Question

In a Wittig reaction, what is the difference between a betaine intermediate and a ylide?

Answer 1

The Wittig Reaction allows the preparation of an alkene by the reaction of an aldehyde or ketone with the ylide generated from a phosphonium salt. The geometry of the resulting alkene depends on the reactivity of the ylide. If R is Ph, then the ylide is stabilized and is not as reactive as when R = alkyl. Stabilized ylides give (E)-alkenes whereas non-stabilized ylides lead to (Z)-alkenes

Mechanism of the Wittig Reaction

Addition of the ylide to the carbonyl is postulated to lead first to the zwitterionic intermediate betaine, which would then close to form a four-membered cyclic intermediate, an oxaphosphetane. The existence of the betaine hasn't been fully established, although its intermediacy plays an important role in the Schlosser Modification. Betaines may be stabilized by lithium salts leading to side products; therefore, suitable bases in the Wittig Reaction are for example: NaH, NaOMe, NEt₃).

The driving force is the formation of a very stable phosphine oxide:

Reactive ylides give rapid reaction and subsequent rapid ring opening to give the (Z)-alkene: