Question

Phosphorus ylides are not stable in water. How is it possible that the part of this experiment even works?

Reaction is as follows:

4-bromomethylbenzoic acid + triphenylphospine (with acetone over the arrow) -----> 4-carboxybenzyl-triphenyl-phosphonium bromide

4-carboxybenzyl-triphenyl-phosphonium bromide (with 1) formaldehyde + NaOH 2) HCl over arrow) ---> 4-vinylbenzoic acid

Answer 1

Wittig reaction

Phosphorous ylides are not stable in water, they tend to decompose to phosphene oxide and alkane upon reaction with water. However, the wittig reaction proceeds well over here. This is because as the reaction proceeds initially only a small amount of ylide reacts with the carbonyl compound to form the alkene product which is more stable than the starting materials. This thus forces the reaction in forward direction. The stability of the product enables the reaction to proceed in the forwards direction. Such that the reaction is thermodynamically feasible.