Cinnamic acid is an organic compound with the formula C6H5CH=CHCOOH. It is a white crystalline compound that is slightly solublein water, and freely soluble in many organic solvents. Classified as an unsaturated carboxylic acid, it occurs naturally in a number of plants. It exists as both a cis and a trans isomer, although the latter is more common.
Biosynthesis
Cinnamic acid is a central intermediate in the biosynthesis of myriad natural products include lignols (precursors to lignin and lignocellulose), flavonoids, isoflavonoids, coumarins, aurones, stilbenes, catechin, and phenylpropanoids. Its biosynthesis is performed by action of the enzyme phenylalanine ammonia-lyase (PAL) on phenylalanine.
Natural occurrence
It is obtained from oil of cinnamon, or from balsams such as storax. It is also found in shea butter. Cinnamic acid has a honey-like odor; it and its more volatile ethyl ester (ethyl cinnamate) are flavor components in the essential oil of cinnamon, in which related cinnamaldehyde is the major constituent.
Production
The original synthesis of cinnamic acid involves the Perkin reaction, which entails the base-catalysed condensation of acetic anhydrideand benzaldehyde. Rainer Ludwig Claisen (1851–1930) described the synthesis of cinnamate esters by the reaction of benzaldehyde and esters. The reaction is known as the aldol condensation (with accompanying hydrolysis of the anhydride).
It can also be prepared from cinnamaldehyde and benzal chloride.
Another way of preparing cinnamic acid is by the Knövenaegel–Hans condensation reaction. The reactants for this are the corresponding benzaldehyde and malonic acid in the presence of a weak base, followed by acid hydrolysis.