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Haloalkanes and Haloarenes – Class 12 Chemistry notes, important questions and MCQs

NCERT Class 12 Chemistry, chapter 6. Notes, key terms, important questions with answers and MCQs.

Chapter overview

Haloalkanes and haloarenes contain halogen atoms bonded to alkyl or aryl groups. They undergo nucleophilic substitution and elimination, and many have industrial and environmental importance.

Reactions of haloalkanes

SN2: one step, inversion of configuration, favoured by primary halides; rate = k[RX][Nu⁻].
SN1: carbocation intermediate, racemisation, favoured by tertiary halides; rate = k[RX].
Elimination with alcoholic KOH gives alkenes (Saytzeff rule: more substituted alkene is major).

Haloarenes and uses

Haloarenes are less reactive to nucleophilic substitution (resonance, sp² carbon); they show electrophilic substitution at ortho/para positions.
Grignard reagent RMgX is prepared from haloalkanes with Mg in dry ether.
Environmental concerns: CFCs deplete ozone; DDT is persistent.

Key terms

Nucleophile: electron-rich species that attacks an electron-poor carbon
Racemic mixture: equal amounts of two enantiomers, optically inactive
SN2 reaction: one-step bimolecular nucleophilic substitution with backside attack and inversion of configuration

Formulas

Reactivity for SN1: 3° > 2° > 1°
Reactivity for SN2: 1° > 2° > 3°

Important questions with answers

What is a racemic mixture and why is it optically inactive?2 marks

An equimolar mixture of two enantiomers. The rotation caused by one is exactly cancelled by the other, so the net rotation is zero.

Compare SN1 and SN2 mechanisms on three points.3 marks

SN1: two steps via a carbocation, rate = k[RX] (first order), gives racemisation; favoured by 3° halides and polar protic solvents. SN2: one step with backside attack, rate = k[RX][Nu] (second order), gives inversion of configuration; favoured by 1° halides.

Write equations for (i) Wurtz reaction, (ii) Fittig reaction, (iii) Friedel–Crafts alkylation of chlorobenzene, and explain why DDT and chloroform are harmful.5 marks

(i) 2CH₃Br + 2Na → CH₃–CH₃ + 2NaBr (dry ether). (ii) 2C₆H₅Cl + 2Na → C₆H₅–C₆H₅ + 2NaCl. (iii) C₆H₅Cl + CH₃Cl → (anhydrous AlCl₃) → o- and p-chlorotoluene + HCl (Cl is o/p-directing). DDT is non-biodegradable and accumulates in fatty tissues, causing biomagnification along food chains. Chloroform slowly oxidises in air and light to poisonous phosgene (COCl₂), so it is stored in dark bottles filled to the top; it also affects the nervous system and liver.

Quick revision

SN2 gives inversion; SN1 gives racemisation.
Haloarenes are o/p directing.
CFCs harm the ozone layer.

Exam tips

Aqueous KOH → substitution (alcohol); alcoholic KOH → elimination (alkene).
Order of reactivity: SN1 3° > 2° > 1° (carbocation stability); SN2 1° > 2° > 3° (steric hindrance).

Practice MCQs

Alcoholic KOH with haloalkanes mainly gives
1. alkenes
2. alcohols
3. ethers
4. alkanes
Show answer
Answer: (a) alkenes. Elimination.
Grignard reagent is
1. RX
2. RNa
3. RLi only
4. RMgX
Show answer
Answer: (d) RMgX. Alkyl magnesium halide.
SN1 reaction is fastest for
1. tertiary halides
2. primary halides
3. methyl halides
4. vinyl halides
Show answer
Answer: (a) tertiary halides. Stable carbocation.
Inversion of configuration occurs in
1. SN1
2. E1
3. SN2
4. free-radical reactions
Show answer
Answer: (c) SN2. Backside attack.
The product of CH₃Br with aqueous KOH is
1. CH₃OH
2. CH₄
3. CH₃OCH₃
4. HCHO
Show answer
Answer: (a) CH₃OH. Substitution by OH⁻.
Chlorobenzene is less reactive towards nucleophiles than chloroethane because
1. C–Cl bond has partial double-bond character
2. Cl is bigger
3. benzene is acidic
4. it is a gas
Show answer
Answer: (a) C–Cl bond has partial double-bond character. Resonance.
Dehydrohalogenation of 2-bromobutane mainly gives
1. 1-butene
2. 2-butene
3. butane
4. butanol
Show answer
Answer: (b) 2-butene. Saytzeff product.
Chlorobenzene is less reactive than chloroethane towards nucleophilic substitution because
1. Cl is more electronegative in benzene
2. it has no Cl
3. benzene is a gas
4. C–Cl has partial double-bond character due to resonance
Show answer
Answer: (d) C–Cl has partial double-bond character due to resonance. Resonance, sp² carbon.

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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.