ADDITION REACTIONS OF CONJUGATED DOUBLE BONDS JOHN R. S~U~PEY, HOWARD COLLEGE, BIRMINGHAM, ALABAMA The addition reactions of compounds possessing a single pair of double bonds are so well explained by the theories of Baeyer,'.* Wunderlich2and Victor MeyerSthat one is surprised a t thecomplex natweof the phenomenon of unsatwation in substances with two pairs of adjacent double bonds. In 1899 Johannes Thiele4 published an article in which he set forth a theory to explain the properties of these compounds that contain what This article has been the he calls a "conjugated system," C=C-C-=. inspiration of so many investigators that it has been considered the beginning of modern interest in valence,=and certainly a study of the addition reactions of conjugated systems will throw light upon several points of theoretical interest keenly alive today. The existence of polarity in a double bond has been shown;' from the reactions of conjugated double bonds much information about the polarity of the unsaturated system can be gained; even a superficial observation of the reactions will disclose the fact that the distribution of valence forces in the system changes with each alteration in either the spatial arrangement or the chemical nature of the groups attached. The literature on the subject of conjugated double bonds is so extensive that it will he necessary to limit the present article to reactions of hydrocarbons containing the system C=C-C=C and to a, &unsaturated aldehydes, nitriles, acids, esters, and ketones. The type of addenda to these six systems has also been limited: the reactions of Grignard's reagents with these type compounds have been the subject of so many investigations that reference to them will have to be omitted. Finally, mention only can he made of the application of Thiele's theory to the problems of substitution in the benzene nucleus; an excellent review of this phase of the subject has recently appeared.? Before entering upon a critical discussion of Thiele's hypothesis it might be well to note two of the unusual physical properties of conjugated double bonds, which mark them as constituting a system of remarkable activity. J. W. Bruhls has shown that the presence of a conjugated system is readily indicated by the exaltation of molecular refraction. He explained the fact that benzene and all its derivatives are exceptions, by assuming that the equally situated ethenoid groups neutralized one another. In supporting this he pointed out that after the insertion of another atom or radical between two of the six carbon atoms of the benzene nucleus (as in the case of tropilidene where CH2 is inserted into the ring), the resulting compound showed the typical exaltation of refraction. K. von Auwersgextended Bruhl's rules of optical behavior to many types
* Numbers refer to bibliography at the end of this article.
of unsaturated compounds. He showed that "crossed" double bonds of
c==C the type C=C
