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

The Wittig Reaction

alkene synthesis using the Wittig reaction
        ylide           aldehyde or ketone                  alkene 

Reaction type:  Nucleophilic Addition then Elimination


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

Related Reactions

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.
addition of Grignard reagent to an aldehyde

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.



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© Dr. Ian Hunt, Department of Chemistry, University of Calgary