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-trimethylphenol, 2,5-dimethylphenol and 2,4-dimethylphenol at room temperature (modified method), to form chromogens with λ max values of 500-505 nm, 510 nm, 500nm and 505-510 nm, respectively. The spectra were identical to those of the chromogens formed by hydroxy-methylfuraldehyde (HMF). ε values for chromogens formed from fructose and mannose with different phenols were as follows: 2,6-dimethylphenol (48000, 23000), 2,3,6-trimethylphenol (38200, 17300), 2,5-dimethylphenol (37400, 16600), 2,4-dimethylphenol (18400, 9360). The absorption spectra of the chromogens formed from hexoses with 3, 4, 5- trimethylphenol, 3,4-dimethylphenol, 3,5-dimethylphenol and 2,3,5- trimethylphenol indicated that apart from HMF, another dehydration product formed was also responsible for the color. The chromogens formed from this product absorbed light in the lower wavelength region (430-440 nm). When the hexoses were dehydrated and condensed simultaneously (conventional method) in hot 75% sulfuric acid, with 2,6-dimethyl phenol and 2, 5-dimethylphenol, chromogens formed had spectra similar to those seen in the modified method. However, fructose gave less color in the conventional method, which can be attributed to reduced availability of phenols due to in situ sulphonation. Mannose was more chromogenic than fructose in the conventional method unlike in the modified method. It is probable, an early intermediate formed during dehydration of mannose, condensed effectively with the substituted phenols and the products underwent facile conversion to HMF derivatives, minimizing formation of side-products. Analysis of the chromogens formed in the conventional method from hexoses with 2,4-dimethylphenol, 3,4-dimethylphenol, 3,5-dimethylphenol and 2,3,5-trimethylphenol, suggested that intermediates formed during acid dehydration may be trapped as condensation products preventing BMF formation.