Past exam of the chemistry B course of the University of Cambridge / 2021 / Paper 1 / B5 / b / i / Solution

Two molecules of the carboxylic acid form a cyclic carboxylic-acid dimer. Let Ar denote the meta-methylphenyl group. One molecule donates its OH hydrogen to the other's carbonyl oxygen, and the second donates back:
The two contacts close an eight-membered hydrogen-bonded ring, shown explicitly below.

Both molecules therefore act once as hydrogen-bond donor and once as hydrogen-bond acceptor, giving two O–H···O=C contacts. This is the conventional self-complementary dimer motif; the schematic is not a prediction of every weaker contact in an unprovided crystal structure.