Addition of HCl, HBr, or HI to Alkynes (1 Equivalent)
Addition of HCl, HBr, or HI to Alkynes (1 Equivalent): An alkyne reacts with one equivalent of HX to form an alkenyl (vinyl) halide.
What happens
An alkyne reacts with one equivalent of HX to form an alkenyl (vinyl) halide. Only one equivalent is added, so the reaction stops at the alkene stage. It is an electrophilic addition, and for a simple unsymmetrical terminal alkyne it generally follows Markovnikov regioselectivity.
Reaction summary
Alkyne + 1 equivalent HX → alkenyl halide (C≡C → C=C with H and X added)
Reagents and conditions
- Starting material: Propyne (terminal alkyne)
- Reagents: HBr
- Conditions: 1 equivalent of HX (HCl, HBr or HI)
- Products: 2-Bromopropene (alkenyl halide, Markovnikov product)
Mechanism
- Protonation: The alkyne π electrons react with H⁺ from HX. A new C–H bond forms and a vinyl carbocation is generated. For a terminal alkyne, protonation occurs in the direction that places the positive charge on the more substituted carbon.
- Halide Attack: The halide (Cl⁻, Br⁻ or I⁻) attacks the positively charged carbon. The result is an alkenyl halide.
- Product: The alkenyl halide: the alkyne has become an alkene and carries one H and one X.
Selectivity
- Regioselectivity: For a terminal alkyne R–C≡C–H, one equivalent of HX generally gives R–C(X)=CH₂: X goes to the MORE substituted carbon and H to the LESS substituted carbon (the Markovnikov product). A symmetrical internal alkyne has no regioselectivity issue because both alkyne carbons are equivalent; an unsymmetrical internal alkyne can give a mixture of regioisomers because the two carbons may have similar substitution.
Common mistakes
- Don't automatically turn the alkyne into an alkane. With one equivalent of HX: C≡C → C=C, not C≡C → C–C.
Memory trick
ONE HX = ONE π BOND GONE. C≡C → (1 eq HX) → C=C.