While certain additive relations hold between some homologous series, yet differences occur which must be referred to the constitution of the molecule. As a general rule, compounds formed Constitutive influences. with a great evolution of heat have high boiling-points, and vice versa. The introduction of negative groups into a molecule alters the boiling-point according to the number of negative groups already present. This is shown in the case of the chloracetic acids:
| CH3CO2H = 118° | Diff. | |
| ClCH2·CO2H = 185° | 67° | |
| Cl2CH·CO2H = 195° | 10° | |
| Cl3C·CO2H = 195° | -200° | 3° |
According to van ’t Hoff the substitution of chlorine atoms into a methyl group occasions the following increments:—
| Cl in CH3 | 66° |
| Cl in CH2Cl | 39° |
| Cl in CHCl2 | 13°. |
The introduction of chlorine, however, may involve a fall in the boiling-point, as is recorded by Henry in the case of the chlorinated acetonitriles:—
| NC·CH3. | NC·CH2Cl. | NC·CHC12. | NC·CC13. | |||
| 81° | 123° | 112° | 83° | |||
| 42° | -11° | -29° |
The replacement of one negative group by another is accompanied by a change in the boiling-point, which is independent of the compound in which the substitution is effected, and solely conditioned by the nature of the replaced and replacing groups. Thus bromine and iodine replace chlorine with increments of about 22° and 50° respectively.
A factor of considerable importance in determining boiling-points of isomers is the symmetry of the molecule. Referring to the esters C9H18O2 previously mentioned, it is seen that the highest boiling-points belong to methyl octoate and octyl formate, the least symmetrical, while the minimum belongs to amyl butyrate, the most symmetrical. The isomeric pentanes also exhibit a similar relation CH3(CH2)4CH3 = 38°, (CH3)2CHC2H5 = 30°, (CH3)4C = 9.5°. For a similar reason secondary alcohols boil at a lower temperature than the corresponding primary, the difference being about 19°. A.E. Earp (Phil. Mag., 1893 [5], 35, p. 458) has shown that, while an increase in molecular weight is generally associated with a rise in the boiling-point, yet the symmetry of the resulting molecule may exert such a lowering effect that the final result is a diminution in the boiling-point. The series H2S = -61°, CH3SH = 21°, (CH3)2S = 41° is an example; in the first case, the molecular weight is increased and the symmetry diminished, the increase of boiling-point being 82°; in the second case the molecular weight is again increased but the molecule assumes a more symmetrical configuration, hence the comparatively slight increase of 20°. A similar depression is presented by methyl alcohol (67°) and methyl ether (-23°).
Among the aromatic di-substitution derivatives the ortho compounds have the highest boiling-point, and the meta boil at a higher, or about the same temperature as the para compounds. Of the tri-derivatives the symmetrical compounds boil at the lowest temperature, the asymmetric next, and the vicinal at the highest.
An ethylenic or double carbon union in the aliphatic hydrocarbons has, apparently, the same effect on the boiling-point as two hydrogen atoms, since the compounds CnH2n+2 and CnH2n boil at about the same temperature. An acetylenic or triple linkage is associated with a rise in the boiling-point; for example, propargyl compounds boil about 19.5° higher than the corresponding propyl compound.