Part 1: MCQs (Multiple Choice Questions)

MCQs: Basic Concepts and Classification of Hydrocarbons

1. Hydrocarbons contain only:

A) Carbon and oxygen
B) Carbon and hydrogen
C) Hydrogen and oxygen
D) Carbon and nitrogen

Answer: B) Carbon and hydrogen

2. The general formula of alkanes is:

A) CₙH₂ₙ
B) CₙH₂ₙ₋₂
C) CₙH₂ₙ₊₂
D) CₙHₙ

Answer: C) CₙH₂ₙ₊₂

3. Which of the following is a saturated hydrocarbon?

A) Ethene
B) Ethyne
C) Ethane
D) Benzene

Answer: C) Ethane

4. The general formula of alkenes is:

A) CₙH₂ₙ₊₂
B) CₙH₂ₙ
C) CₙH₂ₙ₋₂
D) CₙHₙ₊₂

Answer: B) CₙH₂ₙ

5. The first member of alkane series is:

A) Ethane
B) Methane
C) Propane
D) Butane

Answer: B) Methane

6. The first member of alkene series is:

A) Methene
B) Ethene
C) Propene
D) Butene

Answer: B) Ethene

7. The first member of alkyne series is:

A) Methyne
B) Ethyne
C) Propyne
D) Butyne

Answer: B) Ethyne

8. Benzene belongs to:

A) Alkanes
B) Alkynes
C) Aromatic hydrocarbons
D) Cycloalkanes

Answer: C) Aromatic hydrocarbons

MCQs: Hybridisation and Structure

9. Carbon atoms in methane are:

A) sp hybridised
B) sp² hybridised
C) sp³ hybridised
D) dsp² hybridised

Answer: C) sp³ hybridised

10. The bond angle in methane is approximately:

A) 90°
B) 109.5°
C) 120°
D) 180°

Answer: B) 109.5°

11. Carbon atoms in ethene are:

A) sp hybridised
B) sp² hybridised
C) sp³ hybridised
D) unhybridised

Answer: B) sp² hybridised

12. The geometry of ethene molecule is:

A) Linear
B) Tetrahedral
C) Trigonal planar
D) Pyramidal

Answer: C) Trigonal planar

13. Carbon atoms in ethyne are:

A) sp hybridised
B) sp² hybridised
C) sp³ hybridised
D) sp³d hybridised

Answer: A) sp hybridised

14. The bond angle in ethyne is:

A) 109.5°
B) 120°
C) 180°
D) 90°

Answer: C) 180°

15. A double bond consists of:

A) Two sigma bonds
B) Two pi bonds
C) One sigma and one pi bond
D) One sigma only

Answer: C) One sigma and one pi bond

16. A triple bond consists of:

A) One sigma and two pi bonds
B) Two sigma and one pi bond
C) Three sigma bonds
D) Three pi bonds

Answer: A) One sigma and two pi bonds

MCQs: Nomenclature and Isomerism

17. The IUPAC name of CH₃–CH₂–CH₃ is:

A) Methane
B) Ethane
C) Propane
D) Butane

Answer: C) Propane

18. The IUPAC name of CH₂=CH₂ is:

A) Ethane
B) Ethene
C) Ethyne
D) Methene

Answer: B) Ethene

19. The IUPAC name of CH≡CH is:

A) Ethane
B) Ethene
C) Ethyne
D) Methyne

Answer: C) Ethyne

20. Compounds having the same molecular formula but different structures are called:

A) Homologues
B) Isomers
C) Isotopes
D) Radicals

Answer: B) Isomers

21. Butane shows:

A) Chain isomerism
B) Optical isomerism
C) Geometrical isomerism
D) Position isomerism only

Answer: A) Chain isomerism

22. Cis-trans isomerism is shown due to:

A) Free rotation around single bond
B) Restricted rotation around double bond
C) Presence of ionic bond
D) Hydrogen bonding

