Which of the following best describes the product formed when an $\mathrm{aldehyde}$ reacts with a $\mathrm{phosphonium}\mathrm{ylide}$ in a $\mathrm{Wittig}$ reaction?

• A. A $\mathrm{ketone}$ formed by nucleophilic addition to the $\mathrm{aldehyde}$
• B. A $\mathrm{primary}\mathrm{alcohol}$ formed by reduction of the $\mathrm{aldehyde}$
• C. An $\mathrm{alkene}$ with the $\mathrm{double}\mathrm{bond}$ formed between the $\mathrm{carbon}$ of the former $\mathrm{carbonyl}\mathrm{group}$ and the $\mathrm{carbon}$ of the $\mathrm{ylide}$
• D. An $\mathrm{ether}$ formed by nucleophilic substitution at the $\mathrm{carbonyl}\mathrm{carbon}$

Answer: (C)

Verified step by step guidance

1. Recall that a Wittig reaction involves the reaction of an aldehyde or ketone with a phosphonium ylide to form an alkene.
2. Understand that the phosphonium ylide acts as a nucleophile, attacking the electrophilic carbonyl carbon of the aldehyde, leading to the formation of a four-membered oxaphosphetane intermediate.
3. Recognize that this intermediate then undergoes a [2+2] cycloelimination to produce an alkene and a triphenylphosphine oxide byproduct.
4. Note that the double bond in the product forms between the carbon of the original carbonyl group (from the aldehyde) and the carbon of the ylide.
5. Conclude that the product of the Wittig reaction with an aldehyde is an alkene, not a ketone, alcohol, or ether.