412 TRANSACTIONS OF SECTION B. 
the following formule, and propionic acid with bromine and red phosphorus 
under Volhard’s conditions ‘° gives a-bromopropionic acid, 
CH,Br‘CO-CH,CH, and CH,-CO-CHBr'CH, ! = CH,-CHBr-CO-OH 
the halogen occupying what is termed the a-position with reference to the 
carbonyl radical. Why is substitution in the methyl group easier when this 
radical is present in acetone or acetic acid than it is in methane, is one question 
that may be asked. A second will inquire whether the carbonyl group has a 
directing influence, and, if so, by what means is it exercised. a 
It has been supposed by Werner that the distribution of valency is disturbed 
by the introduction of the oxygen atom of the carbonyl group into the molecule 
of the hydrocarbon; that this oxygen atom absorbs much of the valency of the 
carbon atom of the carbonyl group, leaving less to bind its neighbour or neigh- 
bours, which results in their having free valency, and thereby attaching sub- 
stituents to themselves. This explanation, if accepted for the bromination of 
ketones and acids, also for the chlorination of ketones, does not account for the 
results recorded by Michael and by Montemartini in the case of carboxylic 
acids. Michael has found that the f-chloro-, not the a-chloro- acid is the 
chief product (60-65 p.c.) when homologues of acetic acid are chlorinated 17; 
and Montemartini states that if the radical CH occur in any part of the carbon 
chain the exchange of hydrogen for chlorine takes place in that position, how- 
ever distant it may be from the carbonyl group of the acid.13 
CH; 
CH, * CH, * CHCI: CH, CO-OH COL: CHp CH, CO + OH 
(Michael) CH,“ (Montemartini) 
At present there seems to be no clue to the reason why chlorine and bromine in 
these reactions behave alike towards ketones and not towards acids. 
An alternative explanation of this reaction, which has come to be widely 
accepted, is based on the remarkable property called desmotropy or dynamic 
isomerism, which certain of these carbonyl compounds exhibit. A desmotropic 
compound may exist in two or more forms, and its peculiar isomerism is known 
to depend on the mobility of a hydrogen atom in the complex *CH,’CO° whereby 
an equilibrium is set up of the type: 
*CH,:CO: — *CH:C(OH): 
Ketonic form Enolic form 
Of these two forms, the enolic is the more unsaturated, and presumably the more 
reactive."* Lapworth, making use of this desmotropic relationship, supposes that 
when the ketone reacts with halogen in dilute aqueous solution three changes 
*° J. Volhard, Annalen, 1887, 242, 141; Ber., 1888, 21, 1904. 
“ L. Van Raymenant, Bull. Acad. roy. Belg., 1900, 724. For the chloro- 
ketobutanes, cf. idem; Kling, Compt. rend. 1905, 140, 312; Bull. Soc. chim., 
1905 [iii.], 38, 322. 
** A. Michael, Ber., 1901, 34, 4035, 4046. 
** C. Montemartini, Gazz. chim. ital., 1897, 27 [ii.], 368; 1898, 28 [ii.], 290. 
** It may be of interest to note that the long controversy respecting the com- 
position of ordinary ethyl acetoacetate CH,'CO’CH,CO-‘OEt, the first of 
these desmotropic compounds to be discovered, has been brought to an end 
by the isolation of each desmotropic form at temperatures sufficiently low to 
inhibit the desmotropic change. From refractometric observations with mixtures 
of the pure isomerides, Knorr concludes that this ester at the ordinary 
temperature contains about two per cent. of the enolic form, whereas from 
bromination experiments with the ester itself, which may possibly be accom- 
panied by a disturbance of the equilibrium, K. H. Meyer infers that the 
amount may be as much as seven per cent. (L. Knorr, O. Rothe, and H. 
Averbeck, Ber., 1911, 44, 1138; K. H. Meyer, Annalen, 1911, 380, 222; K. H. 
Meyer and P. Kappelmeier, Ber., 1911, 44, 2718.) 
