Haloalkanes and Haloarenes — Class 12 MCQs with Answers

Haloalkanes and Haloarenes — important questions & MCQs with answers (Class 12 Chemistry)

6 solved questions from this chapter's difficulty levels, each with its answer and explanation. Tap an option to check yourself.

0 of 6 answered

1. Q1 of 6Easy

   General formula of haloalkanes:

   ACₙHₙXBCₙH₂ₙXCCₙH₂ₙ₊₁XDCₙHX
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   Answer: CₙH₂ₙ₊₁X

   Explanation: Replacing one H of the alkane CₙH₂ₙ₊₂ with a halogen keeps the saturated skeleton, giving CₙH₂ₙ₊₁X — the CₙH₂ₙX distractor wrongly implies a degree of unsaturation.

2. Q2 of 6Easy

   Nucleophilic substitution involves:

   AReplacement of leaving group by nucleophileBAdding waterCHeat onlyDPolymerisation
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   Answer: Replacement of leaving group by nucleophile

   Explanation: Nucleophilic substitution means an electron-rich nucleophile displaces the leaving group on carbon, e.g. OH⁻ replacing Br⁻ — a bond-breaking/bond-making swap, not simple heating or polymerisation.

3. Q3 of 6Medium

   Sn1 leads to:

   ARacemic mixture from chiral substrateBPure inversionCNo reactionDPolymer
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   Answer: Racemic mixture from chiral substrate

   Explanation: SN1 goes through a planar sp² carbocation that a nucleophile can attack from either face, so one enantiomer of R–X gives a racemic (50:50) mixture, not the clean inversion of SN2.

4. Q4 of 6Medium

   Sn2 reaction is:

   APhotochemical onlyBTwo-step via carbocationCFree radicalDSingle-step concerted (bimolecular)
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   Answer: Single-step concerted (bimolecular)

   Explanation: SN2 is single-step and bimolecular: the nucleophile attacks the backside of C–X as the leaving group departs, so rate depends on both R–X and nucleophile — unlike the two-step carbocation route of SN1.

5. Q5 of 6Hard

   Haloarenes are less reactive than haloalkanes in nucleophilic substitution because of:

   AResonance and partial double-bond character of C–XBHigher massCHigher temperatureDNo reason
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   Answer: Resonance and partial double-bond character of C–X

   Explanation: A lone pair on X conjugates into the ring, giving C–X partial double-bond character that shortens and strengthens the bond, so haloarenes resist the backside attack (SN2) that haloalkanes undergo easily.

6. Q6 of 6Hard

   E2 elimination requires:

   AAcidBWeak base onlyCStrong base, anti-periplanar H and leaving groupDNo base
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   Answer: Strong base, anti-periplanar H and leaving group

   Explanation: E2 is concerted: a strong base removes a β-hydrogen as the leaving group departs in one step, needing anti-periplanar (180°) alignment of H and the leaving group so their orbitals overlap as the π bond forms.

Haloalkanes and Haloarenes notes: key concepts (Class 12 Chemistry)

Haloalkanes and Haloarenes covers halogen compounds on sp³ and aromatic carbon: their preparation, the SN1 (carbocation, racemisation) and SN2 (concerted, inversion) substitution mechanisms, E1/E2 elimination with Zaitsev's rule, the low reactivity of haloarenes, optical isomerism, and named reactions such as Wurtz, Fittig, Finkelstein and Swarts.

Classification

Halogen on sp³ carbon (haloalkanes, further 1°/2°/3°) or on an aromatic ring (haloarenes); allylic, benzylic and vinylic types differ sharply in reactivity.

Nature of the C–X bond

The C–X bond is polar (X more electronegative), so carbon is electrophilic; bond strength falls C–F > C–Cl > C–Br > C–I, making iodides the most reactive.

SN2 mechanism

One-step, bimolecular; the nucleophile attacks the carbon from the side opposite the leaving group, giving inversion. Fastest for 1° (least steric hindrance).

SN1 mechanism

Two-step, unimolecular; slow ionisation to a carbocation then fast nucleophile attack. Fastest for 3° (stable carbocation), favoured by polar protic solvents.

Stereochemistry of substitution

SN2 gives complete inversion of configuration (Walden inversion); SN1 gives a racemic mixture because the planar carbocation is attacked from both faces.

Elimination reactions

Dehydrohalogenation with alcoholic KOH removes H and X (β-elimination) to give an alkene; Zaitsev's rule predicts the more substituted (more stable) alkene as major.

Substitution vs elimination

Strong small nucleophiles and low temperature favour substitution; strong bulky bases and heat favour elimination — the same reagent can do either.

Low reactivity of haloarenes

Resonance gives the C–X bond partial double-bond character and the sp² carbon holds the halogen tightly, so haloarenes resist nucleophilic substitution.

Grignard reagent

R–MgX formed from R–X and Mg in dry ether; a strongly nucleophilic reagent that must be kept away from water and adds to carbonyls to give alcohols.

Optical isomerism

A carbon with four different groups is chiral and rotates plane-polarised light; enantiomers rotate it in opposite directions (dextro/laevo).

Reactivity order of leaving groups

A weaker C–X bond and more stable X⁻ make a better leaving group: I⁻ > Br⁻ > Cl⁻ > F⁻, so R–I reacts fastest in substitution.

Polyhalogen compounds

Chloroform (CHCl₃, forms poisonous phosgene in air), iodoform, carbon tetrachloride, freons (ozone depletion) and DDT (persistent pesticide) are important uses.