The 2 acid exists in cis- and trans- forms; these are produced simultaneously in the reduction of A or A 1.5 dihydroterephthalic acids by sodium amalgam.
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.
The A 4 acid exists in cis- and transforms. The trans-variety is produced by reducing phthalic acid, and the cis-acid by reducing 2.4 dihydrophthalic acid.
Stereo-isomerism may occur: the simplest examples are the dibasic acids, where a cis- (maleinoid) form and a trans- (fumaroid) form have been observed.
Of the dihydrophthalic acids, five are known in the ortho-series, two of which are stereo-isomers of the cis- and trans-type, and a similar number are known in the para-series.