Answer: B) Restricted rotation around double bond

MCQs: Preparation and Reactions

23. Hydrogenation of ethene produces:

A) Methane
B) Ethane
C) Ethyne
D) Benzene

Answer: B) Ethane

24. Catalyst used in hydrogenation of alkenes is:

A) NaOH
B) Ni
C) H₂SO₄
D) KMnO₄

Answer: B) Ni

25. Wurtz reaction is used for preparation of:

A) Alkenes
B) Alkynes
C) Higher alkanes
D) Benzene

Answer: C) Higher alkanes

26. Dehydrohalogenation of alkyl halides gives:

A) Alkane
B) Alkene
C) Alcohol
D) Ketone

Answer: B) Alkene

27. Alkenes mainly undergo:

A) Substitution reactions
B) Addition reactions
C) Neutralisation reactions
D) Decomposition reactions

Answer: B) Addition reactions

28. Alkanes mainly undergo:

A) Addition reactions
B) Substitution reactions
C) Polymerisation
D) Condensation

Answer: B) Substitution reactions

29. Benzene mainly undergoes:

A) Electrophilic substitution
B) Nucleophilic addition
C) Elimination
D) Hydrolysis

Answer: A) Electrophilic substitution

30. Ethene reacts with bromine water and:

A) Produces gas
B) Decolourises bromine water
C) Gives precipitate
D) No reaction occurs

Answer: B) Decolourises bromine water

MCQs: Benzene and Aromatic Hydrocarbons

31. Molecular formula of benzene is:

A) C₆H₁₂
B) C₆H₆
C) C₆H₁₀
D) C₆H₈

Answer: B) C₆H₆

32. Benzene contains how many π electrons?

A) 2
B) 4
C) 6
D) 8

Answer: C) 6

33. Benzene follows:

A) Markovnikov rule
B) Huckel’s rule
C) Boyle’s law
D) Raoult’s law

Answer: B) Huckel’s rule

34. Nitration of benzene produces:

A) Aniline
B) Nitrobenzene
C) Toluene
D) Phenol

Answer: B) Nitrobenzene

35. Catalyst used in Friedel-Crafts alkylation is:

A) Ni
B) AlCl₃
C) KMnO₄
D) H₂SO₄

Answer: B) AlCl₃

36. Benzene + chlorine in presence of FeCl₃ gives:

A) Chlorobenzene
B) Cyclohexane
C) Phenol
D) Benzyl chloride

Answer: A) Chlorobenzene

Answer Key (Quick Revision)

1-B, 2-C, 3-C, 4-B, 5-B, 6-B, 7-B, 8-C
9-C, 10-B, 11-B, 12-C, 13-A, 14-C, 15-C, 16-A
17-C, 18-B, 19-C, 20-B, 21-A, 22-B
23-B, 24-B, 25-C, 26-B, 27-B, 28-B, 29-A, 30-B
31-B, 32-C, 33-B, 34-B, 35-B, 36-A

Part 2: Assertion–Reason, Fill in the Blanks, True/False & Match the Following

A. Assertion–Reason Questions

Directions:

Choose the correct option:

A) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
B) Both Assertion and Reason are true, but Reason is not the correct explanation of Assertion.
C) Assertion is true, but Reason is false.
D) Assertion is false, but Reason is true.

1.

Assertion: Alkanes are less reactive than alkenes.
Reason: Alkanes contain only strong C–C and C–H sigma bonds.

Answer: A

2.

Assertion: Ethene undergoes addition reactions easily.
Reason: Ethene contains a reactive π bond.

Answer: A

3.

Assertion: Benzene mainly undergoes substitution reactions.
Reason: Substitution reactions preserve the aromatic stability of benzene.

Answer: A

4.

Assertion: Ethyne is acidic in nature compared to ethene and ethane.
Reason: Carbon in ethyne is sp hybridised and has higher s-character.

Answer: A

5.

