1949

Nonclassical Ion Controversy

Saul Winstein (1912–1969), Herbert C. Brown (1912–2004), George Olah (b. 1927)

The year 1949 inaugurated a controversy that was to last for decades, eventually involving several Nobel laureates and nearly all of the world’s most eminent physical organic chemists. And it all started with one tiny compound: the norbornyl cation.

It had been clear for some time that positively charged carbon atoms (carbocations) were intermediates in many reactions. But the norbornyl system, a small bicyclic ring, behaved oddly in one of the standard carbocation reactions. Two different isomers of the starting material gave the same product, which made sense if they were going through the same intermediate, but one mysteriously reacted much faster than the other.

Two explanations were advanced. American chemist Saul Winstein proposed a strange-looking “nonclassical” carbocation, with the single positive charge distributed across three carbon atoms. This species would form faster from one isomer than from another. But London-born American chemist Herbert C. Brown (later a Nobel laureate for his work in organoboron chemistry) rejected this idea, believing that no exotic species had to be invented. He held that two standard carbocations were interconverting so quickly that experiments couldn’t distinguish them, and that the rate differences were due to crowding around the reaction center (“steric hindrance,” in the language of chemistry).

Sorting out these ideas strained the chemical and analytical techniques of the time and forced a number of new ones to be invented. A crucial method was found by Hungarian-American chemist George Olah (later a Nobel winner himself), who discovered powerful acid solutions that would allow carbocations to persist long enough to be studied by NMR (nuclear magnetic resonance). His experiments suggested that Winstein was right, and most of the field ended up believing the nonclassical structure, but Brown never threw in the towel.

In 2013, an X-ray crystallography structure of a norbornyl cation was obtained. It showed Winstein’s nonclassical structure, which was, by that time, what most chemists had expected to see. The controversy was long over by then, but attempts to resolve it had resulted in advances in analytical techniques and a much greater understanding of chemical reactions and structures.

George Olah, whose NMR techniques provided strong evidence that the nonclassical ion was, in fact, real. The bitter controversy over this issue advanced physical organic chemistry in many ways.