Intermediates and Side Reaction Products in Food Red No. 3 (Erythrosine)-Reaction Products of Iodine with Resorcinol-

Abstract

Triiodoresorcinol is one of the side reaction products of Food Red No. 3 (Erythrosine, Color Index No. 45430, FD & C Red No. 3). According to the Code of Federal Regulations in the USA, the limit for triiodoresorcinol in FD & C Red No. 3 is not more than 0.2%. However, the method of preparation of this compound is not described. In this work, the preparation of iodoresorcinols and the reaction products of iodine with resorcinol were investigated.Triiodoresorcinol was prepared by Michael's and Claassen's methods. Brown powder (A) and a black-brown paste (B) were obtained by Michael's method. From A, brownishpurple crystals were obtained after fractionation (50-150ml of chloroform fraction) on a silicic acid column. The crystals melted at 130°, and were identified as 2, 4, 6-triiodoresorcinol from the infrared (IR) and nuclear magnetic resonance (NMR) spectra (Figs. 2 and 3). The absorption maximum of 2, 4, 6-triiodoresorcinol was at 293nm (E1%1cm 152, in ethyl alcohol). From B, pale yellow crystals were obtained in the same way (450-550ml of chloroform fraction), mp 90°C; the structure of this compound was concluded to be 2, 4-diiodoresorcinol from the IR and NMR spectra (Figs. 4 and 5). The absorption maximum of 2, 4-diiodoresorcinol was at 286nm (E1%1cm 73, in ethyl alcohol). No crystalline product could be obtained by Claassen's method. The colorless crystals obtained by Nicolet's method, mp 149°C, were confirmed to be 4, 6-diiodoresorcinol by analysis of the IR and NMR spectra (Figs. 6 and 7). The absorption maximum of 4, 6-diiodoresorcinol was at 296nm (E1%1cm, in ethyl alcohol).Food Red No. 3 is prepared by iodination of fluorescein obtained by condensation of 2 mole of resorcinol and 1mol of phthalic anhydride. Reaction of iodine with resorcinol was carried out in 50w/v% sodium hydroxide solution, as well as iodination of fluorescein. Reaction of iodine (3 g-atom) with resorcinol (1mol) yielded five compounds, which could be separated by thin layer chromatography (TLC) on a silica gel plate developed with a mixture of chloroform-glacial acetic acid (4:1). The spot was detected by spraying the chromatoplate with diazotized sulfanilic acid solution. Ultraviolet absorption and IR spectra of the five compounds isolated by TLC were measured. The spectra of spots a (Rf 0.79), b (Rf 0.69) and d (Rf 0.45) were identical with those of 2, 4, 6-tri-, 2, 4-di- and 4, 6-di-iodoresorcinols, respectively. Spots c (Rf 0.53) and e (Rf 0.35) are thought to be 2-mono-and 4-mono-iodoresorcinols, respectively. A very small amount of 4, 6-diiodoresorcinol was formed in the reaction under the above conditions.