Molecular packing modes. Part IV. Crystal and molecular structures of three cinnamic acids

Abstract

The crystal structures of β-methyl-cis-, (III), β-chloro-cis-, (II), and β-chloro-trans-cinnamic, (I), acids have been determined from three-dimensional counter data by Patterson and Fourier methods. (I): a= 19·588, b= 5·374, c= 16·683 Å, β= 103·62°, Z= 8, space group A2/a, R 0·054 (1873 reflections); (II): a= 5·797, b= 24·247, c= 7·240 Å; β= 121·56°, Z= 4, space group P2₁/c, R 0·046 (1917 reflections); (III): a= 7·504 b= 7·442, c= 17·056 Å, β= 107·00°, Z= 4, space group P2₁/c, R 0·066 (1987 reflections). The σ values of the experimental bond lengths are <0·01 Å, except for the C–H bonds where they are 0·03 Å.

The electron-density distribution in the planes of the paired carboxy-groups and the lengths of the C–O bonds suggests that the packing of these groups is disordered in the two cis-cinnamic acids but not in β-chloro-trans-cinnamic acid.