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Hydrocarbons – Class 11 Chemistry notes, important questions and MCQs

NCERT Class 11 Chemistry, chapter 9. Notes, key terms, important questions with answers and MCQs.

Chapter overview

Hydrocarbons contain only carbon and hydrogen. Alkanes, alkenes, alkynes and aromatic compounds differ in bonding and reactions.

Aliphatic hydrocarbons

Alkanes (CₙH₂ₙ₊₂) undergo free-radical substitution and combustion; Wurtz reaction joins alkyl halides.
Alkenes (CₙH₂ₙ) undergo electrophilic addition; Markovnikov's rule; peroxide effect (anti-Markovnikov) only with HBr.
Alkynes (CₙH₂ₙ₋₂); terminal alkynes are weakly acidic; ozonolysis locates double bonds.

Aromatic hydrocarbons

Benzene: planar, resonance-stabilised; Hückel's rule (4n + 2) π electrons for aromaticity.
Electrophilic substitution: nitration, halogenation, sulphonation, Friedel–Crafts alkylation and acylation.
Directive influence: –OH, –CH₃ are ortho/para directing; –NO₂, –COOH are meta directing.

Key terms

Markovnikov's rule: in HX addition, H goes to the carbon with more hydrogens
Aromaticity: extra stability of planar cyclic conjugated systems with (4n + 2) π electrons
Ozonolysis: reaction of an alkene with ozone followed by Zn/H₂O that splits the double bond into two carbonyl compounds, locating its position

Important questions with answers

Why is benzene highly stable though it has three double bonds?2 marks

Its six π electrons are delocalised over the ring (resonance), giving extra stability (aromaticity) and equal C–C bond lengths; so it prefers substitution to addition.

Why are terminal alkynes acidic? Show with a reaction.3 marks

The C–H hydrogen of ethyne is attached to an sp carbon, which has 50% s-character and is more electronegative, so the H can be removed as H⁺. HC≡CH + 2Na → NaC≡CNa + H₂ (or with NaNH₂).

Explain the mechanism of addition of HBr to propene with and without peroxide, giving the major product in each case.5 marks

Without peroxide (ionic, Markovnikov): H⁺ adds to the CH₂ end to form the more stable secondary carbocation CH₃–C⁺H–CH₃; Br⁻ attacks it to give 2-bromopropane. With peroxide (free-radical, anti-Markovnikov): the peroxide forms Br• radicals; Br• adds to CH₂ to give the more stable secondary radical CH₃–C•H–CH₂Br, which takes H from HBr to give 1-bromopropane. The peroxide effect works only with HBr, because H–Cl is too strong and the I• radical too unreactive.

Quick revision

Peroxide effect only with HBr.
Benzene prefers substitution over addition.
–NO₂ is meta directing.

Exam tips

Conformations of ethane: staggered is more stable than eclipsed.
Conformations of ethane (staggered, eclipsed) interconvert freely; geometrical (cis–trans) isomers of alkenes do not, because the C=C cannot rotate.

Practice MCQs

General formula of alkenes is
1. CₙH₂ₙ₊₂
2. CₙH₂ₙ₋₂
3. CₙH₂ₙ
4. CₙHₙ
Show answer
Answer: (c) CₙH₂ₙ. One double bond.
Benzene mainly undergoes
1. addition
2. electrophilic substitution
3. elimination
4. polymerisation
Show answer
Answer: (b) electrophilic substitution. Keeps aromaticity.
Addition of HBr to propene (no peroxide) gives mainly
1. 1-bromopropane
2. propane
3. 2-bromopropane
4. 1,2-dibromopropane
Show answer
Answer: (c) 2-bromopropane. Markovnikov.
Number of π electrons in benzene is
1. 4
2. 2
3. 8
4. 6
Show answer
Answer: (d) 6. Three double bonds.
Which shows geometrical (cis–trans) isomerism
1. propene
2. but-1-ene
3. but-2-ene
4. ethene
Show answer
Answer: (c) but-2-ene. Each double-bond carbon has two different groups.
Which group is meta directing
1. –OH
2. –NO₂
3. –CH₃
4. –NH₂
Show answer
Answer: (b) –NO₂. Electron withdrawing.
Anti-Markovnikov addition (peroxide effect) is seen with
1. HBr
2. HCl
3. HI
4. HF
Show answer
Answer: (a) HBr. Only HBr.
Ozonolysis of an alkene gives only propanone (acetone). The alkene is
1. but-2-ene
2. 2,3-dimethylbut-2-ene
3. propene
4. 2-methylpropene
Show answer
Answer: (b) 2,3-dimethylbut-2-ene. Symmetrical, (CH₃)₂C=C(CH₃)₂.

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Written by Kundali IQ from standard, well-established facts to help with revision. This is not an official NCERT publication; always read your textbook too.