Question

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

Answer 1

Hint: Let us know that an alkenyl halide is just an alkenyl with a halide linked to it. One of the carbon atoms in a double bond is called an alkenyl. Haloalkanes are another name for alkenyl halides. A chemical with one or more halogen atoms linked to an alkynyl group is known as an alkynyl halide or haloalkyne. Fluoride, chlorine, bromide, and iodine are examples of halides found in group $17$ of the periodic table.

Complete answer:
Bond strength, cation instability, and steric hindrance are the key reasons why alkenyl and alkynyl halides are so unfavourable for SN and E reactions. It can be explained with examples as follows.
\[{{\text{S}}_{\text{N}}}{\text{1}}\] and \[{\text{E1}}\] reactions:
We can discuss this with the help of an example. Vinyl bromide is a common alkenyl halide.
Because the \[{\text{C - Br}}\] link is \[{\text{s}}{{\text{p}}^2}\] hybridised, it is stronger than the \[{\text{s}}{{\text{p}}^{\text{3}}}\] hybridised bond in ethyl chloride, this is a poor substrate for \[{{\text{S}}_{\text{N}}}{\text{1}}\] and \[{\text{E1}}\] reactions. The resulting vinyl cation has a vacancy in a \[{\text{s}}{{\text{p}}^2}\] orbital (rather than an \[{\text{s}}{{\text{p}}^{\text{3}}}\] ), making it less stable than an ethyl cation.
Because the sp hybridised \[{\text{C - Br}}\] bond is stronger and the acetylene cation is less stable than the vinyl cation, bromoacetylene is an even poorer substrate.
\[{{\text{S}}_N}2\] reactions:
The nucleophile attacks the backside of the nucleophile in \[{{\text{S}}_N}2\] reactions.
Because the nucleophile is resisted by the electrons of the pi bond, the reaction with vinyl bromide is slow.The H atom or another group trans to the leaving group will sterically obstruct the nucleophile's approach.
There is significantly stronger repulsion from the pi clouds in bromoacetylene.

Note:
It can be noted that the carbon-hydrogen and carbon-bromine bonds must be antiperiplanar in an E$2$ elimination. Vinyl halides fulfil this requirement. Alkenyl halides are converted to alkynes by strong bases like \[{\text{NaN}}{{\text{H}}_{\text{2}}}\] in liquid ammonia. Chloroethene (vinyl chloride) is the most common alkenyl halide, and it can be made in a variety of ways. High-temperature chlorination of ethene is the most cost-effective commercial preparation.