The structure of benzoquinone. A quantitative X-ray investigation

Abstract

One of the principal results of recent X-ray studies of aromatic compounds has been the proof that the benzene ring has the form of a regular plane hexagon, the distance from centre to centre of the carbon atoms being 1·41 A. It is not easy to state concisely to what limits of accuracy this has been established, because the precision of the intensity measurements varies somewhat from compound to compound according to the experimental difficulties. The accuracy of the results can probably best be judged by inspection of the contoured maps of electron density which have been obtained by Fourier analysis from the various structures. In favourable cases quite a small departure from regularity in the structure, say of the order of 0·03 A, would be detected with some certainty. This work has shown that the carbon atom in aromatic compounds differs in size as well as in the disposition of its valency bonds from the aliphatic carbon atom. As yet there has been no evidence of the alternating double and single bonds of the Kekulé formula, but, of course the X-ray work would not be expected to reveal any permanent difference in the links in the true aromatic compounds, because all the chemical evidence goes to show that the six corners of the ring are equivalent, and if the links are of two kinds their positions must interchange with great rapidity. But if the configuration is stabilized by the introduction of divalent groups or by other means, then it might be possible to detect a distortion in the shape of the ring by X-ray methods. We should expect a contraction at the double bonds and corresponding alteration of the valency angles. The effects would certainly be small but the present technique of X-ray analysis should be capable of revealing them.