56 PHYSICAL PROPERTIES AND COMPOSITION 



Here is to be noted also : 



(a) When one negative group replaces another the in- 

 fluence is about the same in different compounds : 



e. g. if chlorine be replaced by bromine the boiling point 

 is raised, usually by 22. 



(/3) When one positive group replaces another the same is 

 true, the replacement of H by CH 3 being the most frequent 

 instance. 



(y) If a negative group replaces a positive one, e. g. 

 chlorine, hydrogen, the influence is quite different according 

 as there are already negative groups present or not. In 

 the former case l i. e. on introducing 



Cl in CH 3 the rise averages 66 

 Cl in CH 2 C1 ,, 39 



Cl in CHC1 2 13 



A very striking example due to the researches of Henry 

 on chlorinated acetonitril is 



NCCH 3 81 NCCH 2 C1 123 NCCHC1 2 112 NCCC1 3 83 

 + 42 11 29 



Here then the introduction of chlorine, instead of hydrogen, 

 causes a fall in the boiling point. 



(b) Influence of Molecular Symmetry. 



Besides the action of strong affinities the symmetry of 

 the molecule is of influence, and in general depresses the 

 boiling point 2 . This is noticeable in the list of isomeric 

 esters given on p. 52, since the boiling point is highest 

 for the most unequal division of the carbon atoms : 



Methyl octoate 192-9 Octyl formate 198-1, 



whilst there is a minimum for 



Amyl butyrate 184-8. 

 This is equally true of the simple hydrocarbons : 



CH 3 CH 2 CH 2 CH 2 CH 3 38 



(CH 3 ) 2 CHCH 2 CH 3 30 



(CH 3 ) 4 C 9.5 



1 Van 't Hoff, Ansichten uber Org. Chem. 2. 20, 22, 26. 



2 Earp. Phil Mag. (5) 35. 458, 1893 ; Nernst, Theoretische Chemie, 1898, 314. 



