Crystal structures of three co-crystals of 1,2-bis­(pyridin-4-yl)ethane with 4-alk­oxy­benzoic acids: 4-eth­oxy­benzoic acid–1,2-bis­(pyridin-4-yl)ethane (2/1), 4-n-propoxybenzoic acid–1,2-bis(pyridin-4-yl)ethane (2/1) and 4-n-but­oxy­benzoic acid–1,2-bis­(pyridin-4-yl)ethane (2/1)

The crystal structures of three hydrogen-bonded co-crystals of 4-alk­oxy­benzoic acid–1,2-bis­(pyridin-4-yl)ethane (2/1), namely, 2C(9)H(10)O(3)·C(12)H(12)N(2), (I), 2C(10)H(12)O(3)·C(12)H(12)N(2), (II), and 2C(11)H(14)O(3)·C(12)H(12)N(2), (III), have been determined at 93, 290 and 93 K, respectively. In (I), the asymmetric unit consists of one 4-eth­oxy­benzoic acid mol­ecule and one half-mol­ecule of 1,2-bis­(pyridin-4-yl)ethane, which lies on an inversion centre. In (II) and (III), the asymmetric units each comprise two crystallographically independent 4-alk­oxy­benzoic acid mol­ecules and one 1,2-bis­(pyridin-4-yl)ethane mol­ecule. In each crystal, the two components are linked by O—H⋯N hydrogen bonds, forming a linear hydrogen-bonded 2:1unit of the acid and the base. Similar to the structure of 2:1 unit of (I), the units of (II) and (III) adopt nearly pseudo-inversion symmetry. The 2:1 units of (I), (II) and (III) are linked via C—H⋯O hydrogen bonds, forming tape structures.