CINNAMIC ACID, or PHENYLACRYLIC ACID, is present in Peru and Tolu balsams, in storax and in some gum benzoins, combined with benzyl alcohol as an ester. Its formula is It can be prepared by the reduction of phenyl propiolic acid with zinc and acetic acid, by heating benzal malonic acid, by the condensation of ethyl acetate with benzalde hyde in the presence of sodium ethylate or by the so-called "Perkin reaction" ; the latter being the method commonly employed. In this process benzaldehyde, acetic anhydride and anhydrous sodium acetate are heated to about 180° C. ; the mixture is made alkaline with sodium carbonate, and excess of benzaldehyde removed by a current of steam. The residual liquor is acidified with hydro chloric acid, when cinnamic acid is precipitated, and recrystallized from hot water. Cinnamic acid crystallizes in needles or prisms, melting at 133° C. It exists in two stereoisomeric varieties one of which is trimorphous. On nitration it gives a mixture of ortho- and para-nitrocinnamic acids, the former of which is of historical importance, as by converting it into orthonitrophenyl propiolic acid A. Baeyer was enabled to carry out a complete synthesis of indigo (q.v.).
Cinnamic acid and its esters find employment in the produc tion of perfumes. Its sodium salt is applied in tuberculosis; its cresol and guaiacol esters and ethyl ditrome cinnamate have also been used therapeutically.