Structural diversity, but no polymorphism, in a homologous family of co-crystals of urea and α,ω-dihydroxyalkanes

A family of stoichiometric (2 : 1 molar ratio) co-crystals formed between urea and α,ω-dihydroxyalkanes of even chain length [HO(CH2)nOH, n = 2m, m = 3–8] is shown to exhibit three well-defined structure types, which are rationalized on the basis of specific hydrogen-bonding motifs. In spite of the structural diversity observed for different α,ω-dihydroxyalkane chain lengths, there is no evidence that any member of this family of co-crystals exhibits polymorphism (i.e. none of the α,ω-dihydroxyalkane/urea systems is observed to exist in more than one of the well-defined co-crystal structure types).