STRUCTURES IN SOLUTION OF YELLOW AND WHITE FORMS OF DIMETHYL 3,6-DICHLORO-2,5-DIHYDROXYTEREPHTHALATE

The puzzling dependence of the color (and spectra) of solutions of dimethyl 2,5-dihydroxy-3,6-dichloro-terephthalate (I) on the nature of the solvent has now been found to be due to the presence of three species in a solvent-dependent equilibrium. One (Y-I) has intramolecular hydrogen bonds to the carbonyl oxygen atoms at each end of the molecule; the second (W-I) has two intramolecular hydrogen bonds to the chlorine atoms; and the third (H-I) is a hybrid with one hydrogen bond to a carbonyl group and the other to a chlorine atom. Application of a statistical treatment of the uv spectra of solutions in cyclohexane, carbon tetrachloride, chloroform, and mixtures of these with ethanol gives ratios of these three species which are in agreement with results from infrared studies. Replacement of the hydroxyl protons by deuterons is shown to shift the hydrogen-bonded carbonyl stretching vibration (in chloroform solution) to lower frequency but not the unbonded carbonyl vibration. © 1969, American Chemical Society. All rights reserved.