o-Ethoxy-trans-cinnamic acid has been found in three polymorphic forms, alpha, beta and gamma , with the form obtained depending on the solvent used to grow the crystal [1, 2 & 3]. The alpha and beta forms react in UV light to form a dimer product unique to the crystal form, 2,2'-diethoxy-alpha-truxillic acid and 2,2'-diethoxy-beta-truxinic acid respectively. No other product is obtained for either crystal form. The gamma form is light stable. This selectivity in the reaction products is due to lattice control over reaction pathways with the reacting double bonds needing to be between 3.6 and 4.1Å apart in order for the reaction to occur. In order to provide information on the ability of the molecular packing to control these reactions, comparisons in the molecular packing between the various crystal types and products of o-Ethoxy-trans-cinnamic acid are to be presented.

References

1. Cohen, M.D. and Schmidt, G.M.J. (1964). J. Chem. Soc. pp. 1996 - 2000
2. Cohen, M.D., Schmidt, G.M.J. and Sonntag, F.I. (1964). J.Chem.Soc. pp. 2000 - 2013.
3. Schmidt, G.M.J. (1964). J. Chem. Soc. pp. 2014 - 2021.