Q34P - Organic Chemistry (Mcmurry)

Question

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

Steps involved for addition of primary amine are-

• Nucleophilic addition

• Proton transfer

• Protonation

• Water is eliminated to form an iminium ion

• Deprotonation.

Step 1: Nucleophilic Addition of Amines

Aldehydes and ketones on reaction with primary amines form imine derivatives (Schiff bases), and elimination of water occurs in the reaction. During the imine formation, the carbonyl oxygen is completely removed. The nitrogen of the primary amine reactant replaces the carbonyl oxygen to form an imine C=N bond and during this, both hydrogens of the amine group are lost.

Most aldehydes and ketones react with secondary amines to give products known as enamines (alkene+ amine). In the nucleophilic addition of secondary amines, a different product is obtained because there is no second hydrogen on nitrogen, so hydrogen is removed from an adjacent carbon forming a C=C bond.

Step 2: Mechanism (a)

Steps involved for the addition of primary amine are-

• Nucleophilic addition

• Proton transfer

• Protonation

• Water is eliminated to form an iminium ion

• Deprotonation.

Step 3: Mechanism (b)

Steps involved for the addition of secondary amine are-

• Nucleophilic addition

• Proton transfer

• Protonation

• Water is eliminated to form an iminium ion.

• Water then removes hydrogen from adjacent carbon to form an alkene bond pushing two electrons from the C=N double bond onto the positively charged nitrogen, generating a neutral enamine and hydronium.

Step 4: Mechanism (c)

Since it’s a primary amine, again the steps involved in the mechanism are

• Nucleophilic addition

• Proton transfer

• Protonation

• Water is eliminated to form an iminium ion

• Deprotonation

Step 5: Mechanism (d)

It’s a secondary amine given so the steps involved will be-

• Nucleophilic addition

• Proton transfer

• Protonation

• Water is eliminated to form an iminium ion.

• Water then removes hydrogen from adjacent carbon to form an alkene bond pushing two electrons from the C=N double bond onto the positively charged nitrogen, generating a neutral enamine and hydronium.

Step 6: Explanation for common feature

All these reactions are nucleophilic addition reactions and follow the same pathway, where the nucleophilic part of the reagent adds to the electrophilic carbonyl carbon to form an intermediate and then product is formed.