Investigation of the colour reaction of phenols with MBTH, II: Properties of the isolated products of the reaction with phenol, 2,6-dimethylphenol and 4-methylphenol

Abstract

The following model compounds for the study of the oxidative coupling reactions of 2-hydrazono-3-methylbenzothiazoline (MBTH) with phenolic compounds were prepared: thep-coupling products of the reagent with phenol and 2,6-xylenol and theo-coupling product withp-cresol. The proposed structure of the dyes was confirmed. In strongly polar and ionic media the azobetaine form of the products predominates, and in non-polar media the azine structure. The principal physico-chemical properties of the products were determined and the results contributed to formulation of the course of the coupling reaction, and to optimization of the conditions for determination of small amounts of phenolic substances. The violeto-coupling product is less stable than the redp-coupling products, and intramolecularly hydrogen-bonded six-membered ring is assumed to exist in the structure. In acid medium the dyes are mainly monoprotonated, and partly diprotonated in concentrated sulphuric acid medium. The products are stable up to about pH 11. These properties make extraction into low-polarity solvents possible. The dye derived from 2,6-xylenol andp-cresol has not hitherto been prepared. The principal spectral characteristics, γmax(ɛ), of the products in ethanol medium are: 499 nm (4.7 × 104 1·mole−1 cm−1) for the phenol derivative, 486 nm (5.0 × 104) for the 2,6-xylenol product, and γmax 540 nm for thep-cresol product.