Wittig Reaction

What is Wittig Reaction?

Wittig reaction is an organic chemical reaction wherein an aldehyde or a ketone is reacted with a Wittig Reagent (a triphenyl phosphonium ylide) to yield an alkene along with triphenylphosphine oxide.

This Reaction is named after its discoverer, the German chemist Georg Wittig. He was also awarded the 1979 Nobel Prize in Chemistry for this discovery. An example of the Wittig Reaction is provided below.

Wittig Reaction

This reaction is a very common method used in the organic synthesis of alkenes. One of the prime advantages of alkene synthesis is that the site of a double bond is precisely fixed in comparison to the mixtures of differently located double bonds formed by alcohol dehydration.

Wittig Reagent Preparation

Wittig Reagent is a term used to refer to an organo-phosphorus ylide. Compounds wherein two adjacent atoms contain complete octet electronic configurations and hold opposite charges are called Ylides.

Since phosphorus exhibits the ability to hold more than 8 electrons in the valence shell, the following resonance structures can be drawn :

Wittig Reagent Isomer

The ylide from wherein the phosphorus is positively charged and the carbon is negatively charged significantly contributes towards the Wittig Reaction.

Since the phosphorus atom stabilizes the carbanion, the acidity of the compound increases. Therefore, bases like butyl lithium which are very strong must be used in the formation of the Ylide. The reaction detailing the preparation of a phosphorus ylide is provided below.

\([Ph_{3}P^{+}CH_{2}R]X^{-} + C_{4}H_{9}Li \rightarrow Ph_{3}P=CHR + LiX + C_{4}H_{10}\)

Wittig Reaction Mechanism

The Wittig reaction mechanism proceeds via three steps. These steps are:

Step 1: The negatively charged carbon belonging to the ylide is nucleophilic. This carbon proceeds to execute a nucleophilic attack on the carbonyl carbon of the aldehyde or ketone. This leads to the formation of a charge separated (and dipolar) intermediate called a betaine. This step can be illustrated as follows:

Wittig Reaction Mechanism Step 1

Step 2: The betaine intermediate which is formed in step 1 is now subject to the formation of a new oxygen phosphorus bond, yielding another intermediate which has a four-membered ring structure. This step is illustrated below:

Wittig Reaction Mechanism Step 2

Step 3: In the four-membered ring intermediate, the carbon-oxygen bond and the carbon-phosphorus bonds are cleaved. The oxygen takes both the bonding electrons and forms a new double bond with the phosphorus which lost the bonding pair of electrons to the carbon atom. A new carbon-carbon double bond is formed with this electron pair as well, yielding the required alkene product. This step is illustrated below.

Wittig Reaction Mechanism Step 3

Advantages and Limitations of Wittig Reaction

Some advantages of the Wittig reaction include:

  • Alkenes can be synthesized from aldehydes or ketones via this method.
  • Tolerance of carbonyl compounds with many kinds of functional groups like OH group, OR group, etc. by the Wittig reagent.
  • The geometry of the double bond can easily be predicted if the Ylide’s nature is known.

A few limitations of the Wittig reaction are:

  • Both the E and the Z double bond isomers can be formed.
  • The reaction speed is very slow when sterically hindered ketones are used. The yield is also low for these reactions.
  • Aldehydes can easily undergo oxidation, decomposition, or even polymerization.

Some of these limitations can be overcome by following a variation of the Wittig reaction such as the Schlosser modification.

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