Alcohol → Alkyl Halide with HX (HCl/HBr/HI in CH₂Cl₂ or neat) — SN1 vs SN2, Rearrangements, and Stereochemistry

Protonating an alcohol converts a poor leaving group (–OH) into water, letting chloride, bromide, or iodide displace it. Which pathway wins hinges on substitution, resonance, nucleophile strength, and temperature: tertiary and most secondary sites ionise (SN1) whereas primary and methyl centres demand a concerted SN2 displacement. HBr and HI are strong enough to do the job directly; HCl often needs ZnCl₂ (Lucas conditions) to activate sluggish systems. Keep an eye on the classic traps—carbocation rearrangements and E1/E2 elimination.

Need a complementary toolbox? Compare this acid-driven route with PBr₃ halogenation, SOCl₂ chlorination, or the Appel reaction when you need milder SN2 control or halide swaps.

Quick Summary

• Reagents: Concentrated HBr or HI (often CH₂Cl₂ or neat); HCl typically needs ZnCl₂ (Lucas) for primary/secondary substrates. Keep reactions cold-to-rt to suppress elimination.
• Pathway: Tertiary, benzylic, allylic, and many secondary alcohols proceed via SN1 (carbocation, possible rearrangement, racemisation). Primary and methyl alcohols react via SN2 (backside attack, inversion).
• Halide strength: HI > HBr ≫ HCl; Cl⁻ is sluggish in protic media without ZnCl₂ assistance.
• Stereochemistry: SN1 → racemisation (usually with slight inversion bias). SN2 → net inversion.
• Exam traps: Watch for 1,2-hydride/alkyl shifts, neopentyl sluggishness, Lucas turbidity logic, and E1/E2 by-products under heat.

Mechanism — SN1 Path (4 Steps)

Step S1.1 — Protonation of the alcohol

Step S1.2 — Ionization to the carbocation

Step S1.3 — Nucleophilic capture and stereochemical outcome

Step S1.4 — Product separation and HX regeneration

Mechanism — SN2 Path (3 Steps)

Step S2.1 — Protonation of the alcohol

Step S2.2 — Backside attack with concerted water loss

Step S2.3 — Workup and product isolation

Worked Examples

Primary Alcohol → Isobutyl Bromide (SN2)

Cold HBr or HI in CH₂Cl₂

Backside attack inverts the α-carbon; no rearrangement occurs. Chloride needs ZnCl₂ support to match these rates.

Secondary Alcohol → Bromocyclohexane (SN1)

Room-temperature HBr

Solvolysis to the cyclohexyl carbocation lets Br⁻ capture with racemisation; rearrangements are minimal because the secondary cation sits in a stable chair.

Mechanistic Checklist

• Decide SN1 vs SN2 by substitution and conditions: tertiary ≥ benzylic/allylic ≈ secondary → SN1; primary/methyl → SN2 unless β-branching creates neopentyl-type drag.
• Rearrangements (1,2-hydride or alkyl shifts, ring expansion) are SN1-only; SN2 never rearranges.
• Stereochemistry: SN1 gives racemisation with slight inversion bias from ion pairs; SN2 delivers clean inversion.
• Halide nucleophilicity in protic media: I⁻ > Br⁻ ≫ Cl⁻ ≫ F⁻. Pair HCl with ZnCl₂ to achieve practical rates on primary/secondary alcohols.
• Competing elimination (E1/E2) escalates with heat, bulky substrates, or weak nucleophile/strong acid balances; moderate temperatures and excess halide suppress it.
• Lucas test logic: tertiary alcohols turn cloudy immediately (fast SN1); secondary take minutes; primary require heat or fail at room temperature with HCl.

Alcohols + HX: The Complete Mechanistic Playbook

What this reaction does—in one line

ROH + HX → RX + H₂O (plus potential alkene from elimination). The acid installs water as a leaving group, halide then replaces it.

Big-picture roadmap (SN1 vs. SN2 vs. elimination)

• SN2 (one-step displacement): Favoured by methyl and unhindered primary alcohols, especially with HBr or HI. Clean inversion, no rearrangement.
• SN1 (carbocation): Favoured by tertiary, benzylic/allylic, and many secondary alcohols under strongly acidic, ionising conditions. Racemisation with possible rearrangements.
• Elimination (E1/E2): Competes at secondary/tertiary centres under heat or when halide is weak. Expect Zaitsev alkenes in mixtures.

Universal first step: activate the leaving group

Protonate the alcohol (ROH + H⁺ ⇌ ROH₂⁺) to turn hydroxyl into water. Lucas aid: ZnCl₂ complexes with less reactive alcohols to boost chloride delivery. HX strength: HI > HBr > HCl ≫ HF.

SN2 pathway — who, when, and stereochemistry

• Who: Methyl and primary alcohols (avoid heavily β-branched or neopentyl systems).
• When: Cold temperatures, strong nucleophiles (HBr/HI; HCl+ZnCl₂).
• How: Backside attack of X⁻ as water leaves in one concerted step.
• Stereochemistry: Walden inversion at the reacting carbon.
• Pitfalls: Neopentyl and β-branched primaries are slow; consider alternative reagents (PBr₃, SOCl₂, Appel) if rearrangement is unacceptable.

