60 OUTLINES OF PHYSICAL CHEMISTRY
boiling point rises by about 20° for the addition of each
CH 2 .
Formic acid boils at 99° Acetic acid „ 118° Propionic acid ,, 140°, and so on.
This regularity has only been observed for the first six or seven members of the series. It is also to be generally noticed that the change of boiling point brought about by the addition of CH 2 tends to diminish as the substance becomes richer in carbon.
For the aromatic hydrocarbons C n H 2n - 6 the introduc- tion of a CH 2 group into a lateral chain produces a normal rise of about 20° in the boiling point. But if the addition takes place directly on to the benzene nucleus, the boiling point is raised by about 30°.
Example. — Benzene boils at 80*4° Toluene „ 110-3°.
The three xylenes boil from 137° to 143°.
4. Where the substances differ not only in molecular weight but have also different constitutions, then, for a given difference in these two factors, there is a correlative difference in the boiling points.
Example. — An acid generally boils about 40° higher than the alcohol from which it is obtained by oxidation.
Formic acid boils at 99° Acetic acid boils* at 118°
Methyl alcohol „ 60° Ethyl alcohol „ 78°
Benzoic acid boils at 250°
Benzyl alcohol „ 207°.
But that is now sufficient for Kopp's law as developed by L. Meyer. The law is not without exceptions any more than the following rule taken from the works of Naumann and L. Henry.
5. When we consider isomeric substances derived from the dame hydrocarbon by substitution of one or more hydrogen atoms, by certain elements (CI, Br, I, 0), or by
�� �
