The Wittig reaction is the overall substitution of a C=O bond to a C=C bond. It is a reaction between a cabonyl compound (aldehyde or ketone only) and a species known as a phosphonium ylid. An ylid is a species with a positive and negative charges on adjacent atoms. A phosphonium ylid carries its positive charge on phosphorus. ‘Click’ on the image below to view the 3D mechanism for the formation of one such phosphonium ylid:
Below is an example of a typical Wittig reaction. The phosphonium ylid is treated with the carbonyl compound (cyclohexanone) to give the alkene in 85% yield. ‘Click’ on the image below to view the 3D mechanism for the formation of the alkene:
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