Preparation of Alkynes

Preparation of Alkynes

Alkynes are generally prepared through elimination reactions, primarily starting from dihalides. Here's a breakdown of the common methods:

1. Dehydrohalogenation of Vicinal or Geminal Dihalides:

   • Starting Material: Vicinal dihalides (halogens on adjacent carbons) or geminal dihalides (halogens on the same carbon).
   • Reagents: A strong base is required, typically an alkali metal hydroxide (like NaOH or KOH) for the first elimination step, followed by an even stronger base, like sodium amide (NaNH₂) in liquid ammonia (NH₃), for the second elimination. Alcoholic KOH can sometimes be used for the first elimination.
   • Example: The reaction of 1,2-dibromopropane with 2 equivalents of NaNH₂ to produce propyne.
     
   • Mechanism: Two consecutive E2 elimination reactions occur. Each elimination removes HX (a hydrogen halide). The first elimination forms a haloalkene, and the second elimination forms the alkyne.
     

2. Dehalogenation of Tetrahalides:
   

   • Starting Material: Tetrahalides (four halogens on adjacent carbons).
   • Reagents: A metal like zinc (Zn) or magnesium (Mg) is used.
   • Example: The reaction of 2,2,3,3-tetrabromobutane with zinc dust will produce 2-butyne.
     
     
     
   • Mechanism: The metal removes the halogens, resulting in the formation of the triple bond.
     
3. Preparation of Alkynes from Alkenes 
   
   
   • Starting Material: Alkenes.
   • Reagents: Br₂ then strong base (NaNH₂).
   • Example: Addition of Br2 to trans-but-2-ene then double elimination.
     
     
   • Mechanism: Halogenation followed by 2 consecutive E2 eliminations.