It is an example of Favorsky rearrangement reaction. When `alpha`-halogenocyclohexanone (i.e., six membered ring) is treated with alkali (NaOH) followed by acidification forms cyclopentane carboxylic acid ( a five membred ring carboxylic acid).
Mechanism : Base abstracts acidic `alpha`-H atom from `alpha`-haloketone, followed by `S_(N^(2))` displacement of `Cl^(-)` to give a bicycloketone. Ring opening followed by acidification gives cyclopentane carboxylic acid.
