The Infra-Red Absorption of Carboxylic Acids in Solution I. Qualitative Features

Abstract

The absorption spectra of solutions of acetic, benzoic and trichloracetic acids in carbon tetrachloride have been studied at various concentrations and temperatures in the neighborhood of 3μ, 6μ, and 7μ. At each of these regions is a double band, one component of which is due to the monomeric, the other to the dimeric form of the molecule. The relatively small shift in each case indicates that the molecule is not radically altered on association. The absorptions in the three regions studied were due respectively to the O–H, the C=O and the C–O bonds of the COOH group. It has been possible to make a first estimate of these bond distances in the monomer and dimer, as follows (in A units): $$\begin{array}{cccc}& \mathrm{O}-\mathrm{H}& \mathrm{C}=\mathrm{O}& \mathrm{C}-\mathrm{O}\\ \mathrm{Monomer}& {\text{0.9}}_{81}& {\text{1.1}}_{95}& {\text{1.2}}_{98}\\ \mathrm{Dimer}& {\text{1.0}}_4& {\text{1.2}}_{11}& {\text{1.2}}_{85}\end{array}$$ The values for the C=O and C–O distances are in satisfactory agreement with those deduced by x‐ray methods for oxalic acid but not with those obtained by electron diffraction for formic acid. The general appearance and the temperature variation of the O–H ``association'' band in these acids appears to be very similar to that of the corresponding band in the alcohols. This is evidence in favor of Badger's explanation of the width of such bands rather than Errera's.