Another form of isomerism is occasioned by spatial arrangements, many of the reduced terephthalic acids existing in two stereo-isomeric forms. Baeyer explains this by analogy with fumaric and maleic acids: he assumes the reduced benzene ring to lie in a plane; when both carboxyl groups are on the same side of this plane, the acids, in general, resemble maleic acids, these forms he denotes by rcis-cis, or shortly cis-; when the carboxyl groups are on opposite sides, the acids correspond to fumaric acid, these forms are denoted by rcis-trans, or shortly trans-.
The A 1.5 acid is obtained by boiling the cis- and trans-A 2.5 acids with water, which are obtained on reducing terephthalic acid with sodium amalgam in faintly alkaline solution.
There are three isomers: (1) ortho, or phthalic acid; (2) meta, or isophthalic acid; (3) para, or terephthalic acid.
Ladenburg (Ann., 1875, 179, p. 163) to be symmetrical trimethyl benzene; terephthalic acid, the remaining isomer, must therefore be the para-compound.