1911 Encyclopædia Britannica/Cinnamic Acid
CINNAMIC ACID, or Phenylacrylic Acid, C9H8O2 or C6H6.CH:CH.COOH, an acid found in the form of its benzyl ester in Peru and Tolu balsams, in storax and in some gum-benzoins. 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 benzaldehyde in the presence of sodium ethylate or by the so-called “Perkin reaction”; the latter being the method commonly employed. In making the acid by this process benzaldehyde, acetic anhydride and anhydrous sodium acetate are heated for some hours to about 1800 C, the resulting product is made alkaline with sodium carbonate, and any excess of benzaldehyde removed by a current of steam. The residual liquor is filtered and acidified with hydrochloric acid, when cinnamic acid is precipitated, C6H5CHO+CH3COONa = C6H5CH:CH.COONa + H2O. It may be purified by recrystallization from hot water. Considerable controversy has taken place as to the course pursued by this reaction, but the matter has been definitely settled by the work of R. Fittig and his pupils (Annalen, 1883, 216, pp. 100, 115; 1885, 227, pp. 55, 119), in which it was shown that the aldehyde forms an addition compound with the sodium salt of the fatty acid, and that the acetic anhydride plays the part of a dehydrating agent. Cinnamic acid crystallizes in needles or prisms, melting at 133°C; on reduction it gives phenyl propionic acid, C6H5.CH2.CH2.COOH. Nitric acid oxidizes it to benzoic acid and acetic acid. Potash fusion decomposes it into benzoic and acetic acids. Being an unsaturated acid it combines directly with hydrochloric acid, hydrobromic acid, bromine, &c. 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 the complete synthesis of indigo (q.v.). Reduction of orthonitrocinnamic acid gives orthoaminocinnamic acid, C6H4(NH2)CH:CH.COOH, which is of theoretical importance, as it readily gives a quinoline derivative. An isomer of cinnamic acid known as allo-cinnamic acid is also known.
For the oxy-cinnamic adds see Coumarin.