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Chemistry · Unit 15

Alkyl Halides: MDCAT Notes and MCQs

Nomenclature & classification

  • IUPAC: halogen as prefix — CH3CH2Cl chloroethane; (CH3)2CHBr 2-bromopropane; (CH3)3CCl 2-chloro-2-methylpropane.
  • Primary (1°), secondary (2°), tertiary (3°) depending on the carbon bearing X.

Structure & reactivity

  • C–X bond is polar (Cδ+–Xδ−) → carbon is electrophilic and attacked by nucleophiles.
  • Reactivity depends mainly on bond strength: R–I > R–Br > R–Cl > R–F (C–I weakest, longest; I− best leaving group).

Nucleophilic substitution

SN2SN1
StepsOne step, concerted; transition state with 5 groupsTwo steps: slow ionization → carbocation, then fast attack
Kinetics2nd order: rate = k[RX][Nu−] (bimolecular)1st order: rate = k[RX] (unimolecular)
StereochemistryInversion of configuration (backside attack; Walden inversion)Racemization (planar carbocation attacked from both sides)
Favoured by1° (CH3X > 1° > 2°), strong nucleophile, polar aprotic solvent3° (stable carbocation), polar protic solvent

Examples: RX + aq. KOH → ROH; + KCN → RCN; + NH3 → RNH2; + RONa → ether (Williamson).

Elimination (β-elimination / dehydrohalogenation)

  • Reagent: alcoholic KOH (strong base) removes H from β-carbon and X from α-carbon → alkene.
  • E2: one step, 2nd order, needs anti-periplanar H and X; strong base, favoured for 2°/3°. E1: two steps via carbocation, 1st order, 3° substrates.
  • Saytzeff's rule: the more substituted alkene is the major product (2-bromobutane → but-2-ene major).
  • Substitution vs elimination: high temperature, strong/bulky base, alcoholic solvent, 3° RX → elimination; aqueous base, 1° RX → substitution.

Grignard reagent: RX + Mg in dry ether → RMgX — used to make alcohols, acids, alkanes.

PMDC learning outcomes for this unit
  • Alkyl halides: 15.1 Name alkyl halides using IUPAC system.
  • Alkyl halides: 15.2 Discuss the structure and reactivity of RX.
  • Alkyl halides: 15.3 Describe the mechanism and types of nucleophilic substitution reactions.
  • Alkyl halides: 15.4 Describe the mechanism and types of elimination reactions.

Alkyl Halides MCQs with answers

Practise and track your score →
Q1
The most reactive alkyl halide towards nucleophilic substitution is:
AR–F
BR–I
CR–Cl
DR–Br
Show answer
Answer: B (R–I). The C–I bond is weakest and I⁻ is the best leaving group.
Q2
SN2 reactions proceed with:
ACarbocation formation
BInversion of configuration
CRacemization
DRetention only
Show answer
Answer: B (Inversion of configuration). Backside attack inverts the configuration (Walden inversion).
Q3
SN1 reactions are fastest with:
AMethyl halides
BPrimary halides
CTertiary halides
DSecondary halides
Show answer
Answer: C (Tertiary halides). Tertiary halides form the most stable carbocations.
Q4
The rate law for an SN1 reaction is:
ARate = k[RX]
BRate = k[Nu⁻]
CRate = k[RX]²
DRate = k[RX][Nu⁻]
Show answer
Answer: A (Rate = k[RX]). Only the substrate is involved in the slow ionization step.
Q5
Ethyl bromide with alcoholic KOH mainly gives:
AEthene
BDiethyl ether
CEthanol
DEthane
Show answer
Answer: A (Ethene). Alcoholic KOH causes dehydrohalogenation (elimination).
Q6
According to Saytzeff's rule, dehydrobromination of 2-bromobutane mainly gives:
AButane
BButan-2-ol
CBut-2-ene
DBut-1-ene
Show answer
Answer: C (But-2-ene). The more substituted (more stable) alkene is the major product.
Q7
The IUPAC name of (CH₃)₃C–Cl is:
A2-Chloro-2-methylpropane
B2-Chlorobutane
CTert-butyl chloride
D1-Chlorobutane
Show answer
Answer: A (2-Chloro-2-methylpropane). Longest chain = propane with methyl and chloro at C-2.
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