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Nature volume 135, pages 247249 (16 February 1935) | Download Citation



THE term ‘Tautomerism’ was introduced by van Laar in 1885 in order to describe those cases in which the compounds represented by two different structural formulæ are found to be “not isomeric but identical”. A pioneer example of this phenomenon was discovered in benzene, since Kekulé, so long ago as 1870, was content to regard the single and double bonds in his formula as merely an instantaneous picture of a moving system, in which the double and single bonds were incessantly changing places. In this way he was able to account for the fact that the 1:2 and 1:6 di-derivatives of benzene are, as Laar expresses it, “not isomeric but identical”. Kekulé's migrating bonds would now be described as a flow of valency electrons, and the phenomenon would perhaps be described as electromerism, but his fundamental ideas are still valid. Indeed, at the present day, chemists would probably be unanimous in saying that all the diverse formulæ which have been assigned to benzene represent merely different aspects or phases of the same molecule, which they would not attempt to isolate.

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