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Haloalkanes

Part of the CUET UG study roadmap. Chemistry topic chem-021 of Chemistry.

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Haloalkanes

🟢 Lite — Quick Review (1h–1d)

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A haloalkane (alkyl halide) is an alkane derivative in which one or more hydrogen atoms on an sp³-hybridised carbon are replaced by a halogen atom (F, Cl, Br, I). The general formula of a saturated mono-haloalkane is CₙH₂ₙ₊₁X, in many papers written as R–X, where R is an alkyl group and X is a halogen.

  • Classification is by the carbon bearing the halogen: primary (1°) has one C attached, secondary (2°) has two, tertiary (3°) has three.
  • SN2 reactivity follows 1° > 2° > 3°, while SN1 reactivity reverses to 3° > 2° > 1° (carbocation stability).
  • Boiling points rise with molecular mass and polarisability: RI > RBr > RCl > RF for the same alkyl group; for isomers, 1° > 2° > 3° (branching lowers BP).
  • CUET UG weight: ~3% of Chemistry, in standard papers 1–2 MCQs from Haloalkanes & Haloarenes.
Must-know comparisonSN2SN1
StepsOne step (concerted)Two steps via carbocation
KineticsRate ∝ [RX][Nu⁻]Rate ∝ [RX] only
StereochemistryWalden inversionRacemisation
Best substrate1° (methyl fastest)3°, allyl, benzyl

🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Structure and Classification

The C–X bond is polar because halogens are more electronegative than carbon (C: 2.55, F: 3.98, Cl: 3.16, Br: 2.96, I: 2.66 on the Pauling scale). Bond polarity drives dipole moments (~1.87 D for CH₃Cl) and gives the carbon a partial positive charge (δ⁺C–Xδ⁻), making it susceptible to nucleophilic attack.

A carbon bearing the halogen can carry one, two, or three other carbon substituents, defining it as 1°, 2°, or 3°. Allylic (next to C=C) and benzylic (next to a benzene ring) halides behave like 3° substrates in SN1 because the intermediate carbocation is resonance-stabilised. Vinyl and aryl halides, where X is bonded to an sp² carbon, are essentially inert to SN1 and SN2.

Preparation Methods

  • Free-radical halogenation of alkanes (hydrogen reactivity: 3° > 2° > 1°; halogen reactivity: F₂ > Cl₂ > Br₂ > I₂).
  • From alcohols: HX (Lucas test for 1°/2°/3°), PCl₃, PCl₅, or SOCl₂.
  • Halogen exchange: Finkelstein (NaI/acetone → RI) and Swarts (AgF/Hg₂F₂ → RF).
  • Sandmeyer reaction: ArN₂⁺X⁻ + CuCl/HBr → ArCl/ArBr.
  • Hunsdiecker: RCOOAg + Br₂ → RBr + CO₂ + AgBr.
  • Wurtz coupling: 2 R–X + 2 Na → R–R + 2 NaX (Wurtz–Fittig gives R–Ar with Ar–X).

Nucleophilic Substitution and Elimination

  • SN2 (bimolecular): a single concerted step in which the nucleophile attacks from the side opposite to X (backside attack), giving Walden inversion. Favoured by 1° substrates, polar aprotic solvents, and strong nucleophiles.
  • SN1 (unimolecular): rate-determining ionisation of R–X to a carbocation, followed by nucleophilic capture. Gives a racemic mixture from a chiral substrate. Favoured by 3°/allyl/benzyl substrates and polar protic solvents.
  • β-elimination (E1/E2) with alcoholic KOH produces alkenes; Saytzeff’s rule says the more substituted alkene dominates unless a bulky base (e.g., t-BuOK) is used, in which case the Hofmann (less substituted) product wins.
Preparation reactionReagentsProduct
Free-radical halogenationCl₂/hνR–Cl + HCl
Alcohol → alkyl halideSOCl₂R–Cl + SO₂ + HCl
FinkelsteinNaI/acetoneR–I + NaCl↓
SwartsAgF or Hg₂F₂R–F + AgCl/HgCl
SandmeyerCuCl/HCl or CuBr/HBrAr–Cl or Ar–Br
Wurtz2 Na, dry etherR–R + 2 NaX

Important Distinctions

  • C–X bond strength decreases down the group: C–F (485) > C–Cl (339) > C–Br (276) > C–I (238 kJ mol⁻¹), which is why RF is kinetically the least reactive despite the largest polarity.
  • CHCl₃ and CCl₄ are denser than water but remain poor solvents for ionic species because they are non-polar — a frequent misconception in MCQs.

🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Stereochemistry and Mechanism Nuances

SN2 proceeds through a trigonal-bipyramidal transition state with the nucleophile and leaving group 180° apart; an SN2 reaction on (R)-2-bromobutane gives (S)-2-butanol after hydrolysis. SN1 on a chiral 2° halide in standard papers yields a 50:50 (racemic) mixture because the planar carbocation can be attacked from either face, although ion-pair effects (contact ion pair → inverted, solvent-separated → racemic) frequently produce partial retention, giving an enantiomeric excess of 5–20 % rather than perfect 50:50.

  • Markownikoff vs anti-Markownikoff: ionic addition of HX to alkenes places X on the more substituted carbon; the peroxide (Kharasch) effect reverses this for HBr only via a radical chain (HCl and HI do not work because H–Cl is too strong and H–I gives an unfavourable propagation step).
  • Grignard reagents: R–X + Mg in dry ether gives R–Mg–X; aryl and vinyl halides need THF because their C–X bonds are stronger and less reactive.
  • CFCs and the ozone layer: CF₂Cl₂ photolyses at ~220 nm in the stratosphere, releasing Cl• that catalytically destroys O₃; this links haloalkanes directly to environmental chemistry questions.

Common Traps in CUET MCQs

  1. Confusing haloalkanes (sp³ C–X) with haloarenes (sp² C–X on benzene); aryl halides are inert to SN1/SN2.
  2. Marking 3° as fastest in SN2 — it actually favours SN1/E1.
  3. Forgetting that the anti-Markownikoff rule is HBr-specific and radical-mediated.
  4. Predicting salt solubility in CHCl₃/CCl₄ despite density.

Practice Prompts

  1. Identify the major product when (R)-3-bromo-3-methylhexane reacts with aqueous KOH — predict whether the reaction follows SN1 or SN2 and justify with one line about substrate class.
  2. Rank CH₃CH₂F, CH₃CH₂Cl, CH₃CH₂Br, CH₃CH₂I in order of (a) boiling point and (b) SN2 reactivity, and explain why the two orders are not identical.

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