SN1 pathway — who, when, stereochemistry, rearrangements

• Who: Tertiary ≫ secondary; benzylic/allylic primaries/secondaries thanks to resonance.
• When: rt or gentle heat with concentrated HX; HCl often needs ZnCl₂.
• How: Protonation → H₂O loss → carbocation (possible rearrangement) → capture by X⁻.
• Stereochemistry: Racemisation, usually with slight inversion bias from contact ion pairs.
• Rearrangements: 1,2-hydride shifts (secondary → tertiary), 1,2-alkyl shifts (relieve strain or boost substitution), and ring expansions are test favourites.

Allylic and benzylic alcohols — resonance-enabled twists

Stabilised cations form quickly, so even primary allylic/benzylic alcohols follow SN1. Expect mixtures when resonance allows cation/alkene migration.

Competing elimination — when alkenes win

Tertiary and many secondary alcohols heated with strong acid favour dehydration (E1). Maintain moderate temperatures and high [X⁻] to bias substitution; watch for Zaitsev alkenes otherwise.

Choosing X — practical notes you’re expected to remember

• HBr is the workhorse; HI is even faster.
• Avoid generating HI with H₂SO₄ + NaI (oxidises I⁻ to I₂); use H₃PO₄ or directly supplied HI.
• HCl is sluggish; pair with ZnCl₂ (Lucas reagent), especially for 1°/2° substrates.
• HF is rarely useful (weak acid, poor F⁻ nucleophilicity).

Special/tricky cases professors love to test

• Neopentyl alcohols: SN2 is painfully slow; strong acids may trigger rearranged SN1 products.
• Ring expansion: Protonated cyclobutanols ionise to strained cations that expand before capture.
• Secondary → tertiary shifts: Hydride or methyl migration often precedes halide capture.
• Allylic migration: Allylic systems can shift the double bond during capture.
• Ion-pair stereochemistry: Partial racemisation with inversion bias is common.
• Competing dehydration: Hot HX pushes E1, especially for tertiary substrates.

Kinetics you can quote under exam pressure

• SN1: rate ≈ k [ROH₂⁺] (first order in substrate).
• SN2: rate ≈ k [ROH₂⁺][X⁻] (second order; sensitive to sterics and nucleophile strength).

Quick practice diagnoses (mini-worked checks)

• 2-Butanol + HBr (rt) → racemic 2-bromobutane; warming increases elimination.
• Ethanol + HI (reflux) → ethyl iodide via SN2.
• Neopentyl alcohol + HCl (rt) → rearranged tert-butyl chloride.
• Cyclobutanol + HBr (warm) → bromocyclopentane via ring expansion.
• Benzyl alcohol + HCl/ZnCl₂ → benzyl chloride rapidly via stabilised cation.

One-page decision checklist (fast exam mode)

1. Classify the carbon (methyl/1°/2°/3°; allylic/benzylic? neopentyl?).
2. Protonate –OH → –OH₂⁺ to activate the leaving group.
3. Choose the pathway:
   • Methyl/1° (unhindered) + HBr/HI or HCl+ZnCl₂ → SN2 (inversion).
   • 3°/2°/allylic/benzylic or β-branched → SN1 (racemisation, possible rearrangements).
4. Scan for rearrangements (only SN1).
5. Monitor conditions: heat or weak [X⁻] increases elimination.
6. In situ HI? Use non-oxidising acid; avoid H₂SO₄.

Edge Cases & Exam Traps

• Neopentyl systems: Primary yet sterically blocked; SN2 is glacial. Strong acid can prompt rearranged SN1 products.
• Ion-pair stereochemistry: Product often shows slight inversion enrichment even in SN1 due to contact ion pairs.
• Lucas test logic: Immediate turbidity signals tertiary SN1; sluggish secondary or inert primary helps classify substitution class.
• Elimination creep: Heat, dilute halide, or bulky carbocations funnel E1/E2, giving Zaitsev alkenes.
• Solvent/nucleophile strength: Cl⁻ underperforms in protic media without ZnCl₂; upgrade to Br⁻/I⁻ or alternative reagents for reliable SN2.
• HI quirks: HI is strong and nucleophilic enough to cleave ethers under forcing conditions—avoid misassigning products if an ether is present.

Practical Tips

• Keep tertiary and benzylic systems cool to favour substitution over elimination; warm only as needed for recalcitrant substrates.
• Choose halide deliberately: HBr is the generalist, HI is fastest, HCl typically demands ZnCl₂ (Lucas) to compete.
• Quench promptly and neutralise acid; prolonged exposure can trigger solvolysis or elimination.
• Handle HX with respect—fuming HI/HBr and concentrated HCl/ZnCl₂ require gloves, goggles, and good ventilation.

Exam-Style Summary

• Protonate ROH → ROH₂⁺.
• Tertiary/secondary/benzylic/allylic → SN1 (carbocation; rearrangements possible; racemisation).
• Primary/methyl → SN2 (backside attack; inversion; no rearrangement).
• HBr/HI usually sufficient; HCl often needs ZnCl₂. Watch for elimination at elevated temperatures.

Interactive Toolbox

• Mechanism Solver — Pick "HBr/HCl/HF/HI" to inspect the SN1 and SN2 frames generated above.
• Reaction Solver — test custom alcohol substrates under HX conditions to predict substitution vs elimination outcomes.
• IUPAC Namer — confirm systematic names for the alkyl halide products you generate.