A Brief History of Curly Arrows

The inventor of Curly Arrows is usually recognised to be Robert Robinson (1886-1975), winner of the Nobel Prize for Chemistry in 1947. Robinson's main interests were in the study of the structures, properties, and synthesis of plant dyestuffs (anthocyanins), sex hormones, and alkaloids. His discoveries led to the successful production of synthetic antimalarial drugs, and he contributed towards the investigation of penicillin. He also took a great interest in fundamental questions concerning the mechanisms of organic chemical reactions. You can read more about him at the FECS, and UCL.

The first use of curly arrows to represent the movement of electrons is in a paper entitled An Explanation of the Property of Induced Polarity of Atoms and an Interpretation of the Theory of Partial Valences on an Electronic Basis, by William Ogilvy Kermack and Robert Robinson, published in the Journal of the Chemical Society, 1922, 121, 427.

Robinson had just recently come to adopt the then quite new idea, first proposed by G.N. Lewis, that a (covalent) chemical bond was formed by two shared electrons. The relevant part of the paper is concerned with the observed addition reactions of conjugated dienes, which it explains by noting that unsaturated atoms share more electrons in common than saturated ones, and proposing that there will therefore be a greater mobility of electrons.

5. Primary Conjugation The term "primary conjugation" is used to denote a process occurring in the course of a reaction resulting in addition to the terminal members of an unsaturated system of more than two atoms. (a) Conjugation of Ethylene Linkings.—The representation of the active phase and conjugation of butadiene on the partial valency theory is and on the Thomson and Lewis-Langmuir theory as now interpreted Similarly, the conjugation of three double bonds, supposing such to occur, would be represented thus:— Therefore the notation which has been used in the development of the theory of partial valency acquires a physical meaning if we assume that the symbols imply electrons to the number of four, three, two, and one, respectively, held in common by two atoms.

Representing an electron pair by a line, the following exhibits the covalency changes leading to the negatively charged p-position. This figure is in strict accordance with the views on the electronic explanation of conjugation which were expressed by Kermack and Robinson.

You can read more about the history of Curly Arrows in a recent article entitled "Who really invented the curly arrow?", Education in Chemistry, 2001, 38, 102, and The Fontana History of Chemistry, W.H. Brock, Fontana, London, 1992, gives more information about the history of this period in the development of organic chemistry.

 

Aberdeen Connections

There are two local connections for those of us at Aberdeen University. The first is that the first phase of the building we presently occupy, the Meston Building, was opened by Sir Robert Robinson in 1952.

 

The second local connection is William Ogilvy Kermack (1898-1970), Robinson's co-author on the first "curly arrow" paper mentioned above. Kermack was blinded in a laboratory accident in 1924, but nevertheless succeeded in continuing with a career in science, although he did not continue to collaborate with Robinson. He authored with McKendrick two very important papers (1927 and 1932) on the Mathematical Theory of Epidemics which are still regularly cited to this day. Later, he joined Aberdeen University as Professor of Biochemistry in 1948, and remained in this post until his retirement in 1968. Students from his last years have vivid recollections of his regular visits, with his assistant, to the laboratory classes high in Marischal College, and of his inimitable lecturing style.

 

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