A century of curly arrows

To chemists of a certain generation, Peter Sykes’ classic A Guidebook to Mechanism in Organic Chemistry  (first published in 1961, but still going strong in the 1980s at least) conjures up one enduring image: curly arrows. Page after page shows organic molecules reconfiguring themselves via this schematic relocation of electrons, giving the impression of chemical reactions as electronic gymnastics displays. As the student quickly learns, a curly arrow with just one barb at its head depicts the movement of a single electron, while twin barbs denote the relocation of an electron pair. 

It’s the habit of the student to receive all instruction as eternal knowledge. But the curly arrow, which enjoys its centenary this year, deftly represents a rather momentous shift in thinking about how chemical change occurs. Once it became clear in the 19th century that organic molecules may have complex but specific molecular shapes, chemists sought to account for why their reactions did not seem to produce random permutations of the constituent atoms. Add bromine to butadiene, say, and the product dibromobutene has bromines either at the 1,2 or 1,4 positions of the C₄  chain, in particular proportions. How come? 

In 1922, the English chemist Robert Robinson and his collaborator William Kermack, a Scot, offered an explanation. They suggested that the way electrons can hop between adjacent atoms will dictate which bonds might form and which might not, and illustrated the notion with curly arrows. Only gradually, however, did the need become apparent to distinguish carefully between the movement of single and pairs of electrons, leading to the arrowhead convention.