Walden inversion

In the Walden inversion, the backside attack by the nucleophile in an SN2 reaction gives rise to a product whose configuration is opposite to the reactant.

In the next step the hydroxyl group was replaced by chlorine to the other isomer of chlorosuccinic acid 3 by reaction with phosphorus pentachloride.

The intermediates are the carboxyl dianion A which gives an intramolecular nucleophilic substitution by the β-carboxylate anion to produce a four-membered β-lactone ring B.

The α-carboxyl group is also reactive but in silico data suggests that the transition state for the formation of the three-membered α-lactone is very high.

A hydroxyde ion ring-opens the lactone to form the alcohol C and the net effect of two counts of inversion is retention of configuration.

Montage, using ball-and-stick models , of the three steps in an S N 2 reaction . The nucleophile is green, the leaving group is red and the three substituents are orange.
The S N 2 reaction causes inversion of stereochemical configuration, known as Walden inversion.
Walden cycle