Question

Starting from chloroform how would you prepare propyne?

Answer 1

Starting from chloroform, propyne can be prepared by the following steps:

1. Reaction with sodium amide (NaNH₂): Treat chloroform (CHCl₃) with 3 equivalents of sodium amide in liquid ammonia. This leads to the formation of sodium carbene intermediate, which then undergoes elimination to give sodium acetylide (NaC ≡ CH).
2. Alkylation with methyl iodide: React the sodium acetylide with methyl iodide (CH₃I). The nucleophilic acetylide ion attacks methyl iodide, resulting in the formation of propyne (CH ≡ C – CH₃).

The overall reaction can be summarized as:

\[\mathrm{CHCl}_3 \xrightarrow[\text{liq. NH}_3]{3, \mathrm{NaNH}_2} \mathrm{NaC}\equiv \mathrm{CH} \xrightarrow[\quad]{\mathrm{CH}_3I} \mathrm{CH}_3-\mathrm{C}\equiv \mathrm{CH}\]

This method uses chloroform as the source of the acetylide ion after conversion via sodium amide, followed by methylation to yield propyne. This is a classical method for preparing terminal alkynes and their alkylated derivatives.