Question

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

Short answer

Answer

Mechanism of rearrangement reaction

Step-by-step solution

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.

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.

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