula.

Answer: An organic compound containing the carboxyl group (–COOH) is known as carboxylic acid. These compounds possess acidic properties.
The general formula for carboxylic acids is CₙH₂ₙ₊₁COOH or RCOOH.

2. Write the names of:

(a) first three members of carboxylic acid series.

Answer: The first three members of the carboxylic acid series are:

(i) Formic acid (IUPAC name: Methanoic acid)
(ii) Acetic acid (IUPAC name: Ethanoic acid)
(iii) Propionic acid (IUPAC name: Propanoic acid)

(b) three compounds which can be oxidised directly, or in stages to produce acetic acid.

Answer: Three compounds which can be oxidised directly, or in stages to produce acetic acid (ethanoic acid) are:

(i) Ethane (oxidises to ethyl alcohol, then ethanal, then ethanoic acid)
(ii) Ethyl alcohol (Ethanol) (oxidised to ethanal, then ethanoic acid)
(iii) Ethanal (Acetaldehyde) (oxidised to ethanoic acid)

3. (a) Give the structural formulae of acetic acid.

Answer:

(b) IUPAC name of acetic acid.

Answer: The IUPAC name of acetic acid is ethanoic acid.

(c) What is glacial acetic acid?

Answer: The anhydrous acetic acid on cooling forms a crystalline mass resembling ice; its melting point is 17°C and for this reason, it is called glacial acetic acid.

4. Vinegar is greyish in colour with a particular taste. Explain.

Answer: The presence of a colouring matter gives vinegar a greyish or reddish or brownish colour while the presence of some other organic compound imparts it the usual taste and flavour.

5. Complete:

(a) Vinegar is prepared by the bacterial oxidation of ………

Answer: Vinegar is prepared by the bacterial oxidation of ethanol.

(b) The organic acid present in vinegar is ………

Answer: The organic acid present in vinegar is ethanoic acid.

(c) The next higher homologue of ethanoic acid is ………

Answer: The next higher homologue of ethanoic acid is propanoic acid.

6. How is acetic acid prepared from (a) ethanol and (b) acetylene?

Answer: (a) From Ethanol

Ethanol can be converted to ethanoic acid (acetic acid) through oxidation. The reaction is carried out using a strong oxidizing agent, such as acidified potassium dichromate (K₂Cr₂O₇). The process occurs in two steps: ethanol is first oxidized to ethanal (acetaldehyde), which is then immediately oxidized further to ethanoic acid.

Oxidation of Ethanol to Ethanal
C₂H₅OH + [O] → CH₃CHO
(Ethanol) (Ethanal / Acetaldehyde)
Reagent used: K₂Cr₂O₇
Oxidation of Ethanal to Ethanoic Acid
CH₃CHO + [O] → CH₃COOH
(Ethanal / Acetaldehyde) (Ethanoic Acid / Acetic Acid)
Reagent used: K₂Cr₂O₇

(b) From Acetylene

This is a two-stage industrial process.

Conversion of Acetylene to Ethanal
First, acetylene is hydrated to form ethanal (acetaldehyde). This is done by passing acetylene gas through a 40% sulfuric acid (H₂SO₄) solution at 60°C in the presence of 1% mercury(II) sulfate (HgSO₄) as a catalyst.C₂H₂ + H₂O → CH₃CHO
(Acetylene) (Ethanal / Acetaldehyde)
Catalysts: H₂SO₄ (dilute), HgSO₄
Oxidation of Ethanal to Ethanoic Acid
The ethanal produced is then oxidized to ethanoic acid. This is achieved by passing a mixture of ethanal vapour and air (oxygen) over a manganese acetate catalyst at 70°C.2CH₃CHO + O₂ → 2CH₃COOH
(Ethanal / Acetaldehyde) (Ethanoic Acid / Acetic Acid)
Conditions: Heat (Δ) and a catalyst

7. What do you notice when acetic acid reacts with

(a) litmus,

Answer: When acetic acid reacts with litmus, it turns moist blue litmus red.

(b) metals,

Answer: When acetic acid reacts with active metals (i.e., Zn and Mg), hydrogen gas is evolved.

(c) alkalies,

Answer: When acetic acid reacts with alkalis, it forms salt and water.

(d) alcohol?

Answer: When ethanoic acid (acetic acid) reacts with alcohol in the presence of a dehydrating agent like conc. H₂SO₄, it forms an ester, which has a pleasant fruity smell.

8. Acetic acid is a typical acid. Write one equation in each case for its reaction with

(a) a metal,

Answer: 2CH₃COOH + Zn → (CH₃COO)₂Zn + H₂↑ (Zinc acetate)

(b) a base/alkali,

Answer: CH₃COOH + NaOH → CH₃COONa + H₂O (Sod. acetate)

(c) a carbonate,

Answer: 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂↑

(d) a bicarbonate.

