Polymorphism of Cinnamic and α-Truxillic Acids: New Additions to an Old Story^†

X-ray structure determinations have been carried out and analyzed for two conformational polymorphs of α-truxillic acid (2). Polymorph I (cell parameters known since 1959) is isolated upon recrystallization of 2 from acetic acid−petroleum ether solution, while polymorph II (fully characterized in 1993) is the product of a one-phase solid-state reaction of tail-irradiated α-cinnamic acid. For polymorph II, two redeterminations of mixed crystals containing solid solutions of α-cinnamic acid and 2 (68.1 and 89.5%, respectively) were performed. Analysis of the structural parameters of the 1993 and present redeterminations of the polymorph II structures, as well as comparison to data from the Cambridge Structural Database, confirm that claims of errors in the 1993 structure determination of polymorph II are unfounded. X-ray quality crystals of the β-polymorph of trans-cinnamic acid (cell parameters known since 1964) have been obtained from Et2O/petroleum ether solutions; the crystal structure is quite similar to that of 4-chlorocinnamic acid.