Alkyne Hydrohalogenation Hx

Alkyne Reactions: Hydrohalogenation with HX (HCl, HBr, HI)

Alkyne Reactions: Hydrohalogenation with HX (HCl, HBr, HI)

Alkynes undergo electrophilic addition with hydrogen halides to produce either vinyl halides (with one equivalent of HX) or geminal dihalides (with excess HX). The ionic pathway follows Markovnikov's rule: the proton adds to the carbon that leads to the more substituted vinylic cation, and the halide anion captures that cationic carbon. A second equivalent of HX repeats the sequence across the resulting C=C bond, placing both halides on the same carbon (geminal addition). Internal alkynes commonly give E/Z mixtures of vinyl halides because the first addition is not stereospecific.

Working with alkenes instead? Review the companion guide on alkene hydrohalogenation for the less saturated case and rearrangement examples.

Anti-Markovnikov additions are much rarer: only HBr in the presence of peroxides (ROOR) engages a radical chain that places Br on the terminal carbon. HCl and HI do not show a practical peroxide effect in alkyne chemistry.


Quick Summary

  • Reagents/conditions: HX (HCl, HBr, HI) in inert solvents such as AcOH, Et2O, or CH2Cl2; 0 deg C to room temperature (reflux if sluggish).
  • 1 equiv HX: Markovnikov vinyl halide. Internal alkynes usually furnish an E/Z mixture.
  • Excess HX: Geminal dihalide; both halides reside on the more substituted carbon.
  • Mechanism: pi-protonation -> vinylic cation (or tight ion pair) -> halide capture; repeat under excess HX.
  • Regiochemistry: Governed by vinylic cation stability (substitution, resonance).
  • Stereochemistry: First addition not stereospecific (E/Z); second addition erases alkene stereochemistry.
  • Radical caveat: Only HBr/ROOR provides anti-Markovnikov bromoalkenes; HCl/HI remain ionic.


Mechanism (5 Ionic Steps)

Step 1: pi protonation forms the Markovnikov vinylic cation
Step 1 - HX protonates the alkyne; H adds to the less substituted carbon so the vinylic cation sits on the more substituted carbon.

The pi bond donates into HX. Protonation follows Markovnikov orientation because the carbocation-like center is best stabilized on the more substituted/benzylic/allylic carbon.

Step 2: Halide capture delivers the vinyl halide
Step 2 - X- attacks the vinylic cation (or intimate ion pair), furnishing the Markovnikov vinyl halide.

The halide anion attacks the positively charged carbon. Internal alkynes typically give E/Z mixtures because the attack is not geometry constrained.

Step 3: Second protonation under excess HX
Step 3 - With excess HX, the newly formed C=C is protonated a second time, regenerating the cation next to X.

If you continue adding HX, the alkene behaves like the familiar alkene hydrohalogenation: protonation again places H on the less substituted carbon, leaving the cation adjacent to the existing halide.

Step 4: Second halide capture installs the geminal dihalide
Step 4 - X- captures the cation again, installing a second halide on the same carbon (geminal dihalide).

The ionic capture places both halides on the carbon that best stabilizes the cation. Mixed HX sequences (e.g., HCl then HBr) lead to mixed geminal dihalides.

Step 5: Product set decision point
Step 5 - Outcome depends on HX equivalents: stop at the vinyl halide (1 equiv) or push to the geminal dihalide (excess).

Controlling stoichiometry is key: quench after the first equivalent for vinyl halides, or supply excess HX (and time) to reach the geminal dihalide.

Radical variant (HBr only): In the presence of peroxides (ROOR) or light, HBr can add anti-Markovnikov via a vinyl radical chain, halting after one equivalent because vinyl radicals are less reactive toward a second addition. HCl and HI do not participate in a useful peroxide effect with alkynes.