Answer: CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂↑

9. What do you observe when acetic acid is added to :

(a) sodium bicarbonate,

Answer: When acetic acid is added to sodium bicarbonate, it liberates brisk effervescence of carbon dioxide.

(b) ethyl alcohol in the presence of sulphuric acid,

Answer: When acetic acid is warmed with ethyl alcohol in the presence of conc. sulphuric acid, a pleasant fruity smell of ethyl acetate (an ester) is produced.

(c) neutral FeCl₃ solution ?

Answer: On adding acetic acid to neutral iron (III) chloride solution, a wine red colour is produced.

10. Name:
(a) compound formed when acetic acid and ethanol react together.

Answer: The compound formed when acetic acid and ethanol react together is ethyl acetate (an ester).

(b) substance used to change acetic acid to acetic anhydride.

Answer: The substance used to change acetic acid to acetic anhydride is phosphorus pentoxide, upon heating.

Exercise

MCQs

1. Which of the following statements is wrong about alkanes ?

(a) They are all saturated hydrocarbons.
(b) They can undergo addition as well as substitution reaction.
(c) They are almost non polar in nature.
(d) On complete combustion give out carbon dioxide and water.

Answer: (b) They can undergo addition as well as substitution reaction.

2. The organic compound obtained as the end product of the fermentation of sugar solution is :

(a) Methanol
(b) Ethanol
(c) Ethane
(d) Methanoic acid

Answer: (b) Ethanol

3. An organic compound undergoes addition reactions and gives a red colour precipitate with ammoniacal cuprous chloride. Therefore, the organic compound could be :

(a) Ethane
(b) Ethene
(c) Ethyne
(d) Ethanol

Answer: (c) Ethyne

4. An organic weak acid is :

(a) Formic acid
(b) Sulphuric acid
(c) Nitric acid
(d) Hydrochloric acid

Answer: (a) Formic acid

5. The organic compound mixed with ethanol to make it spurious is:

(a) Methanol
(b) Methanoic acid
(c) Methanal
(d) Ethanoic acid

Answer: (a) Methanol

6. The functional group present in acetic acid is :

(a) Ketonic >C = O
(b) Hydroxyl – OH
(c) Aldehydic – CHO
(d) Carboxyl – COOH

Answer: (d) Carboxyl – COOH

7. The unsaturated hydrocarbons undergo :

(a) a substitution reaction
(b) an oxidation reaction
(c) an addition reaction
(d) none of the above

Answer: (c) an addition reaction

8. The number of C-H bonds in ethane molecule are:

(a) Four
(b) Six
(c) Eight
(d) Ten

Answer: (b) Six

9. Hydrocarbon which is a greenhouse gas is :

(a) Butane
(b) Ethylene
(c) Ethane
(d) Methane

Answer: (d) Methane

10. The I.U.P.A.C. name of acetylene is :

(a) propane
(b) propyne
(c) ethene
(d) ethyne

Answer: (d) ethyne

11. The organic compound having a double carbon-carbon bond is:

(a) C₄H₁₀
(b) C₅H₁₀
(c) C₃H₄
(d) C₃H₈

Answer: (b) C₅H₁₀

12. An example of a cyclic organic compound is :

(a) Pentene
(b) Butene
(c) Benzene
(d) Propene

Answer: (c) Benzene

13. The IUPAC name of methyl acetylene is :

(a) Propyne
(b) Ethene
(c) Propane
(d) Ethyne

Answer: (a) Propyne

14. The structures of four hydrocarbons are shown below:

How many isomers of butene are there?

(a) 1
(b) 2
(c) 3
(d) 4

Answer: (b) 2

15. Which of the following is the best reagent to distinguish between ethylene and acetylene ?

P Bromine water
Q Ammoniacal silver nitrate solution
R Acidified potassium dichromate solution

(a) Only P
(b) Only Q
(c) Both P and Q
(d) Both Q and R

Answer: (b) Only Q

16. Which of the following molecules contains >C = C< bond between adjacent carbon atoms ?

P C₄H₈
Q C₃H₄
R C₃H₆

(a) Only P
(b) Only R
(c) Both P and R
(d) Both P and Q

Answer: (c) Both P and R

17. Assertion (A): Carbon shows catenation.
Reason (R) : Catenation is the property of self linking by covalent bonds.

(1) Both A and R are true and R is the correct explanation of A.
(2) Both A and R are true but R is not the correct explanation of A.
(3) A is true but R is false.
(4) A is false but R is true.