An X-Ray Study of the p-n-Alkoxybenzoic Acids. Part VII. Crystal Structures of Related Forms of p-n-Hexoxy- and p-n-Octoxy-Benzoic Acids

Abstract

X-ray structure analyses have been carried out at room temperature for a crystalline modification of p-n-hexoxybenzoic acid (60BAC) which undergoes four solid-solid transitions on heating before yielding a nematic phase, and for a form of p-n-octoxybenzoic acid, different from that previously reported, which undergoes a single solid-solid transition before yielding a smectic phase. In both crystals the molecules are present as centrosymmetric hydrogen-bonded dimers. The alkyl chains are in the all-trans extended conformation but the dimers have a non-planar conformation arising from a gauche-relation of the ether oxygen and C(γ) about the C(α)-C(β) bond of the chain. This molecular conformation is the same as that found in other crystalline modifications of homologous members. The dimers in each structure are arranged in identical stacks along the crystal b-axes, and in projection down that axis are arranged in parallel staggered rows in a head-to-tail fashion in 8OBAC, but in an alternating head-to-head and tail-to-tail sequence in 6OBAC. Chains in a given stack interact laterally with the antiparallel chains or dimerized cores in neighboring stacks. C₁₃H₁₈O₃ (6OBAC). Monoclinic, P2₁/c, a = 14.689(3), b = 4.957(2), c = 21.888(4) Å, β = 128.68(2)°, Z = 4. C₁₅H₂₂O₃ (8OBAC). Triclinic, Pī, a = 13.442(4), b = 4.869(2), c = 12.192(2)Å, α = 88.58(2), β = 116.76(2), γ = 88.91(2)°, Z = 2.