Q.69. - Organic Chemistry

Question

Question: Devise a stepwise mechanism for the following reaction. (Hint: The mechanism begins with the conjugate addition of $\mathit{O}\mathit{H}$ .)

Step-by-Step Solution

Answer

Answer

Mechanism of rearrangement reaction

Step 1: Rearrangement reactions

A reaction in which a single compound rearranges to break the existing bonds and form new bonds within itself is called the rearrangement reaction.

Rearrangement reactions occur under acidic or basic conditions.

The product of a rearrangement reaction will be a structural isomer of the starting molecule.

Step 2: Rearrangement in ketones

Rearrangement reactions in ketones occur due to the formation of enols from ketones. The abstraction of a proton by base from beta position produces the enol.

In alpha-beta unsaturated ketones, the protons are abstracted from the fourth position (Michael attack) causing enol formation.

The mechanism of rearrangement reaction is retro aldol condensation.

Step 3: Mechanism of rearrangement

The reaction is initiated when hydroxide attacks the fourth position of an alpha-beta unsaturated ketone.

The enol abstracts proton from water followed by removal of hydroxide ion.

Intramolecular aldol condensation occurs forming a five-membered ring.

Dehydration gives the final product.

Mechanism of the given reaction