Q59E - Organic Chemistry (Mcmurry)

Question

Show how you might use a Wittig reaction to prepare the following alkenes. Identify the alkyl halide and the carbonyl components.

a. 

b. 

Step-by-Step Solution

Answer

a. 

b. 

Wittig reaction

A Wittig reaction involves the formation of an alkene product when an aldehyde or ketone reacts with an alkyl halide. The triphenylphosphonium ylide in this reaction is known as the Wittig reagent.

The mechanism for the formation of alkene from alkyl halide by Wittig reaction

In a Wittig reaction, alkyl halide in reaction with triphenylphosphine gives phosphonium salt. This salt, further on reaction with butyllithium, gives ylide.

The reaction of triphenylphosphonium ylide with aldehyde or ketone gives an alkene product.

                             The mechanism for Wittig reaction

The alkyl halide and carbonyl compound required to form compound (a)

1. The given compound (alkene) is formed by the Wittig reaction when alkyl halide reacts with an aldehyde. The alkyl halide and aldehyde needed for this reaction are as follows,

                              Formation of alkene compound 

The alkyl halide and carbonyl compound required to form compound (b)

b. A given alkene product is formed by a Wittig reaction when ${\text{C}}_{\text{6}}{\text{H}}_{\text{5}}{\text{CH}}_{\text{2}}\text{Br}$ (alkyl halide) reacts with a ${\text{C}}_{\text{5}}{\text{H}}_{\text{10}}\text{O}$ (ketone). The alkyl halide and ketone needed for this reaction are as follows,

                                          Formation of alkene compound