Mechanistic Checklist

  • Markovnikov orientation: protonation generates the more substituted (or resonance-stabilized) vinylic cation.
  • 1 equiv HX -> vinyl halide (expect E/Z mixtures for internal alkynes).
  • Excess HX -> geminal dihalide; mixed HX sequences give mixed geminal dihalides.
  • No rearrangements: vinylic cations do not undergo hydride/methyl shifts like classical carbocations.
  • Radical anti-Markovnikov addition is only reliable for HBr/ROOR.
  • Report stereochemistry carefully; E/Z ratios stem from the first, non-stereospecific addition.


Worked Examples

Example A - 4-methylpent-2-yne with HCl (ionic path)

Substrate: 4-methylpent-2-yne
4-methylpent-2-yne
Reagent card: HCl
1 equiv HCl -> 3-chloro-4-methylpent-2-ene
1 equiv HCl: 3-chloro-4-methylpent-2-ene (E/Z mixture)
Excess HCl -> 3,3-dichloro-2-methylpentane
Excess HCl: 3,3-dichloro-2-methylpentane (geminal dihalide)

Markovnikov orientation places both chlorides on the tertiary carbon once excess acid is supplied.

Example B - 3-methylbut-1-yne with HI (ionic path)

Substrate: 3-methylbut-1-yne
3-methylbut-1-yne
Reagent card: HI
1 equiv HI -> 2-iodo-3-methylbut-1-ene
1 equiv HI: 2-iodo-3-methylbut-1-ene (E/Z mixture)
Excess HI -> 2,2-diiodo-3-methylbutane
Excess HI: 2,2-diiodo-3-methylbutane (geminal dihalide)

The first addition is not stereospecific (report E/Z). Excess HI furnishes the geminal diiodide.


Multiple Unsaturations & Selectivity

  • Alkenes usually add HX faster than alkynes; protect or sequence additions if both are present.
  • Benzylic/allylic alkynes strongly favour Markovnikov capture at the benzylic carbon.
  • In conjugated systems, the more substituted cation still dominates; draw resonance forms to verify.


Practical Tips & Pitfalls

  • Control equivalents: Stop the reaction promptly if the vinyl halide is desired; excess HX and time push to geminal dihalides.
  • Solvent choice: Acetic acid and halogenated solvents stabilize ionic intermediates; avoid peroxides unless pursuing the radical HBr variant.
  • E/Z reporting: Internal alkynes afford mixtures-specify ratios or label as "E/Z mixture".
  • Safety: Concentrated HX (especially HBr, HI) are highly corrosive; work in a hood with appropriate PPE and neutralisation protocols.
  • Radical vs ionic: Only enable radical conditions (ROOR, light) when you intentionally want anti-Markovnikov HBr addition; this mode does not extend to HCl or HI.


Exam-Style Summary

Alkyne + HX (ionic) -> Markovnikov addition. One equivalent gives the vinyl halide (often an E/Z mix); excess yields the geminal dihalide with both X on the same carbon. No rearrangements occur because vinylic cations do not undergo 1,2-shifts. Anti-Markovnikov addition is limited to HBr under radical conditions.


Interactive Toolbox

  • Reaction Solver - switch between 1 equiv and excess HX to visualise vinyl vs geminal outcomes.
  • Mechanism Solver - step through the ionic mechanism frames above.
  • IUPAC Namer - confirm systematic names for both vinyl halides and geminal dihalides.


FAQ / Notes

Why does the first addition give E/Z mixtures? Vinylic cation capture by X- is not constrained to either face, so both geometries form unless the substrate is biased.

Can I stop at the vinyl halide? Yes-use exactly one equivalent of HX, quench promptly, and avoid heating. Excess HX and longer times promote the second addition.

Does rearrangement occur? No. Vinylic cations (and ion pairs) do not undergo hydride/methyl shifts like classical sp2 carbocations.

Is anti-Markovnikov addition viable? Only for HBr with radical initiators; ionic HCl/HI pathways dominate under normal conditions.