Why are alkenyl and alkynyl halides so bad for SN and E reactions?

Jan 23, 2015

The main reasons are bond strength, cation instability, and steric hindrance.

$\text{S"_"N} 1$ and $\text{E} 1$ Reactions

A typical alkenyl halide is vinyl bromide (CH₂=CH-Br). This is a poor substrate for $\text{S"_"N} 1$ and $\text{E} 1$ reactions because:

• The C-Br bond is sp² hybridized, so it is stronger than the sp³ hybridized bond in ethyl chloride.
• The vinyl cation (CH₂=CH⁺) that would be formed has the vacancy in an sp² orbital (not sp³), so it is less stable than an ethyl cation (CH₃CH₂⁺).

Bromoacetylene (HC≡C-Br) is an even poorer substrate because the sp hybridized C-Br bond is stronger and the acetylene cation (HC≡C⁺) is less stable than the vinyl cation.

$\text{S"_"N} 2$ Reactions

$\text{S"_"N} 2$ reactions involve back-side attack by the nucleophile.

The reaction with vinyl bromide is slow because

• The nucleophile is repelled by the electrons of the π bond.
• The H atom or other group trans to the leaving group will sterically hinder the approach of the nucleophile.

In bromoacetylene, there is even more repulsion from the π clouds.

And it is impossible for the nucleophile to attack the rear of the carbon bearing the leaving group.

$\text{E} 2$ Eliminations

In an $\text{E} 2$ elimination, the C-H and C-Br bonds must be antiperiplanar to each other. Vinyl halides satisfy this condition.

Strong bases like NaNH₂ in liquid ammonia convert alkenyl halides to alkynes.