Question

The Wittig reaction is useful for placing double bonds in less stable positions. For example, the following transformation is easily accomplished using a Wittig reaction.

(a) Show how you would use a Wittig reaction to do this.
(b) Show how you might do this without using a Wittig reaction, and then explain why the Wittig reaction is a much better synthesis.

Accepted Answer

Step 1: Understand the Wittig reaction mechanism. The Wittig reaction involves the reaction of an aldehyde or ketone with a phosphonium ylide to form an alkene. The ylide is typically generated from a phosphonium salt, which is formed by the reaction of a triphenylphosphine with an alkyl halide. Step 2: Identify the starting materials for the Wittig reaction. You need a phosphonium ylide and a carbonyl compound (aldehyde or ketone). Determine the structure of the ylide and the carbonyl compound that will lead to the desired alkene product. Step 3: Outline the Wittig reaction process. The ylide attacks the carbonyl carbon, forming a betaine intermediate. This intermediate then undergoes a rearrangement to form the alkene and triphenylphosphine oxide as a byproduct. Step 4: Consider an alternative method without using the Wittig reaction. One possible method is the use of a dehydration reaction of an alcohol to form an alkene. This involves converting the alcohol to a good leaving group, such as a tosylate, followed by elimination. Step 5: Compare the Wittig reaction with the alternative method. The Wittig reaction is often preferred because it allows for precise control over the position and stereochemistry of the double bond, whereas other methods may lead to mixtures of products or less control over the double bond placement.

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Key Concepts

Wittig Reaction

Phosphonium Ylide

Alternative Synthesis Methods