Question

 Draw The Most Stable Form Of The Major Mixed Claisen Product Formed In The Following Reaction.

Answer 1

The mixed Claisen condensation of two esters is given, and the primary product needs to be delivered. The Claisen condensation takes place in the presence of a base such as sodium ethoxide and results in the formation of $\mathrm{\backslash u03b2}$β ketoester.

The Claisen condensation is a reaction between two esters having α hydrogen in the presence of a base. The final product is a $\mathrm{\backslash u03b2}$β ketoester.

The base abstracts $\mathrm{\backslash u03b1}$α hydrogen from one of the ester to form a carbanion. The carbanion then attacks the other ester molecule and forms an intermediate. Finally, an ethoxide ion is eliminated from the ester molecule to give $\mathrm{\backslash u03b2}$β ketoester.

The carbanion formed is as follows:

The extraction of more acidic $\mathrm{\backslash u03b1}$α hydrogen takes place by base. The hydrogen is abstracted by base from ethyl acetate and not from ethyl isobutyrate since the acidity of the $\mathrm{\backslash u03b1}$α hydrogen is gradually reduced due to two electron donating methyl groups present. Hence, the α hydrogen of ethyl acetate is more acidic and is easily extracted by the base.

The mechanism is as follows:

The carbanion attacks on the carbonyl carbon of ethyl isobutyrate.

The nucleophilic species that is carbanion formed in step 1 attacks the electrophilic carbon of ethyl isobutyrate originating the intermediate as described below:

The final product formed is as follows:

The primary product of the given reaction is as follows:

The elimination of ethoxide ions takes place to form β ketoester as follows:

                 

The primary product of the given reaction is as follows: