A spectrophotometric study of the ionization of p-coumaric acid

Abstract

A spectrophotometric study of the ionized and un-ionized forms of the cis and trans stereoisomerides of p-coumaric acid in ethanol and water solution has been made. The complex spectra were interpreted by calculation of the corresponding incremental second derivative functions. The cis and trans forms have been isolated from a solution irradiated with ultra violet light by paper chromatographic separation. It has been shown by chromatography that unirradiated samples of p-coumaric acid consist principally of the trans isomer with only a trace of the cis isomer present. In plain ethanol solutions, no ionization of cis or trans isomers could be detected, but both isomers were partially ionized in aqueous solution, the cis form to a considerably greater extent than the trans. Addition of a little hydrochloric acid to the aqueous solution almost completely suppressed the ionization of both isomers. Under the conditions employed, anhydrous sodium acetate has been found to induce ionization of the carboxyl group of the trans isomer to an apparently greater extent in ethanol than in aqueous solution. The same general effect was found with sodium acetate and sodium hydroxide upon the spectrum of p-methoxy cinnamic acid. The absorption of the trans isomer of p-coumaric acid at 310 mμ was found to be greater than that of the cis isomer at the same wavelength, by a factor of 1·28.