Intramolecular hydrogen bonding of methoxynaphthoic acids and methyl hydroxynaphthoates

Infra-red OH stretching spectra of 2-methoxy-1-naphthoic (I), 1-methoxy-2-naphthoic (II) and 3-methoxy-2-naphthoic (III) acids were measured under conditions where dimeric association and solvent—solute hydrogen bonding could be neglected. From these spectra, together with their ultra-violet spectra, the following conclusions were obtained: (i) The intramolecular hydrogen bond in III is the most favourable, while it is absent in I. This result is in accordance with the conclusion reported previously by the present authors, and can be explained in terms of the steric effect caused by the peri-hydrogen. (ii) The enthalpies of the hydrogen bond formation of II and III are 2·32 and 2·46 kcal/mole, respectively. (iii) Comparison of the ultraviolet spectra of these acids with their esters reveals that there is a remarkable hydrogen bond effect on the spectrum of III.

The ultra-violet spectra of methyl 2-hydroxy-1-, 1-hydroxy-2- and 3-hydroxy-2-naphthoates were also determined and discussed from the view point of the pseudo-aromatic nature of the chelate ring formed by the intramolecular hydrogen bond.