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Cinnamic acid has the formula C6H5CHCHCOOH and is an odorless white crystalline acid, which is slightly soluble in water.
Cinnamic acid is used in flavorings, synthetic indigo, and certain pharmaceuticals. A major use is in the manufacturing of the methyl, ethyl, and benzyl esters for the perfume industry. Cinnamic acid is a precursor to the sweetener aspartame via enzyme-catalysed amination to phenylalanine. Cinnamic acid can dimerize in non-polar solvents resulting in different linear free energy relationships.
Cinnamic acid exists as trans and cis isomers, but the trans form is the one most often found in nature and is the article of commerce. It is obtained from cinnamon bark and balsam resins such as storax. It was first isolated in 1872 by F. Beilstein (of Handbook of Organic Chemistry fame) and A. Kuhlberg. It is synthesized by the Perkin reaction between Ac2O and PhCHO.
trans-Cinnamic acid and cis-Cinnamic acid
trans- Cinnamic acid is more stable and has a melting point of 133°C, and a boiling point of 300°C. The cis-isomer forms have melting points of 68°, 58°, and 42°C because they have three polymorphic forms; the first form, allocinnamic acid, exhibits the highest degree of stability. Cis-cinnamic acid is a naturally occurring compound, presumably converted from trans- cinnamic acid in higher plants.