Q31E - Organic Chemistry (Mcmurry)

Question

Predict the product(s) and provide the mechanism for each reaction below. What does each mechanism have in common?

Step-by-Step Solution

Answer

a) 

b)

c)

d)

Step1: Nucleophilic Addition of Grignard- reagent

Grignard reagent react with the carbonyl (C=O) carbon in aldehydes or ketones to give alcohols. The substituents on the carbonyl determine the nature of the product alcohol. Formaldehyde gives primary alcohols, aldehydes give secondary alcohols and ketones give tertiary alcohols. 

The presence of acidic proton assures the formation of desired alcohol from an intermediate metal alkoxide.

Mechanism: The nucleophilic C of the organometallic reagent is added to the electrophilic carbon of the carbonyl group.

Electrons from the carbon of carbonyl, move to the electronegative O generating an intermediate metal alkoxide complex. Then protonation of the alkoxide oxygen creates an alcohol product.

Nucleophilic Addition of Grignard- reagent

Step 2: Nucleophilic Addition of HCN

Hydrogen cyanide is added to aldehydes and ketones forming cyanohydrins.

 In the first step, the cyanide ion acts as a nucleophile and forms a C-C bond with the electrophilic carbon of the carbonyl group, the electrons of the pi bond between the carbonyl carbon and oxygen are shifted onto oxygen forming an alkoxide ion intermediate.

In the second step, the alkoxide ion is protonated by HCN which regenerates the cyanide ion.

Mechanism 

Nucleophilic Addition of HCN

Step 3: Nucleophilic Addition of Sodium borohydride

Aldehydes are reduced with sodium borohydride ${\mathbf{NaBH}}_4$ to give primary alcohols, while ketones are reduced to give secondary alcohols.

Hydride ions from sodium borohydride act as a nucleophile.

Mechanism

Nucleophilic Addition of Sodium borohydride

Step 4: Nucleophilic Addition of Grignard- reagent

As already discussed, Grignard reagent react with the carbonyl (C=O) carbon in aldehydes or ketones to give alcohols.

Nucleophilic Addition of Grignard- reagent

Step 5: Explanation for common feature

All these reaction are nucleophilic addition reactions where the nucleophilic part of the reagent adds to the electrophilic carbonyl carbon to form an intermediate and then occurs protonation.