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cinnamic acid[sə′nam·ik ′as·əd]
(also β-phenylacrylic acid), a fatty-aromatic unsaturated carboxylic acid, C6H5CH=CHCOOH; colorless crystals. Two geometric isomers of cinnamic acid are known—the trans-isomer and cis-isomer. The trans-isomer is more stable and has a melting point of 133°C, and a boiling point of 300°C. The cis-isomer forms three polymorphic forms with melting points of 68°, 58°, and 42°C, respectively; the first form, allocinnamic acid, exhibits the highest degree of stability.
Cinnamic acid is contained in Peru balsam and Tolu balsam (mainly in the form of esters), as well as in certain essential oils. In industry trans-cinnamic acid is usually prepared by condensation of acetone with benzaldehyde followed by oxidation of the product benzylidene acetone, C6H5CH = CHCOCH3, with hypochlorous acid. The cis-form is obtained from the byproducts of the cocaine industry. Esters of cinnamic acid serve as aromatic substances in perfumes and cosmetics.