Traditionally, Diels-Alder [4+2] cycloaddition reactions have been carried out in solvents. This thermally induced reaction forms a new six membered ring. In contrast, [2+2] cycloaddition reactions are photochemically induced. Two new sigma bonds are formed to produce a four membered ring.
trans-Cinnamic acid, the investigated compound in this experiment, undergoes a [2+2] cycloaddition reaction upon absorption of light to form a photodimer. This solventless photodimerization is unique in that only a single stereoisomer is produced. This experiment will allow students to explore work in a rapidly growing field, "crystal engineering." Green Chemistry concepts such as solventless reactions, reduced waste production, atom economy, and reduced energy sources can be highlighted. The link to the laboratory procedures includes pre- and post-lab questions.
Summary prepared July 2005 by Rie Somlai, Department of Chemistry and Biochemistry at Delta State University.
Doxsee, K. M.; Hutchison, J. E. Green Organic Chemistry - Strategies, Tools, and Laboratory Experiments, Print 2004; pp 206-210.