Assertion: Methane has tetrahedral geometry.
Reason: Carbon in methane is sp³ hybridised.

Answer: A

6.

Assertion: Cis and trans forms of but-2-ene exist.
Reason: Rotation around C=C bond is restricted.

Answer: A

7.

Assertion: Benzene is more stable than expected for a molecule with three double bonds.
Reason: Its π electrons are delocalised over the entire ring.

Answer: A

8.

Assertion: Alkenes decolourise bromine water.
Reason: They undergo electrophilic addition reactions.

Answer: A

9.

Assertion: All hydrocarbons are insoluble in water.
Reason: Most hydrocarbons are non-polar.

Answer: B

10.

Assertion: Wurtz reaction is used to prepare alkanes.
Reason: Alkyl halides react with sodium metal in dry ether.

Answer: A

B. Fill in the Blanks

1.

Hydrocarbons contain only ______ and ______ atoms.

Answer: Carbon, hydrogen

2.

The general formula of alkanes is ______.

Answer: CₙH₂ₙ₊₂

3.

Ethene contains a ______ bond between carbon atoms.

Answer: Double

4.

Ethyne contains ______ sigma and ______ pi bonds.

Answer: One, two

5.

Carbon atoms in benzene are ______ hybridised.

Answer: sp²

6.

The first member of the alkane series is ______.

Answer: Methane

7.

The first member of the alkene series is ______.

Answer: Ethene

8.

The first member of the alkyne series is ______.

Answer: Ethyne

9.

The process of addition of hydrogen to an unsaturated compound is called ______.

Answer: Hydrogenation

10.

Benzene follows ______ rule for aromaticity.

Answer: Huckel’s

11.

The catalyst used in Friedel-Crafts reaction is ______.

Answer: AlCl₃

12.

Alkenes generally undergo ______ reactions.

Answer: Addition

13.

Alkanes generally undergo ______ reactions.

Answer: Substitution

14.

Bromine water test is used to detect ______.

Answer: Unsaturation

15.

Polymerisation of ethene gives ______.

Answer: Polyethylene

C. True or False

1.

Methane is an unsaturated hydrocarbon.

Answer: False

2.

Alkenes contain at least one C=C bond.

Answer: True

3.

Alkynes have general formula CₙH₂ₙ₊₂.

Answer: False

4.

The bond angle in ethyne is 180°.

Answer: True

5.

Benzene contains six π electrons.

Answer: True

6.

Alkanes undergo electrophilic addition reactions.

Answer: False

7.

Ethene can be converted into ethane by hydrogenation.

Answer: True

8.

Wurtz reaction uses sodium metal and dry ether.

Answer: True

9.

Benzene easily undergoes addition reactions under normal conditions.

Answer: False

10.

Terminal alkynes contain acidic hydrogen.

Answer: True

11.

All carbon atoms in methane are sp² hybridised.

Answer: False

12.

The molecular formula of benzene is C₆H₆.

Answer: True

D. Match the Following

Set 1

Column A	Column B
1. Methane	A. C₂H₂
2. Ethene	B. C₆H₆
3. Ethyne	C. CH₄
4. Benzene	D. C₂H₄

Answers:

1 → C
2 → D
3 → A
4 → B

Set 2

Column A	Column B
1. Alkane	A. C≡C
2. Alkene	B. C=C
3. Alkyne	C. Electrophilic substitution
4. Benzene	D. C–C

Answers:

1 → D
2 → B
3 → A
4 → C

Set 3

Column A	Column B
1. Wurtz reaction	A. AlCl₃
2. Hydrogenation	B. Na/dry ether
3. Friedel-Crafts reaction	C. Ni/Pt/Pd
4. Ozonolysis	D. O₃

Answers:

