1. Addition of HBr to Ethyne: React ethyne (acetylene) with hydrogen bromide (HBr) in the presence of a Lewis acid catalyst, such as aluminum bromide (AlBr3) or copper(I) bromide (CuBr). This reaction follows Markovnikov's rule, resulting in the formation of 1-bromo-1-butene.

Step 2: Formation of 1-Butyne

2. Elimination of HBr: Treat 1-bromo-1-butene with a strong base, such as potassium hydroxide (KOH) or sodium hydroxide (NaOH), in an alcoholic solution. This reaction leads to the elimination of hydrogen bromide (HBr) and the formation of 1-butyne.

Step 3: Conversion of 1-Butyne to Hexyne

3. Addition of Ethyne: React 1-butyne with another molecule of ethyne in the presence of a copper(I) acetylide catalyst, such as copper(I) chloride (CuCl) and triethylamine (NEt3). This reaction results in the formation of 4-octyne through a coupling reaction.

Step 4: Isomerization of 4-Octyne to Hexyne

4. Isomerization: Heat 4-octyne in the presence of a suitable catalyst, such as potassium hydroxide (KOH) or sodium hydroxide (NaOH), to induce isomerization. This process leads to the rearrangement of the carbon-carbon triple bond, resulting in the formation of hexyne.

The overall reaction scheme can be summarized as follows:

Ethyne (Acetylene) + Hydrogen Bromide (HBr) → 1-Bromo-1-butene

1-Bromo-1-butene + Base (KOH or NaOH) → 1-Butyne

1-Butyne + Ethyne (Acetylene) → 4-Octyne

4-Octyne → Hexyne