A ketone or aldehyde is converted into an alkene by means of the Wittig reaction. In this process, a phosphorus ylide, R2C-P (C6H15) (also called a phosphorane), adds to a ketone or aldehyde to yield a dipolar intermediate called a betaine. (An yield pronounced ill-id is a neutral, dipolar compound with adjacent plus and minus charges. A betaine pronounced bay-ta-een is neutral, dipolar compound with nonadjacent charges.)
The betain intermediate is not isolated; rather, it spontaneously decomposes through a four-memberediate ring to yield alkene and triphenylphoshine oxide, (Ph)3P=O. The net replacement of the carbonyl oxygen atom by the R2C= group originally bounded tovphosphorus (Figure 19.13).
The phosphorus ylides necesssry for Wittig reaction are easily prepared by SN2 reaction of primary (and some secondary) alkylvhalides with triphenylphosphine, follwed by treatment base. Triphenylphosphine,