1 → B
2 → C
3 → A
4 → D

E. One Word / Very Short Answer Questions

1. What is the simplest hydrocarbon?

Answer: Methane

2. Which hydrocarbon has formula C₂H₂?

Answer: Ethyne

3. Which bond causes reactivity in alkenes?

Answer: π bond

4. Name the catalyst used in hydrogenation.

Answer: Nickel

5. What type of reaction occurs in methane chlorination?

Answer: Free radical substitution

6. Name the aromatic hydrocarbon containing six carbon atoms.

Answer: Benzene

7. What is the hybridisation of carbon in ethyne?

Answer: sp

8. Which rule explains aromaticity?

Answer: Huckel’s rule

9. What is the product of complete hydrogenation of benzene?

Answer: Cyclohexane

10. Which reaction converts ethene into polyethylene?

Answer: Polymerisation

Part 3: Short Answer Questions, Reaction-Based Questions & Long Answer Questions

A. Short Answer Questions (2–3 Marks)

1. What are hydrocarbons? Give two examples.

Answer:

Hydrocarbons are organic compounds containing only carbon and hydrogen atoms.

Examples:

Methane (CH₄)
Ethene (C₂H₄)

2. Differentiate between saturated and unsaturated hydrocarbons.

Answer:

Saturated Hydrocarbons	Unsaturated Hydrocarbons
Contain only single bonds	Contain double or triple bonds
Less reactive	More reactive
Example: Alkane	Example: Alkene, Alkyne

3. Why are alkenes more reactive than alkanes?

Answer:

Alkenes contain a π bond in addition to a sigma bond. The π bond is weaker and easily breaks during reactions, making alkenes more reactive.

4. Why does methane have tetrahedral geometry?

Answer:

Carbon in methane is sp³ hybridised. The four hybrid orbitals arrange themselves to minimize repulsion, producing tetrahedral geometry with a bond angle of 109.5°.

5. Why is rotation restricted around a double bond?

Answer:

A double bond contains a π bond formed by sideways overlap of p-orbitals. Rotation would break this overlap, so free rotation is not possible.

6. What is a homologous series?

Answer:

A homologous series is a group of organic compounds having:

Same general formula
Similar chemical properties
Successive members differing by –CH₂– group

Example:

CH₄, C₂H₆, C₃H₈

7. Explain chain isomerism with an example.

Answer:

Chain isomerism occurs when compounds have the same molecular formula but different carbon skeletons.

Example:

C₄H₁₀:

n-Butane
Isobutane

8. Why does benzene undergo substitution reactions instead of addition reactions?

Answer:

Benzene is aromatic and has high stability due to delocalisation of π electrons. Addition reactions would destroy aromaticity, so benzene prefers substitution reactions.

9. What is aromaticity?

Answer:

Aromaticity is the special stability shown by cyclic, planar compounds having delocalised π electrons and following Huckel’s rule.

10. State Huckel’s rule.

Answer:

A cyclic planar compound is aromatic if it contains: $(4n+2)\pi \text{ electrons}$ (4n+2)π electrons

where n is a whole number.

B. Reaction-Based Questions

1. Write the reaction for hydrogenation of ethene.

Answer: $CH_2=CH_2+H_2 \xrightarrow{Ni} CH_3-CH_3$ CH2=CH2+H2NiCH3−CH3

Ethene → Ethane

2. Write the chlorination reaction of methane.

Answer: $CH_4+Cl_2 \xrightarrow{hv} CH_3Cl+HCl$ CH4+Cl2hvCH3Cl+HCl

3. Write the Wurtz reaction for preparation of ethane.

Answer: $2CH_3Cl+2Na \xrightarrow{dry\ ether} C_2H_6+2NaCl$ 2CH3Cl+2Nadry etherC2H6+2NaCl

4. Write the reaction of ethene with bromine.

Answer: $CH_2=CH_2+Br_2 \rightarrow CH_2Br-CH_2Br$ CH2=CH2+Br2→CH2Br−CH2Br

Product:

