Alkynes, with their triple bonds, are eager to react. They undergo electrophilic addition, forming vinylic carbocations along the way. This process mirrors alkene reactions but with unique twists in stability and product formation.

HX and X2 additions to alkynes follow specific rules. Markovnikov's rule guides where protons land, while stereochemistry determines the final product's shape. These reactions transform alkynes from sp to sp2 hybridization, changing their very nature.

Addition of HX and X2 to Alkynes

Electrophilic addition to alkynes

• Electrophiles add to alkynes in a two-step process involving a vinylic carbocation intermediate
  • Step 1: Electrophile (proton or halogen) adds to one of the alkyne carbons forming a vinylic carbocation
    • Regioselectivity determined by the stability of the vinylic carbocation intermediate (more stable carbocation favored according to Markovnikov's rule)
  • Step 2: Nucleophile (halide ion or solvent) attacks the vinylic carbocation leading to the final product
    • Stereochemistry of the product determined by the approach of the nucleophile from either the top or bottom face of the planar vinylic carbocation
• The reaction involves breaking one of the pi bonds in the alkyne

Mechanism of HX alkyne addition

• Proton ($H^+$) from HX adds to one of the alkyne carbons forming a vinylic carbocation intermediate
  • Proton adds to the carbon that will yield the more stable carbocation (Markovnikov's rule)
• Halide ion ($X^-$) attacks the vinylic carbocation leading to the final product
• Similar two-step mechanism to alkene addition involving carbocation intermediate
  • Alkynes yield vinylic carbocations while alkenes yield alkyl carbocations
• Vinylic carbocations more stable than primary alkyl carbocations but less stable than secondary and tertiary alkyl carbocations (stability: 3° > 2° > vinylic > 1°)
  • Increased stability of vinylic carbocations compared to primary alkyl carbocations due to resonance stabilization (positive charge delocalized between the two carbons of the vinylic system)

Products of alkyne HX and X2 reactions

• Addition of HX to alkynes (hydrohalogenation):
  1. Markovnikov's rule: Proton adds to the carbon that will yield the more stable carbocation intermediate
     • Major product: Halogen attached to the less substituted carbon (more hydrogens)
     • Minor product: Halogen attached to the more substituted carbon (fewer hydrogens)
  2. Stereochemistry: Product is a mixture of E and Z isomers due to approach of the nucleophile from either face of the planar vinylic carbocation
• Addition of X2 to alkynes:
  • Stereochemistry: Anti addition with halogen atoms adding to opposite faces of the alkyne in a concerted mechanism
  • Product: Vicinal dihalide with anti stereochemistry (halogens on opposite sides of the double bond)

Hybridization changes during alkyne reactions

• Alkynes start with sp hybridization
• After addition reactions, the product has sp2 hybridization
• This change in hybridization affects the geometry and reactivity of the molecule