The Wittig reaction forms what type of product from aldehydes/ketones and phosphonium ylides?

• A. Alkanes
• B. Alcohols
• C. Ethers
• D. Alkenes

Answer: (D)

The main idea is that the Wittig reaction takes a carbonyl compound and a phosphonium ylide and makes an alkene. The carbonyl carbon and the ylide carbon come together to form a new C=C bond, while the oxygen from the carbonyl ends up in a phosphine oxide as a byproduct. The process goes through a betaine intermediate and a four-membered oxaphosphetane, which collapses to give the alkene plus the oxide of the phosphine. Because the carbon atoms that become the double-bonded carbons are derived from the carbonyl carbon and the ylide, the product is an alkene rather than an alcohol, ether, or alkane. Stereochemistry of the resulting alkene can depend on the ylide type, with stabilized ylides tending to give more E alkenes and unstabilized ylides often giving more Z alkenes.