The study of hydrogen bonding in substituted benzoic acids by ultraviolet absorption spectra

Knight, Arthur R. (1960) The study of hydrogen bonding in substituted benzoic acids by ultraviolet absorption spectra. Masters thesis, Memorial University of Newfoundland.

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Abstract

The known dependence on solute concentration of the ultraviolet absorption spectrum of benzoic acid is re-examined semi-quantitatively and a critical concentration, above which the intensity of absorption becomes independent of concentration, is found to be exhibited by this acid and its substituted analogues, and is taken as a measure of the relative strengths of the dimeric hydrogen bonds. The critical concentration is found to be unaffected by the addition of small amounts of ether to obtain solution of the acid in cyclohexane. -- The critical concentrations of a number of substituted benzoic acids are determined and the resulting conclusions as to the relative strengths of the dimeric bonds are interpreted in terms of mesomeric and inductive effects and current theories of hydrogen bonding. To explain the observed spectral effects in the m-substituted acids, it is necessary to postulate an electromeric interaction which differs from the usual inductive effect. The proposed effect is refered to as an Ie effect. -- The data show that o-methoxybenzoic acid forms a stronger intramolecular bond than does o-fluorobenzoic acid, suggesting that intramolecular hydrogen bonds of this type are not predominantly electrostatic in nature. -- Experimental methods for the spectral examination of concentrated solutions are also described.

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