Question

A phosphorus ylide is a Wittig reagent and is prepared using an alkyl halide and triphenyl phosphide. Using an appropriate example, explain why a tertiary alkyl halide cannot be used to prepare an phosphorus ylide. Show the reaction mechanism of the reaction as well in your explanation.

Answer 1

[AI-generated answer — Bot GPT 3.5]

A tertiary alkyl halide cannot be used to prepare a phosphorus ylide because the reaction mechanism involves a nucleophilic substitution reaction, which is hindered in the case of tertiary alkyl halides due to steric hindrance.

Let's consider the reaction between a secondary alkyl halide, such as bromobutane, and triphenyl phosphide to form a phosphorus ylide. The reaction proceeds as follows:

Step 1: Triphenyl phosphide acts as a nucleophile and attacks the electrophilic carbon of bromobutane, resulting in the formation of an intermediate where the bromine atom has been replaced by a phosphorus atom bonded to the carbon chain.

Step 2: In the presence of a strong base, such as n-butyllithium, deprotonation occurs to give the desired phosphorus ylide. This involves the acidic hydrogen on the carbon adjacent to the phosphorus being removed by the base, resulting in the formation of the phosphorus ylide.

However, if a tertiary alkyl halide were used instead of a secondary alkyl halide, the nucleophilic attack by the triphenyl phosphide would be hindered due to steric hindrance caused by the bulkiness of the alkyl groups on the tertiary carbon. This would make it difficult for the triphenyl phosphide to approach the electrophilic carbon atom and carry out the substitution reaction efficiently.

Therefore, a tertiary alkyl halide cannot be used to prepare a phosphorus ylide using the Wittig reaction due to steric hindrance, which prevents the nucleophilic substitution from occurring effectively.