18 Dr. S. Young on the Boiling-Points 



associating than "for non -associating substances; also that 

 among the associating substances they are greatest for the 

 alcohols and least for the acids, although the factor of 

 association in the liquid state is very high for both classes of 

 compounds. 



In order to arrive at an explanation of these facts, the 

 effect of replacing hydrogen by chlorine may first be con- 

 sidered. The boiling-point of hydrogen chloride has not yet 

 been determined, but it must be about — 80° 0. Thus, by 

 replacing an atom of hydrogen in the hydrogen molecule 

 by chlorine, the boiling-point is raised about 173° from 

 20 o, 4 abs. to about 193° abs., or approximately in the ratio of 

 1 : 9*5. On replacing an atom of hydrogen in methane by 

 chlorine, the rise of boiling-point is 141 o, from 108 o, 3 to 

 249°* 3, or in the ratio 1 : 2*3. Ascending the series of 

 paraffins the rise of boiling-point, due to the replacement of 

 a terminal hydrogen atom by chlorine, diminishes rapidly 

 at first and then more slowlv, until with octane the rise is 

 only58°'4 from 398°'6 to 457°'0, the ratio being 1:1-147. 

 Thus the influence of the chlorine atom becomes relatively 

 smaller as the formula weight of the alkyl group increases. 



Consider, now, the effect of replacing a hydrogen atom 

 by a hydroxy 1 group. In the formation of water from 

 hydrogen gas the boiling-point is raised no less than 352°* 6 

 from 20°'4 to 373°, or in the ratio of 1 : 18'3 ; in the case 

 of methane the rise is 229°'4 from 108 c> 3 to 337°'7, or in the 

 ratio of 1 : 3'12 ; with octane the rise is 65°*4, from 398°*6 

 to 464°, the ratio being 1 : 1*164 ; and with hexdecane it is 

 only 56°*5 from 560°'5 to 617, in the ratio 1 : 1-101. 



It will be seen that in the case of hydrogen the influence 

 of the hydroxyl group is enormously greater, and in the 

 case of methane much greater than that of the chlorine 

 atom in raising the boiling-point, but that on ascending the 

 series of paraffins to octane, the influence of the hydroxyl 

 group diminishes more rapidly until it is little greater than 

 that of the chlorine atom, and it is probable that with hex- 

 decane it would be somewhat less. This is, no doubt, to be 

 explained by the fact that the molecules of water and of the 

 lower alcohols are highly associated in the liquid, but not 

 in the gaseous state, and that, in order to vaporize the 

 liquids, this molecular attraction must be overcome, and the 

 temperature must therefore be raised. The molecular associa- 

 tion diminishes, however, as the series of alcohols is ascended, 

 and is probably slight in the case of octyl alcohol. If so, 

 it would appear that the effect of the hydroxyl group — apart 

 from association — in raising the boiling-point is not very 

 different from, and is probably somewhat less than, that of 



