text: Sir Christopher Kelk Ingold  (28 October 1893 – 8 December 1970) was a British chemist based in Leeds and London. His groundbreaking work in the 1920s and 1930s on reaction mechanisms and the electronic structure of organic compounds was responsible for the introduction into mainstream chemistry of concepts such as nucleophile, electrophile, inductive and resonance effects, and such descriptors as S1, S2, E1, and E2. He also was a co-author of the Cahn–Ingold–Prelog priority rules. Ingold is regarded as one of the chief pioneers of physical organic chemistry.