Answer:
The phosphorus ylide reacts with the aldehyde or ketone to make an oxaphosphetane.
Explanation:
The Wittig reaction is a reaction that occurs between a phosphorus ylide and an aldehyde or ketone. The final products are an alkene and triphenyl phosphine oxide.
The first step in the reaction is the attack of the phosphorus ylide on the aldehyde or ketone. This is followed by attack of oxygen on phosphorus to form a [2+2] cycloaddition product (oxaphosphetane) which decomposes to form the alkene and triphenylphosphine oxide.