Hydrohalogenation Hbr Hcl

Alkene Reactions: Hydrohalogenation using Hydrohalic Acids

Hydrohalogenation of Alkenes (HBr, HCl): Mechanism, Regiochemistry, and Carbocation Rearrangements

Hydrohalogenation is the electrophilic addition of a hydrogen halide (HX) across a carbon–carbon double bond to form an alkyl halide. Under standard protic conditions, HBr and HCl add with Markovnikov regiochemistry through a discrete carbocation intermediate that may undergo 1,2-hydride or 1,2-methyl shifts prior to halide capture. Under radical (peroxide/hν) conditions, HBr can add anti-Markovnikov via a chain mechanism; this pathway is not synthetically useful for HCl or HI in typical instructional settings. Use the Reaction Solver for rapid regiochemistry predictions and the Mechanism Solver to export stepwise arrow-pushing artwork.

Introduction

Hydrohalogenation is the electrophilic addition of HX to produce an alkyl halide. Markovnikov protonation yields a carbocation that may rearrange by 1,2-hydride or 1,2-alkyl migration before halide capture. Radical peroxide conditions for HBr furnish anti-Markovnikov products via a chain mechanism; analogous anti-Markovnikov pathways for HCl or HI are generally not observed under ordinary laboratory conditions.

Quick Summary

  • Reagents: HBr or HCl supplied as concentrated acid, aqueous HX, or dry gas in polar protic solvents (H₂O, ROH, AcOH).
  • Outcome: Markovnikov addition (H → less substituted vinylic carbon; X → more substituted carbon).
  • Mechanism (ionic): Protonation → carbocation (possible 1,2-shift) → halide capture.
  • Rearrangements: 1,2-hydride and 1,2-methyl (alkyl) shifts when they generate more stable carbocations (e.g., secondary → tertiary; resonance-stabilized).
  • Exception: Anti-Markovnikov addition with HBr under radical (ROOR or hν) conditions; HCl and HI lack practical radical hydrohalogenation pathways.
  • Stereochemistry: Carbocation formation erases stereochemical information; new stereocenters form as racemic mixtures.
  • Related guides: Compare with acid-catalyzed hydration and radical HBr/ROOR hydrohalogenation.

Mechanism (Ionic, Stepwise)

Step 1: protonation of the alkene; π bond donates to H+ while the H–X bond heterolyzes to X-.
Step 1 — Protonation (rate- and regioselectivity-determining).

Step 1 — Protonation. π(C=C) → H⁺; simultaneously H–X → X⁻. Protonation happens at the alkene carbon that furnishes the more stabilized carbocation (greater substitution, resonance, hyperconjugation). Example: CH3-CH=CH-CH3 + HBr → secondary carbocation at C3 after C2–H bond formation.

Step 2: optional 1,2-hydride or alkyl shift migrates the positive charge to a more substituted carbon.
Step 2 — Optional 1,2-rearrangement (hydride or alkyl migration).

Step 2 — Optional 1,2-rearrangement. If an adjacent σ bond can migrate, the carbocation shifts prior to halide capture. Hydride shift ASCII: Cβ–H → Cα⁺. Alkyl (methyl) shift ASCII: Cβ–Cγ → Cα⁺.

  • The adjacent carbon must bear the migrating group (β-H for hydride shift, β-alkyl for methyl/alkyl shift).
  • Migration must increase carbocation stability (secondary → tertiary, benzylic/allylic, resonance).
  • Geometry must permit overlap; avoid severe ring strain or bridgehead violations (Bredt’s rule).
  • Without a suitable migratory group, rearrangement does not occur.
Step 3: halide lone pair attacks the carbocation, forming the C–X sigma bond.
Step 3 — Nucleophilic capture by halide.

Step 3 — Halide capture. The halide donates a lone pair into the carbocation (X⁻ → C⁺) to form the C–X σ bond and complete the ionic hydrohalogenation sequence.

Mechanism Solver reminder: Recreate these steps for your own substrate by exporting SVGs from the Mechanism Solver. Request format=svg, dpi=300, and atom numbering for clean publication graphics.

