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If you have written "SN2" in an answer, or called hydroxide a nucleophile, you have used Christopher Ingold's words. He did not invent the electrons, but he gave chemists a way to talk about where they go during a reaction. This post covers his career, the vocabulary he built, a worked rate-law example, and the rivalry that shaped his ideas.
Who was Christopher Ingold?
The Royal Society's biographical memoir by Charles Shoppee records that Ingold was born at Forest Gate, London, on 28 October 1893, and grew up on the Isle of Wight after his father's illness. He studied at Hartley University College, Southampton, graduating as an external student of the University of London in 1913. He was better at physics than chemistry at school, but chose chemistry because it was taught there as "a living, growing, and exciting subject". He also admitted he might have done better in his degree if he had played less chess.
According to University College London's history of its chemistry department, he took his DSc in 1921 at Imperial College, spent six years as Professor of Organic Chemistry at Leeds, and joined UCL in 1930, succeeding Robert Robinson. He was head of department from 1937 to 1961 and was knighted in 1958.
Where did "nucleophile" and "SN2" come from?
Ingold's long paper Principles of an Electronic Theory of Organic Reactions appeared in Chemical Reviews in 1934. UCL calls it a famous review that was at first nicknamed the "English heresy", and says that by 1939 his ideas, terminology and nomenclature for reaction mechanisms, such as electrophilic, nucleophilic, inductive, mesomeric, SN1 and SN2, were accepted and used everywhere. Much of the work was done with Edward Hughes.
Their 1935 paper, Mechanism of Substitution at a Saturated Carbon Atom, Part IV, discussed how a molecule's structure and the solvent change the mechanism and kinetics of substitution. That is the question you answer every time you decide between SN1 and SN2 for a haloalkane.
Worked example: telling SN1 from SN2 by rate
The two mechanisms give different rate laws. These rate constants are sample numbers for practice, not measured values.
SN2, for example bromomethane with hydroxide: rate = k[CH₃Br][OH⁻].
Take k = 2.0 × 10⁻³ L mol⁻¹ s⁻¹, [CH₃Br] = 0.10 mol L⁻¹ and [OH⁻] = 0.20 mol L⁻¹.
Rate = 2.0 × 10⁻³ × 0.10 × 0.20 = 4.0 × 10⁻⁵ mol L⁻¹ s⁻¹.
Double [OH⁻] to 0.40 mol L⁻¹: rate = 8.0 × 10⁻⁵ mol L⁻¹ s⁻¹. The rate doubles.
SN1, for example 2-bromo-2-methylpropane: rate = k[(CH₃)₃CBr].
Take k = 1.0 × 10⁻⁴ s⁻¹ and [(CH₃)₃CBr] = 0.10 mol L⁻¹.
Rate = 1.0 × 10⁻⁴ × 0.10 = 1.0 × 10⁻⁵ mol L⁻¹ s⁻¹.
Double [OH⁻]: the rate stays at 1.0 × 10⁻⁵ mol L⁻¹ s⁻¹, because hydroxide is not involved in the slow step, the loss of bromide to form a carbocation.
Note the units. An SN2 rate constant has units of L mol⁻¹ s⁻¹; an SN1 rate constant has units of s⁻¹. If your units do not match the order, the rate law is wrong.
Common mistakes:
Predicting SN1 for a primary haloalkane because the solvent is polar. The stability of the carbocation matters more.
Forgetting that SN2 inverts the configuration at a chiral carbon, while SN1 usually gives a mixture.
Our guide to haloalkanes and how to choose between SN1 and SN2 has more practice, and electronic effects in organic chemistry covers the inductive and mesomeric effects, also Ingold's terms.
Who did Ingold argue with?
Martin Saltzman's history in the Bulletin for the History of Chemistry (1996) describes how, in the mid-1920s, Ingold and Robert Robinson engaged in bitter and at times personal arguments about the mechanism of aromatic substitution. By late 1925, Saltzman writes, Ingold knew Robinson was right and began using many of Robinson's ideas about how electron pairs move in conjugated systems. Saltzman counts 443 papers in Ingold's career. Robinson's side of the story is in our post on Robert Robinson and curly arrows.
What can a student take from Ingold's way of working?
Name things precisely. A good label makes a pattern visible. SN1 and SN2 sort hundreds of reactions into two families.
Let kinetics decide. Ingold and Hughes argued from measured rates, not from what looked neat on paper.
Change your mind when the evidence turns. He took on his rival's ideas once he saw they worked.
If you keep picking the wrong mechanism, the Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning rather than printing an answer.
This week, take five haloalkane questions and write the rate law before you write the product.
