is upon this alone that the strength of the acid depends (see Chap. XII). For the four acids mentioned, Ostwald (14) has determined the dissociation constants to be —
��CH3COOH CH^CIOOOH CHClaCOOH CClaCOOH
��— >
��Bromine, iodine, cyanogen, oxygen, and the nitro group, \^'hen introduced into the radicle of an acid in place of hydrogen, increase the dissociation constant, and consequently the strength, of the acid; substitution by hydrogen or the amino group weakens the acid. In the case of the benzene derivatives, substitution in the ortho position exerts a stronger influence than substitution in the meta or para positions, which act about equally. These regularities are of great interest in organic chemistry, and have been much utilised to solve questions concerning constitution and grouping of the atoms in the molecule.
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