COLOR-REACTIONS OF HEXOSES WITH SUBSTITUTED PHENOLS IN SULFURIC-ACID MEDIUM

Fructose and mannose underwent dehydration in hot 75% sulfuric acid to form products which condensed with 2,6-dimethylphenol, 2,3,6-trimethy lphenol, 2,5-dimethylphenol and 2,4-dimethylphenol at room temperature ( modified method),to form chromogens with lambda max values of 500-5 05 nm, 510 nm, 500 nm and 505-510 nm, respectively. The spectra were i dentical to those of the chromogens formed by hydroxymethylfuraldehyde (HMF). epsilon values for chromogens formed from fructose and mannose with different phenols were as follows: 2,6-dimethylphenol (48000,230 00), 2,3,6-trimethylphenol (38200,17300), 2,5-dimethylphenol (37400,16 600), 2,4-dimethylphenol (18400,9360). The absorption spectra of the c hromogens formed from hexoses with 3, 4, 5-trimethylphenol, 3,4-dimeth ylphenol, 3,5-dimethylphenol and 2,3,5-trimethylphenol indicated that apart from HMF, another dehydration product formed was also responsibl e for the color. The chromogens formed from this product absorbed ligh t in the lower wavelength region (430-440 nm). When the hexoses were d ehydrated and condensed simultaneously (conventional method) in hot 75 % sulfuric acid, with 2,6-dimethyl phenol and 2, 5-dimethylphenol, chr omogens formed had spectra similar to those seen in the modified metho d. However, fructose gave less color in the conventional method, which can be attributed to reduced availability of phenols due to in situ s ulphonation. Mannose was more chromogenic than fructose in the convent ional method unlike in the modified method. It is probable, an early i ntermediate formed during dehydration of mannose, condensed effectivel y with the substituted phenols and the products underwent facile conve rsion to HMF derivatives, minimizing formation of side-products. Analy sis of the chromogens formed in the conventional method from hexoses w ith 2,4-dimethylphenol, 3,4-dimethylphenol, 3,5-dimethylphenol and 2,3 ,5-trinethylphenol, suggested that intermediates forced during acid de hydration may be trapped as condensation products preventing HMF forma tion.