242 PROCEEDINGS OF THE AMERICAN ACADEMY 



CgH.XCH^Br + NaC^H^O. = QH^XCH^C^HjOa + NaBr 



in which X stands for the halogen atom attached to the benzol ring. 

 But even this reaction, although less full of sources of error than most 

 of those available, is not so well adapted to work of this sort as I could 

 wish ; since the action takes place so rapidly that small differences 

 in the time of two experiments produce comparatively large differ- 

 ences in their results. I have tried, therefore, to make the time 

 occupied in starting and stopping the action as short as possible, and 

 have reduced the duration of the addition of the sodic acetate solution, 

 by which the reaction is started, to five seconds ; but I have not been 

 so successful in stopping the reaction promptly, as this was done by pre- 

 cipitating the organic matter with water, of which so large a quantity 

 was needed that the average duration of the addition was fifteen sec- 

 onds. 



This method of stopping the reaction by means of water is not 

 above criticism ; for, although I consider of little or no weight the 

 possible objection that the reaction may continue forming more sodic 

 bromide after the addition of water, yet the presence of even a small 

 amount of the substituted benzylbromide will cause a serious error in 

 the determination of the sodic bromide,* and it is very hard to re- 

 move this completely by filtration ; that it is possible, however, if 

 sufficient care is used, appears from the following experiment : — 



A mixture of parabrombenzylbromide and parabrombenzylacetate 

 was dissolved in a little alcohol, and, after precipitating with water 

 and filtering, 4 c.c. of a standard solution of argentic nitrate added, 

 upon titrating the liquid 5.3 c.c. of the standard solution of potassic 

 sulphocyanate were found to be necessary, the theoretical quantity 

 being 5.27 c.c. 



In selecting a solvent, it was necessary to find one which would dis- 

 solve all the substances entering into the reaction ; since Berthelot and 

 Pean de St. Gilles f have shown that, when two liquids which do not 

 mix are used, the amount of action depends to a large extent on the 



* Tliis point is illustrated by the following experiments : A little parabrom- 

 benzylbromide was dissolved in alcoliol, and precipitated with water ; 2.30 c.c. 

 of a standard solution of argentic nitrate were added, and the mixture allowed 

 to stand fifteen minutes ; on titrating for silver, it was found tliat 0.41 c.c, cor- 

 responding to 0.0069 grm. of AgNOg, liad been lost, while another similar sample 

 which stood for one hour and twenty minutes lost 1.69 c.c, corresponding to 

 0.026 grm. of AgNOg. 



t Ann. Chim. Phys., 3d ser., Ixvi. p. 46; Ixviii. p. 238. 



