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The Wittig Reaction Chemistry 238 Section G5 Experiment 5

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University of Alabama at Birmingham

Organic Chemistry II Lab (CH 238)

wittig reaction mechanism of benzyltriphenylphosphonium chloride

Introduction:
Georg Wittig discovered a method for creating alkenes from ketones and
aldehydes. He did this by using phosphonium ylides, which are neutral molecules with
positive and negative charges on neighboring atoms. He went on to win a Noble Prize in
Chemistry for this Wittig synthesis.1
The Wittig synthesis converts a carbon-oxygen double bond into a carbon-carbon
double bond. The driving force of this reaction is the formation of phosphine oxide
product. This reaction will also form an alkene as a product. This reaction has two main
stages. The first stage is the forming of the phosphonium ylide, and the second stage is
the reaction of the phosphonium ylide with the carbonyl group of an aldehyde or
ketone.1
One downside of this reaction is the possibility of steric hindrance. Products will
be more abundant with aldehydes, due to less steric hindrance. Products are lower with
ketones due to more steric hindrance. An upside to the Wittig reaction is the synthetic
value it has. This reaction can take place under regular conditions due to the fact that
location of the carbon-carbon bond is pre-determined.1
In this experiment, a Wittig reaction was performed. The mechanism for this
reaction is shown in figure 1. In this reaction, an aldehyde, trans-cinnamaldehyde, is
turned into an alkene, 1,4-diphenyl-1,3-butadiene. The alkene product has three
possible isomers which are (E,E)-1,4-diphenyl-1,3-butadiene, (E,Z)-1,4-diphenyl-1,3-
butadiene, (Z,Z)-1,4-diphenyl-1,3-butadiene. Another product from this reaction is also a
phosphine oxide product, triphenylphosphine oxide. All chemicals used in this reaction
are shown in table 1.

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The Wittig Reaction Chemistry 238 Section G5 Experiment 5

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