Volume 2

Recent advances in organic chemistry / by Alfred W. Stewart.

  • Stewart, Alfred W. (Alfred Walter), 1880-1947.
Date:
1931
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Recent advances in organic chemistry / by Alfred W. Stewart. Source: Wellcome Collection.

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    interest, is the problem of cyclic groupings and their influence on physical properties such as refractive index. And here, too, the phenomena of spatial conjugation seem to deserve mention. In the group of cyclic compounds, an unexplained pheno¬ menon meets us at once : the benzenoid character possessed by certain substances. This peculiar series of properties is evidently not produced by one factor alone,' but must be the result of at least three coexisting influences. The alternating system of double and single bonds which is characteristic of the benzene system is to be found in other molecules as well. It occurs, for example, in hexatriene : CH2 : CH . CH : CH . CH : CH2 ; but no aromatic character is shown by this substance. Again, the mere occurrence of a six-membered ring in a com¬ pound confers no benzenoid characteristics on the substance, as is seen in the case of the terpenes or the hexamethylene derivatives. Finally, even the combination of a ring and the alternate grouping of single and double bonds does not suffice to produce aromatic properties, since both such characteristics are present in cyclo-octatetraene : CH=CH—CH=CH CH=CH—CH=CH and yet this substance 1 is not benzenoid in character. It is thus evident that the number of the carbon atoms in the ring must have some influence ; and that to possess the aromatic characteristics a compound must contain : (1) a cyclic structure ; (2) six atoms in the ring; * (3) a symmetrical arrangement of alternate double and single bonds within the ring. Why this particular arrangement should be required and why it and only it can confer aromatic properties upon a hydrocarbon molecule is a point upon which speculation has hitherto failed to throw light. The most plausible solution is to be found in Collie’s benzene space-formula ;2 but even it leaves room for further thought on the subject. 1 Willstatter and Waser, Ber., 191], 44, 3423. * It should be noted that in such substances as pyridine, atoms other than carbon ones can form part of the ring and still the benzenoid character is maintained to a great extent. 2 Collie, J., 1897, 71, 1013.