October 28th marks the 120th birthday of Sir Christopher Ingold. He is known for his pioneering work on reaction mechanisms and the electronic structure of organic compounds. Through the course of his work he developed many new scientific terms that are commonly used today, for example, nucleophilic and electrophilic.

Arguably, his most significant work was on the mechanism of substitution reactions. In his work with alkyl halides, he discovered evidence for two different forms of nucleophilic substitution. One, S_N1, in which the rate of reaction is only dependent on the concentration of the alkyl halide and a second, S_N2, in which the rate of reaction is dependent on the concentration of both the nucleophile and the alkyl halide (see picture).

He was still very active in chemistry after his official retirement in 1960 and it was during this period that he co-authored the Cahn–Ingold–Prelog priority rules. This set of rules are used to name the stereoisomers of a molecule and were developed by Christopher Ingold together with Robert Sidney Cahn and Vladimir Prelog.

Sir Chistopher Ingold is the answer to Guess the Chemist (22), which gave details about Ingold’s life.