Chapter 11: Q64. (page 426)

Question: Draw a stepwise mechanism for the following intramolecular reaction

Short Answer

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Answer

Formation of a resonance-stabilized carbocation

Formation of a new five-membered ring

Formation of the final product

Step by step solution

Step-by-Step SolutionStep 1: Intramolecular reactions

The reactions, which take place by the reacting groups that are present on the same molecule, are known as intramolecular reactions.

The intramolecular reactions leading to the formation of a five or six-membered ring occur at a very fast rate, as compared to similar intermolecular reactions.

Stepwise mechanism for the given intramolecular reaction

The given reaction is an intramolecular ring closure reaction that leads to the formation of another five-membered ring within the molecule. The given reaction takes place in the following steps.

The first step is the donation of a proton by the group, which is then followed by the removal of the water molecule from the five-membered ring.

This results in the formation of a resonance-stabilized carbocation, as shown in the following structure:

Formation of a resonance-stabilized carbocation

In the next step, the intramolecular nucleophilic attack by the acetylene group takes place, which leads to the formation of a new five-membered ring, as shown in the following structures:

Formation of a new five-membered ring

In the next step, there is the abstraction of a proton by the ion, which further leads to the formation of the enol. In the final step, there is an attack of another , which leads to the formation of an intramolecular product.