1,2-Dibromoethane

5. Write the hydration reaction of ethene.

Answer: $CH_2=CH_2+H_2O \xrightarrow{H^+} CH_3CH_2OH$ CH2=CH2+H2OH+CH3CH2OH

Product:

Ethanol

6. Write the polymerisation reaction of ethene.

Answer: $n(CH_2=CH_2) \rightarrow (-CH_2-CH_2-)_n$ n(CH2=CH2)→(−CH2−CH2−)n

Product:

Polyethylene

7. Write the nitration reaction of benzene.

Answer: $C_6H_6+HNO_3 \xrightarrow{H_2SO_4} C_6H_5NO_2+H_2O$ C6H6+HNO3H2SO4C6H5NO2+H2O

Product:

Nitrobenzene

8. Write the Friedel-Crafts alkylation reaction of benzene.

Answer: $C_6H_6+CH_3Cl \xrightarrow{AlCl_3} C_6H_5CH_3+HCl$ C6H6+CH3ClAlCl3C6H5CH3+HCl

Product:

Toluene

9. Write the preparation of ethyne from calcium carbide.

Answer: $CaC_2+2H_2O \rightarrow C_2H_2+Ca(OH)_2$ CaC2+2H2O→C2H2+Ca(OH)2

10. Write the reaction of benzene with hydrogen.

Answer: $C_6H_6+3H_2 \xrightarrow{Ni} C_6H_{12}$ C6H6+3H2NiC6H12

Product:

Cyclohexane

C. Long Answer Questions (4–5 Marks)

1. Explain the structure of benzene.

Answer:

Benzene has molecular formula C₆H₆.

Important features:

It contains a six-membered carbon ring.
All carbon atoms are sp² hybridised.
The molecule is planar.
Each carbon forms three sigma bonds.
Six p-orbitals overlap to form a delocalised π-electron cloud.
It contains six π electrons and follows Huckel’s rule.

Because of electron delocalisation, benzene is highly stable.

2. Explain the preparation methods of alkenes.

Answer:

(a) Dehydrohalogenation of alkyl halides:

Alkyl halides react with alcoholic KOH.

Example: $CH_3CH_2Br \xrightarrow{alc.KOH} CH_2=CH_2+HBr$ CH3CH2Bralc.KOHCH2=CH2+HBr

(b) Dehydration of alcohols:

Alcohols lose water molecules on heating with acid.

Example: $CH_3CH_2OH \rightarrow CH_2=CH_2+H_2O$ CH3CH2OH→CH2=CH2+H2O

3. Explain electrophilic substitution reactions of benzene.

Answer:

Electrophilic substitution is the replacement of hydrogen in benzene by an electrophile.

Steps:

Step 1:

Formation of electrophile.

Step 2:

Electrophile attacks benzene ring forming an intermediate.

Step 3:

Loss of hydrogen ion restores aromaticity.

Examples:

Nitration
Halogenation
Sulphonation
Friedel-Crafts reaction

4. Explain the important reactions of alkenes.

Answer:

Alkenes mainly undergo addition reactions because of the presence of a π bond.

(a) Hydrogenation:

Alkene + H₂ → Alkane

(b) Halogen addition:

Alkene + Br₂ → Dibromo compound

(c) Addition of HX:

Alkene + HCl/HBr → Haloalkane

(d) Oxidation:

Using KMnO₄ or ozone

(e) Polymerisation:

Small alkene molecules form polymers.

5. Explain different types of isomerism shown by hydrocarbons.

Answer:

Hydrocarbons show:

1. Chain Isomerism

Different arrangement of carbon chain.

Example:

Butane and isobutane.

2. Position Isomerism

Different position of multiple bonds.

Example:

But-1-ene and But-2-ene.

3. Geometrical Isomerism

Due to restricted rotation around double bonds.

Example:

cis-but-2-ene and trans-but-2-ene.

