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Introducton to the Wittig Reaction

The Wittig reaction, also known as Wittig olefination, is a chemically active reaction in which an aldehyde carbonyl as well as the ketone carbonyl combines with a ylide (commonly triphenyl phosphonium) primarily referred to as a Wittig reagent, to produce a double bonded alkene and a salt of triphenylphosphine oxide. Georg Wittig, a chemist, established the subject reaction (Wittig) in 1954, which led to him being awarded the chemistry Nobel Prize in 1979. The reaction has found a wide application, notably in organic synthesis, for the preparation of alkenes attributable to its predictability.

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The reaction works for a broad array of alkyl/aryl (R) groups, as well as both the aldehydes and ketones. Whenever the simple alkyl or aryl groups are involved, it fundamentally generates/produces the Z-alkene product, however if the R groups are equivalent, E/Z mixes may result. The generation of a highly stable triphenylphosphine oxide (Ph₃P=O) makes it possible for the reaction to take place. The reaction representation below illustrates how the reaction happens in general:

The functional groups targeted by the Wittig reaction 

The Wittig reaction takes place specifically at the carbonyl functional groups of aldehydes or ketones, but not esters or amides. Furthermore, a somewhat unusual-looking species is the ylide (a group having opposing formal charges on neighbouring atoms), specifically a "phosphonium ylide," since ylides of nitrogen and sulfur also exist.

The mechanism for occurrence of the Wittig reaction

Step 1:

The ylide's negatively charged carbon is nucleophilic. This carbon then performs a nucleophilic attack on the aldehyde or ketone's carbonyl carbon. As a result, a betaine, a charge-separated (as well as dipolar) intermediary, is formed.

Step 2:

The betaine intermediary obtained from stage 1 is then introduced/utilized for the production of a new O-P bonding, which results in the formation of another intermediary with a ring structure that is 4-membered.

Step 3: 

The C-O and C-P bonds are segregated/separated in a ring intermediary that is 4-membered. The oxygen accepts both the bonding electrons and establishes a new double bond with the phosphorus that releases the electrons pair that are ready to bond or attached with the C-atom. Having such pair of electrons, a new C=C double bond is generated, providing the needed alkene molecule.