Probable absence of lattice control of cis–trans-isomerization in 2-ethoxy-cis-cinnamic acid

Abstract

The crystal structure of the title compound has been determined from 1 380 reflections measured by diffractometer. Crystals are orthorhombic, space group Pbca, with a= 10.811(3), b= 12.137(3), c= 15.233(4)Å, Z= 8. The structure was solved by direct methods and refined by least-squares to give R 0.041. The acid is present in the crystal in a non-planar conformation and forms centrosymmetric hydrogen-bonded dimers (O–H ⋯ 0, 2.651 Å). The plane of the double bond, C(1)–C(β)–C(α)–C(7), is inclined to the plane of the phenyl ring at 59.4°, and to that of the carhoxy-group at 12.3°. There is a limiting intramolecular contact between the carbonyl oxygen O(1) and C(6) of the phenyl ring. It has bean postulated that the solid-state cis–trans isomerization of such cinnamic acids is lattice controlled, through interactions between CC bonds in neighbouring molecules in close contact (4.0–4.4 Å). It is shown that this crystal, where isomerization is known to occur, contains no such suitable contacts.