Structural Studies of Enantiomers, Racemates, and Quasiracemates. 2-(3-Bromophenoxy)propionic Acid and 2-(3-Methoxyphenoxy)propionic Acid
The quasiracemate approach for constructing molecular assemblies has provided a fertile ground in which to exploit the crystal packing tendencies of quasienantiomeric components. The present study of quasiracemic behavior reveals that cocrystallization of (S)-3-(2-bromophenoxy)propionic acid and (R)-3-(2-methoxy)propionic acid generates supramolecular motifs organized in space group C2 with near inversion symmetry. This quasiracemic system, first investigated by J. and I. L. Karle in 1966, consists of carboxyl···carboxyl heterodimers that mimic those observed for (±)-Br and (±)-OCH3. These racemates crystallize to give four distinct phases (two polymorphic forms for each Br and OCH3 compound); two of which, (±)-Br−I and (±)-OCH3−I, are isostructural with the quasiracemate. This collection of structures, including those of the enantiopure (S)-Br and (R)-OCH3 compounds, underscores the importance of molecular shape to the construction of supramolecular assemblies.