214 RESEARCHES UPON THE CHEMICAL PROPERTIES OF GASES. 



removable by fractionation, may be completely separated from the ethyl bromide by 

 digestion with sulphuric acid in the cold (as recommended by Riedel, Ber., 1891, 

 p. 105 E). 



The olefiue dibromides can be most easily prepared by the direct union of olefine 

 with bromine. Meyer (Ber., 1891, p. 4248) has, however, shown that in presence of 

 iron (wii"e) as an " iibertrager," bromine attacks ethyl bromide, producing the reaction: 



CjH^Br + 3 Br = C^H.Br, + HBr. ^ 



This process, which requires heating in sealed tubes, in the case of ethyl bromide 

 yields propylene dibromide in the cold from C3H7Br. The method is open to the ob- 

 jection that large volumes of HBr gas are necessarily evolved. Moisture w'holly 

 arrests the reaction. ' • 



Experiments tried in this laboratory with other metals (palladium, magnesium, 

 aluminium) as "brom iibertrager" and at varying temperatures, have failed to give 

 satisfactory results in preparing ethylene dibromide. IsTot only heat, but pressure in 

 sealed tubes is also necessary. 



Allyl iodide, which has served as a more convenient material for the preparation 

 of propylene than propyl iodide, was made by the action of iodine upon glycerine in 

 the presence of both red and yellow phosphorus, by the excellent method described 

 by Behal in Ber., 1887, p. 693 R. 



Iodides are to be preferred to bromides in all cases where KOH is used to pro- 

 duce a reaction, as KI is more soluble in alcohol than KBr. For this reason a larger 

 quantity of KOH is necessary for a given reaction (formation of olefine from alkyl 

 bromide) than in case of the iodine compound. 



