Q63E - Organic Chemistry (Mcmurry)

Question

How would you synthesize the following substances from benzaldehyde and any other reagents needed?

a. 

b. 

c. 

Step-by-Step Solution

Answer

a. 

b.  

c. 

Reactions of aldehyde

Aldehyde reactions include Wittig, Grignard, and reduction reactions. The Wittig reagent does the aldehyde conversion to an alkene, and the Grignard reagent does the aldehyde conversion to a secondary alcohol.

The synthesis of phenylacetaldehyde

a. 

The given product resembles the starting material, but the only difference is the presence of ${\text{ - CH}}_{\text{2}}$ group between aldehyde and aromatic benzene ring. The reaction starts from benzaldehyde, which reacts with the reducing agent, primary alcohol. 

The benzyl alcohol on reaction with ${\text{PBr}}_{\text{3}}$ gives benzyl bromide. On further reaction of benzyl bromide with Mg, ether gives Grignard reagent. The Grignard reagent on reaction with formaldehyde gives primary alcohol, followed by oxidation which gives phenylacetaldehyde product.

Synthesis of compound (a) from phenylacetaldehyde.

An alternate route for the formation of phenylacetaldehyde is as shown below.

                           Synthesis of phenylacetaldehyde by using Wittig reagent

The intermediate alcohol is formed by hydroboration of a double bond formed by the Wittig reaction of benzaldehyde and methylenetriphenylphosphorane.

The synthesis of compound (b)

b. 

The given product contains a secondary amine and a double bond, recognized as enamine. The formation of enamine is done by amine and acetophenone. This acetophenone is formed when benzaldehyde reacts with Grignard reagent (methylmagnesium bromide) and is followed by oxidation ${\text{CrO}}_{\text{3}}{\text{,H}}_{\text{3}}{\text{O}}^{\text{ + }}$. The reaction is shown below.

                        Synthesis of compound (b) from benzaldehyde

The synthesis of compound (c)

c. 

The given product has a trisubstituted double bond, which tells it is a Wittig reaction. The Wittig reagent is formed when benzyl bromide reacts with ylide. This Wittig reagent on reaction with cyclopentanone yields the desired product. The formation of the product is shown by the reaction below.

                   Synthesis of compound (c) from benzaldehyde