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 SN1, SN2, 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. Early life and education Born in London to a silk merchant who died of tuberculosis when Ingold was five years old, Ingold began his scientific studies at Hartley University College at Southampton (now Southampton University) taking an external BSc in 1913 with the University of London. He then joined the laboratory of Jocelyn Field Thorpe at Imperial College, London, with a brief hiatus from 1918-1920 during which he conducted research into chemical warfare and the manufacture of poison gas with Cassel Chemical at Glasgow. Ingold received an MSc from the University of London and returned to Imperial College in 1920 to work with Thorpe. He was awarded a PhD in 1918 and a DSc in 1921. Academic career In 1924 Ingold moved to the University of Leeds where he spent six years as Professor of Organic Chemistry. He returned to London in 1930, and served for 24 years as head of the chemistry department at University College London, from 1937 until his retirement in 1961. During his study of alkyl halides, Ingold found evidence for two possible reaction mechanisms for nucleophilic substitution reactions. He found that tertiary alkyl halides underwent a two-step mechanism (SN1) while primary and secondary alkyl halides underwent a one-step mechanism (SN2). This conclusion was based on the finding that reactions of tertiary alkyl halides with nucleophiles were dependent on the concentration of the alkyl halide only. Meanwhile, he discovered that primary and secondary alkyl halides, when reacting with nucleophiles, depend on both the concentration of the alkyl halide and the concentration of the nucleophile. Starting around 1926, Ingold and Robert Robinson carried out a heated debate on the electronic theoretical approaches to organic reaction mechanisms. See, for example, the summary by Saltzman. Ingold authored and co-authored 443 papers. Honours In 1920, Ingold was awarded the British Empire Medal (BEM) for his wartime research involving "great courage in carrying