Chlorine Substituted Acetic Acids and Salts. Effect of Salification on Chlorine-35 NQR*

The N Q R of a quadrupolar probe nucleus is often used to investigate the effect of substituent in molecules. The inductive effect, based on a partial charge migration along the molecular skeleton is the only one present in saturated aliphatics, the conjugative effect appearing in conjugated molecules, especially aromatics. As the stepwise charge migration mechanism, formerly used to explain the inductive effect, is now believed obsolete, we have wanted to reexamined the case of chlorine substituted acetic acids and salts. The data in literature was extended by observing resonances and determining N Q R frequencies in several acids and salts. The present analysis of the salification of mono-, diand tri-chloroacetic acids, which is equivalent to a deprotonat ion or the substitution of the acid hydrogen by a negative unit charge, shows that a model based on the polarization of the chlorine a tom(s) by the carboxyle group is consistent with experimental results: the polarization energy appears to be proport ional to the N Q R frequency shifts; experimental data show a correlation between the N Q R frequency shifts accompanying salification and the variations of the intrinsic acidity measured in the gas phase; this, in turn shows that there is a proportionali ty between the polarization energy and the variations in the acid free enthalpy of dissociation. From the comparison between fluorine, chlorine, bromine and iodine, it also appears that an alternative mechanism, the polarization of the carboxyl group by the halogen, would be important only in the case of the fluoroacetic acid.