D. Higher Thinking Questions (HOTS)

1. Why is ethyne more acidic than ethene?

Answer:

In ethyne, carbon is sp hybridised and has greater s-character. This stabilises the negative charge formed after removal of hydrogen, making ethyne more acidic.

2. Why does benzene not decolourise bromine water like alkenes?

Answer:

Benzene does not easily undergo addition because it would destroy aromatic stability. Therefore it does not show the usual bromine water test.

3. Why are lower alkanes gases at room temperature?

Answer:

Lower alkanes have low molecular mass and weak intermolecular forces, so they exist as gases.

4. Why do alkenes undergo electrophilic addition reactions?

Answer:

The electron-rich π bond attracts electrophiles, leading to addition across the double bond.

5. Why is benzene called an aromatic compound?

Answer:

Benzene has a stable cyclic structure with delocalised π electrons and follows Huckel’s rule, giving it aromatic character.

Part 4: Case-Based Questions, Competency-Based Questions & Mock Test

A. Case-Based Questions (CBQ)

Case Study 1: Unsaturated Hydrocarbons

A student takes ethene gas and passes it through bromine water. The brown colour of bromine water disappears. Ethene is then hydrogenated in presence of nickel catalyst to form ethane.

Questions:

1. Ethene belongs to which class of hydrocarbons?

A) Alkane
B) Alkene
C) Alkyne
D) Aromatic hydrocarbon

Answer: B) Alkene

2. The reaction between ethene and bromine is:

A) Substitution reaction
B) Addition reaction
C) Elimination reaction
D) Neutralisation reaction

Answer: B) Addition reaction

3. Why does ethene decolourise bromine water?

Answer:

Because ethene contains a reactive π bond which undergoes addition with bromine.

4. Write the reaction of ethene with bromine.

Answer: $CH_2=CH_2+Br_2 \rightarrow CH_2Br-CH_2Br$ CH2=CH2+Br2→CH2Br−CH2Br

5. Name the product formed when ethene reacts with hydrogen.

Answer:

Ethane

Case Study 2: Methane and Free Radical Substitution

Methane reacts with chlorine in the presence of sunlight to form chloromethane. The reaction proceeds through free radical formation.

Questions:

1. Methane belongs to:

A) Alkene
B) Alkyne
C) Alkane
D) Aromatic compound

Answer: C) Alkane

2. The type of reaction between methane and chlorine is:

A) Addition
B) Substitution
C) Polymerisation
D) Oxidation

Answer: B) Substitution

3. What initiates the chlorination reaction?

Answer:

Sunlight/UV radiation breaks chlorine molecules into chlorine radicals.

4. Write the overall reaction.

Answer: $CH_4+Cl_2 \xrightarrow{hv} CH_3Cl+HCl$ CH4+Cl2hvCH3Cl+HCl

5. Why are alkanes comparatively less reactive?

Answer:

Because they contain strong C–C and C–H sigma bonds.

Case Study 3: Benzene and Aromaticity

Benzene is a cyclic hydrocarbon with formula C₆H₆. It contains six carbon atoms arranged in a planar ring. Due to electron delocalisation, benzene is highly stable.

Questions:

1. Benzene is classified as:

A) Saturated hydrocarbon
B) Aromatic hydrocarbon
C) Alkane
D) Alkyne

Answer: B) Aromatic hydrocarbon

2. Number of π electrons in benzene is:

A) 2
B) 4
C) 6
D) 8

Answer: C) 6

3. Which rule explains benzene aromaticity?

Answer:

Huckel’s rule

4. Why does benzene prefer substitution reactions?

Answer:

Substitution reactions maintain aromatic stability, whereas addition reactions disturb aromaticity.

5. Name the product of nitration of benzene.

Answer:

Nitrobenzene

Case Study 4: Alkynes

Ethyne is the simplest alkyne. It contains a carbon-carbon triple bond. Carbon atoms in ethyne are sp hybridised and have linear geometry.