Mechanistic Checklist (Exam Focus)

  • Draw and label the curved arrows: π(C=C) → H⁺ and H–X → X⁻.
  • Depict the carbocation intermediate(s) and note any resonance or hyperconjugation that stabilizes them.
  • If a 1,2-shift is plausible, show the migrating σ bond, label the new carbocation, and justify the stability gain.
  • Finish with halide attack and explicitly state the Markovnikov outcome and racemic mixture when new stereocenters arise.
  • If the conditions include peroxides or light with HBr, describe the radical anti-Markovnikov chain pathway (no carbocation, no rearrangement).

Worked Examples

Example A — Hydride Shift to a Ring Junction

  • Substrate: Exocyclic isopropenylcyclohexane (illustrated below).
  • Reagent: Hydrobromic acid in ionic conditions (no peroxides).
  • Key steps: Protonation forms a secondary carbocation; a β-hydride on the ring migrates (1,2-H shift) to create a tertiary bridgehead carbocation; Br⁻ captures the rearranged cation to give a tertiary bromide.
  • Takeaway: Hydride migration outcompetes direct capture because the rearranged tertiary cation is far more stabilized.
Exocyclic isopropenylcyclohexane substrate
Substrate — exocyclic isopropenylcyclohexane
+
Hydrogen bromide reagent
Reagent — HBr
→
Hydride shift followed by bromide capture final step
Final step — hydride shift then Br⁻ capture

Example B — Methyl Shift to Relieve Carbocation Strain

  • Substrate: Tertiary isopropenylcyclohexane (see left illustration).
  • Reagent: Hydrochloric acid (ionic conditions).
  • Key steps: Protonation first gives a secondary cation; a neighboring methyl migrates (1,2-alkyl shift) to furnish a more substituted carbocation; Cl⁻ capture produces a tertiary chloride.
  • Takeaway: Alkyl shifts compete when they create a markedly more substituted carbocation or relieve crowding.
Tertiary isopropenylcyclohexane substrate
Substrate — tertiary isopropenylcyclohexane
+
Hydrogen chloride reagent
Reagent — HCl
→
Methyl shift followed by chloride capture final step
Final step — methyl shift then Cl⁻ capture

When Multiple Alkenes Are Present

The alkene that protonates to give the most stable carbocation reacts first. Conjugated alkenes that delocalize positive charge often dominate.

With excess HX under ionic conditions, multiple double bonds may be consumed sequentially. For ambiguous substrates, run each alkene through the Reaction Solver and document which C=C was selected.


Practical Tips & Pitfalls

  • Radical conditions: Anti-Markovnikov outcomes require HBr + ROOR (or hν) to initiate a radical chain. HCl and HI generally do not participate productively in analogous chains at standard temperatures.
  • Ring systems/strain: Avoid proposing rearrangements that generate severe ring strain or place a carbocation at a bridgehead (Bredt’s rule).
  • Stereochemistry: Ionic hydrohalogenation is non-stereospecific; racemization is expected whenever a new stereocenter forms.
  • Safety: HBr and HCl are corrosive and fuming; work in a fume hood with appropriate PPE and neutralization protocols.

Exam-Style Summary

  • Regiochemistry: Markovnikov addition under ionic conditions.
  • Mechanism: Protonation → carbocation (±1,2 shift) → halide capture.
  • Exception: Anti-Markovnikov only for HBr under radical/peroxide conditions (no carbocation, no rearrangement).
  • Stereochemical note: Ionic pathway forms racemic mixtures; radical pathway can retain or invert depending on radical capture but is usually not discussed beyond regiochemistry.

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FAQ / Exam Notes

Will both alkenes react in a polyene? The alkene whose protonation forms the more stable carbocation reacts first; justify the choice by comparing carbocation stability (substitution, resonance).

How many equivalents of HX are needed? One equivalent adds across one C=C. With excess HX under ionic conditions, additional alkenes may be consumed sequentially.

How should rearrangements be drawn? Show the migrating σ bond (hydride or alkyl) donating into the carbocation; redraw the new carbocation at the donor carbon, then depict halide capture.