Chemists know that nitric and sulfuric acids are strong acids and that acetic
acid is a weak acid. They would also agree that ethanol is at best a very weak
acid. Acid strength is given directly by the energetics of deprotonation
(heterolytic bond dissociation); for example, for acetic acid:
\\[
\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H} \longrightarrow \mathrm{CH}_{3}
\mathrm{CO}_{2}^{-}+\mathrm{H}^{+}
\\] As written, this is a highly endothermic process, because not only is a
bond broken but two charged molecules are created from the neutral acid. It
occurs readily in solution only because the solvent acts to disperse charge.
Acid strength can be calculated simply as the difference in energy between the
acid and its conjugate base (the energy of the proton is 0 ). In fact, acid
strength comparisons among closely related systems, for example, carboxylic
acids, are quite well described with practical quantum chemical models. This
is consistent with the ability of the same models to correctly account for
relative base strengths (see discussion in Section \(26.8 .3)\)
Another possible measure of acid strength is the degree of positive charge on
the acidic hydrogen as measured by the electrostatic potential. It is
reasonable to expect that the more positive the potential in the vicinity of
the hydrogen, the more easily it will dissociate and the stronger the acid.
This kind of measure, were it to prove successful, offers an advantage over
the calculation of reaction energy, in that only the acid (and not the
conjugate base ) needs to be considered.
a. Obtain equilibrium geometries for nitric acid, sulfuric acid, acetic acid,
and ethanol using the HF/3-21G model, and compare electrostatic potential
maps. Be certain to choose the same (color) scale for the four acids. For
which acid is the electrostatic potential in the vicinity of (the acidic)
hydrogen most positive? For which is it least positive? Do electrostatic
potential maps provide a qualitatively correct account of the relative acid
strength of these four compounds? b. Obtain equilibrium geometries for several
of the carboxylic acids found in the following table using the HF/3-21G model
and display an electrostatic potential map for each.
"Measure" the most positive value of the electrostatic potential associated
with the acidic hydrogen in each of these compounds and plot this against
experimental \(\mathrm{p} K_{\mathrm{a}}\) (given in the preceding table). Is
there a reasonable correlation between acid strengths and electrostatic
potential at hydrogen in this closely related series of acids?
