Chapter 17: Aldehydes and Ketones. Nucleophilic Addition to C=O

The Wittig Reaction
 

ylide           aldehyde or ketone                  alkene

Reaction type:  Nucleophilic Addition then Elimination

Summary

• The Wittig reaction is an important method for the formation of alkenes.
• The double bond forms specifically at the location of the original aldehyde or ketone.
• Ylides are neutral molecules but have +ve and -ve centers on adjacent atoms that are connected by a s bond.
• The ylid is prepared via a two step process:

An SN2 reaction between triphenyl phosphine and an alkyl halide followed by treatment with a strong base such as an organolithium reagent.

Related Reactions

• Dehydration of Alcohols
• Dehydrohalogenation of Alkyl Halides

THE WITTIG REACTION
Step 1: The nucleophilic C of the ylid Wittig reagent adds to the electrophilic C in the polar carbonyl group, electrons from the C=O π bond are used to form a σ bond to the +ve P atom. This creates a cyclic intermeiate called an oxaphosphetane.
Step 2: Decomposition of the intermediate by breaking the C-P and C-O σ bonds leads to the formation of the C=C π bond of the alkene and triphenyl phosphine oxide.

© Dr. Ian Hunt, Department of Chemistry