Question

Trimethylphosphine is a stronger nucleophile than triphenylphosphine, but it is rarely used to make ylides. Why is trimethylphosphine unsuitable for making most phosphorus ylides?

Short answer

After reacting with ethyl iodide, trimethyl phosphine forms the phosphonium salt. Deprotonation of ethyl group or methyl groups form an ylide along with an additional product which makes it unsuitable for making phosphorus ylides.

Step-by-step solution

Wittig reaction

Adding a phosphorus stabilized carbanion to a ketone or aldehyde was given by Wittig. In Wittig reaction, the carbonyl group of a ketone or an aldehyde is converted into a new $\mathrm{C}=\mathrm{C}$bond where no bond was present before.

Reaction of trimethyl phosphine and triphenyl phosphine with ethyl iodide

The structures of trimethyl phosphine and triphenyl phosphine is shown below.

Trimethylphosphine Triphenylphosphine

The reaction of trialkyl phosphine with ethyl iodide is shown below.

The reaction of triphenyl phosphine with ethyl iodide is shown below.

Trialkyl phosphines can participate in the Wittig reaction but they are known to create some other problems. Let us consider that a phosphonium salt is produced by the reaction of trimethylphoshine and ethyl iodide as shown in the reaction above. The phosphorus ylide can be obtained from the deprotonation of the desired ethyl group or also it can be formed from the deprotonation of any of the three methyl groups present. As a result, this gives rise to a phosphorus ylide and an additional product. However, triphenyl phosphine has no protons that can be removed after the phosphonium salt is obtained. Therefore, trimethyl phosphine is unsuitable for making most phosphorus ylides.