Questions:

1. Molecular formula of ethyne is:

A) C₂H₆
B) C₂H₄
C) C₂H₂
D) C₃H₄

Answer: C) C₂H₂

2. Hybridisation of carbon in ethyne is:

A) sp³
B) sp²
C) sp
D) dsp²

Answer: C) sp

3. Bond angle in ethyne is:

Answer:

180°

4. Number of π bonds in ethyne is:

Answer:

Two

5. Why are terminal alkynes acidic?

Answer:

Due to higher s-character of sp hybridised carbon, which stabilises the negative charge.

B. Competency-Based Questions

1. A compound has molecular formula C₄H₈. It contains a double bond.

Answer the following:

(a) Identify the class of compound.

Answer:

Alkene

(b) Write two possible isomers.

Answer:

But-1-ene
But-2-ene

(c) Which type of isomerism is shown?

Answer:

Position and geometrical isomerism

2. A hydrocarbon burns completely to produce carbon dioxide and water.

(a) Which elements are present in this compound?

Answer:

Carbon and hydrogen

(b) What is the general reaction called?

Answer:

Combustion

(c) Write combustion equation of methane.

Answer: $CH_4+2O_2 \rightarrow CO_2+2H_2O$ CH4+2O2→CO2+2H2O

3. A compound contains C=C bond and reacts with HBr.

(a) Which type of reaction occurs?

Answer:

Electrophilic addition reaction

(b) Which rule predicts the major product?

Answer:

Markovnikov’s rule

(c) What happens in presence of peroxide?

Answer:

Anti-Markovnikov addition occurs with HBr.

4. A student wants to distinguish between alkane and alkene.

(a) Which test can be used?

Answer:

Bromine water test

(b) Observation for alkene?

Answer:

Decolourisation of bromine water

(c) Reason?

Answer:

Addition reaction occurs at the double bond.

C. Complete Chapter Mock Test

Section A: MCQs

1. The hybridisation of carbon in ethane is:

A) sp
B) sp²
C) sp³
D) sp³d

Answer: C

2. The formula of propyne is:

A) C₃H₈
B) C₃H₆
C) C₃H₄
D) C₂H₂

Answer: C

3. Which compound shows geometrical isomerism?

A) Propene
B) But-2-ene
C) Ethane
D) Methane

Answer: B

4. Catalyst used in hydrogenation is:

A) AlCl₃
B) Ni
C) KMnO₄
D) H₂SO₄

Answer: B

5. Aromaticity of benzene is due to:

A) Single bonds
B) Six sigma bonds
C) Delocalised π electrons
D) Hydrogen bonding

Answer: C

Section B: Very Short Answer Questions

1. Write the formula of benzene.

Answer: C₆H₆

2. Name the catalyst used in Friedel-Crafts reaction.

Answer: AlCl₃

3. Which bond is responsible for alkene reactivity?

Answer: π bond

4. Give the formula of alkynes.

Answer: CₙH₂ₙ₋₂

5. Name the product of complete hydrogenation of benzene.

Answer: Cyclohexane

Section C: Short Answer Questions

1. Explain Markovnikov’s rule.

Answer:

In addition of an unsymmetrical reagent to an unsymmetrical alkene, hydrogen attaches to the carbon having more hydrogen atoms.

2. Why is benzene stable?

Answer:

Because of delocalisation of π electrons and resonance stabilization.

3. Explain polymerisation of ethene.

Answer:

Many ethene molecules combine to form polyethylene under suitable conditions.

Section D: Long Answer Questions

1. Explain preparation and reactions of alkenes.

Expected points:

Preparation from alcohol dehydration
Preparation from alkyl halides
Hydrogenation
Halogen addition
Addition of HX
Oxidation
Polymerisation

2. Explain electrophilic substitution reactions of benzene.

Expected points:

Formation of electrophile
Attack on benzene ring
Intermediate formation
Removal of H⁺
Restoration of aromaticity