STRUCTURE OF P-HYDROXYBENZOIC ACID AND P-HYDROXYBENZOIC ACID ACETONE COMPLEX (2/1)

p-Hydroxybenzoic acid, C7H6O3, M(r) = 138.1, monoclinic, P2(1)/a, a = 18.508(7), b = 5.228(2), c = 6.342(3) angstrom, beta = 93.22(3)-degrees, V = 612.7(4) angstrom3, Z = 4, D(m)(295K) = 1.525(10), D(x) = 1.497 (1) g cm-3, lambda(Mo Kalpha) = 0.71073 angstrom, mu = 0.11 mm-1, F(000) = 288, T = 295 K, R = 0.041 for 1043 unique reflections. p-Hydroxybenzoic acid-acetone (2/1), 2C7H6O3.C3H6O, M(r) = 334.3, monoclinic, P2(1)/a, a = 24.093 (8), b = 7.232 (2), c = 9.699 (2) angstrom, beta = 92.47 (2)-degrees, V = 1688 (1) angstrom, Z = 4, D(m)(294 K) = 1. 308 (I 0), D(x) = 1.315 (1) g cm-3, lambda(Mo Kalpha) = 0.71073 angstrom, mu = 0.11 mm-1, F(000) = 704, T = 295 K, R = 0.054 for 2183 unique reflections. Pairs of p-hydroxybenzoic acid molecules form cyclic hydrogen-bonded dimers in the pure acid and in the acetone complex. The carboxyl H atom is disordered in the pure para isomer. The dimers are centrosymmetric in the pure acid and involve two crystallographically distinct molecules in the acetone complex. In the pure acid, the dimers are linked together through hydrogen-bonded phenolic groups; these bonds spiral around the twofold screw axes to make up layers of dimers parallel to (401). In the acetone complex, a pair of dimers is linked through hydrogen-bonded phenolic groups, and the acetone molecule is hydrogen bonded to one of the phenolic groups; these bonds spiral around the twofold screw axes to form chains of dimers approximately parallel to (20 1). The monoclinic p-hydroxybenzoic acid crystals appeared to be the lower temperature phase with a melting point of 488.0 (2) K and enthalpy of fusion (DELTAHf) of 30.99 (8) kJ mol-1. In the calorimetric studies, a second peak was sometimes detected approximately 1 K above the melting point of the monoclinic para isomer. The structure of the high-temperature phase of p-hydroxybenzoic acid was not resolved; however, the high-temperature structure is expected to contain no dimers.