aay PRESIDENTIAL ADDRESS. 413 
occur which, for the case of acetone, may be represented by the following 
expressions :— 
CH,*CO-CH, -> CH,:C(OH):CH, 
CH,;‘C(OH):CH,+ Br, -S> CH,*C(OH)Br-CH,Br 
CH,‘C(OH)Br:CH,Br  -> CH,:CO-CH Br + HBr 
the first being one of slow enolisation, accelerated catalytically by halogen acid, 
leading to the production of an unsaturated compound, which then by rapid 
addition of bromine and subsequent elimination of hydrogen bromide conforms 
with Kekulé’s hypothesis. The intermediate compounds, it is true, have not 
been isolated, but a study of the dynamics of the reaction by Lapworth, and 
later by Dawson with his collaborators (using iodine instead of bromine), shows 
that this explanation is in harmony with the data obtained.’* When the reaction 
is applied to carboxylic acids under similar conditions, the view that it takes a 
similar course finds support from an investigation of the dynamics of the 
bromination of malonic acid in aqueous solution.’® 
Whether evidence drawn from reactions found to take place in aqueous 
solution is relevant when bromination is effected by heating a carboxylic acid 
with bromine and red phosphorus may be doubted. Certainly it seems to 
afford no assistance in accounting for the course of chlorination in the acids 
examined by Michael and by Montemartini. Nevertheless, Aschan employs the 
keto-enolic hypothesis '’ to elucidate the results of a recent inquiry into the 
‘mechanism’ of the Volhard reaction ’*; and it may be added that racemisation 
has been found to occur when Jevo-valeric acid is brominated by Volhard’s 
method '“—a result which must follow if enolisation take place, although 
susceptible of another explanation. 
So far as I can form a judgment, no case has been made out for the view 
that substitution of halogen for hydrogen under Volhard’s conditions differs in 
its ‘mechanism’ from substitution in the paraffins. This opinion finds support 
in the discovery just announced by Leuchs *° that, while the chief product of the 
bromination of dextro-B-carboxybenzyl-a-hydrindone 
CH CH, 
CoH DOH CHyCyHyC0jH > Ching SoBrCHy'C,H,COH 
CO CO 
is the racemic compound, no less than 10 p.c. is the dextro-bromo- derivative ; 
therefore, the inference is clear that in the formation of the latter compound, 
if not of both, substitution was effected by a process in which migration of 
the hydrogen atom did not occur. 
Attention may now be directed to the question of ‘direct substitution,’ 
which, in its simplest form, is encountered in the paraffin series. As will be 
gathered from the following selection from among the various theories pro- 
pounded to account for the mechanism of substitution, alternative explanations 
of the intermediate reactions leading up to substitution in these cases involve 
either elimination of the hydrogen atom before introduction of the halogen, or 
addition of the halogen in virtue of the supposed residual valency of both 
molecules, followed by disruption of the complex thus formed into the known 
products of the change. 
15 A. Lapworth, 7ans. Chem. Soc., 1904, 85, 31; H. M. Dawson with May 
S. Leslie, ibid., 1909, 95, 1860; with R. Wheatley, ibid., 1910, 97, 2048; with 
F. Powis, ibid., 1912, 101, 1503. 
* K. H. Meyer, Rer., 1912, 45, 2867. 
©. Aschan, Ber., 1912, 45, 1913; 1913, 46, 2162; K. H. Meyer, Ber., 1912, 
45, 2868. 
“J. Volhard, Jéc. cit. 
% O. Schiitz and W. Marckwald, Ber., 1896, 29, 58, 
** H. Leuchs, Ber., 1913, 46, 2435. 
