Question

Identify the most commonly used reaction to convert aldehydes and ketones to alkenes

• A. Reimer–Tiemann reaction
• B. Wittig reaction
• C. Wolff reaction
• D. Wolff-Kishner reaction

Hint:

To remember carbonyl transformations: - Carbonyl \(\longrightarrow\) Alkene = Wittig Reaction (uses a phosphorus ylide). - Carbonyl \(\longrightarrow\) Alkane = Wolff-Kishner Reduction or Clemmensen Reduction.

Answer 1

The Correct Option is B

Concept: Converting a carbonyl group (\(\text{C}=\text{O}\)) found in aldehydes or ketones into a carbon-carbon double bond (\(\text{C}=\text{C}\)), known as an alkene, is a fundamental transformation in synthetic organic chemistry. This process is called olefination. Let us evaluate each of the named organic reactions to determine its primary synthetic purpose:
• Option (A) Reimer–Tiemann reaction: This reaction is used for the ortho-formylation of phenols. It converts a phenol into a phenolic aldehyde (such as transforming phenol into salicylaldehyde) using chloroform and a strong base like sodium hydroxide. It does not synthesize alkenes from carbonyls.
• Option (B) Wittig reaction: The Wittig reaction is a Nobel Prize-winning transformation used to convert aldehydes or ketones directly into alkenes. It involves reacting the carbonyl compound with a phosphorus ylide (commonly referred to as the Wittig reagent, such as triphenylphosphonium methylide). The mechanism proceeds through a four-membered cyclic intermediate called an *oxaphosphaetane*. This pathway allows for precise control over the location of the newly formed double bond without risk of rearrangement, making it an essential tool for alkene synthesis.
• Option (C) Wolff reaction (Wolff Rearrangement): This reaction converts an \(\alpha\)-diazoketone into a ketene intermediate via loss of nitrogen gas, which then reacts with nucleophiles to form carboxylic acid derivatives. It is part of the Arndt-Eistert homologation sequence and does not produce alkenes from simple carbonyls.
• Option (D) Wolff-Kishner reaction: This reaction is a reduction method used to convert aldehydes or ketones into alkanes (completely reducing the \(\text{C}=\text{O}\) group to a \(\text{CH}_2\) group). It uses hydrazine (\(\text{NH}_2\text{NH}_2\)) and a strong base at high temperatures, removing the oxygen atom entirely without forming a double bond. Conclusion: The Wittig reaction is the standard organic method used to convert aldehydes and ketones into alkenes using phosphorus ylides.