relative to RCOOH, while resonance does not figure in either an alcohol or its
conjugate base. This traditional wisdom was apparently first questioned by Thomas
and Siggel, on the basis of ab initio calculations and photoelectron spectroscopy
[ 44 ]. They concluded that the relatively high acidity of carboxylic acids is largely
inherent in the acid itself, as a consequence of the polarization of the COOH group
caused by the electronegative carbonyl group pulling electrons from the hydrogen
atom, an electrostatic phenomenon. This idea was taken up by Streitwieser and
applied to other acids, e.g. nitric and nitrous acids, dimethyl sulfoxide and dimethyl
sulfone [ 45 ]. The results for carbonyl compounds were interpreted in accord with
another iconoclastic idea, namely that the carbonyl group is better regarded as
C+–O"than as>C¼O[ 46 ]. This polarization interpretation was arrived at largely
with the aid of atoms-in-molecules (AIM) analysis of the electron populations on
the atoms involved (Section 5.5.4), and a simpler variation of AIM (the projection
function difference plot) developed by Streitwieser and coworkers [ 47 ]. Work by
others also supports the view that it is “initial-state electrostatic polarization”
that is largely responsible for the acidity of several kinds of compounds, including
carboxylic acids [ 48 ]. However, Burk and Schleyer asserted that the Thomas–Siggel
method at least [ 44 ], which initiated giving credit to electrostatic destabilization of
the acid, was not valid because their “relaxation energy” term, which supposedly
measured electron delocalization or resonance, does not correspond to what
chemists normally mean by those terms [ 49 ]. Other studies, albeit with different
methodologies, nevertheless assigned major importance to electrostatic factors:
75% for CH 3 COOH, using isodesmic reactions with ab initio energies [ 50 ], and
roughly 62–65% for HCOOH, using the effect of separating the CO and OH by
–CH¼CH– groups and of rotating the CO relative to the rest of the conjugated
system, with DFT energies [ 51 ]. Around the same time as [ 50 , 51 ] lent support to
the importance of electrostatic destabilization of the acid, Exner and Cˇa ́rsky [ 52 ],
using ab initio calculations and isodesmic reactions, published a “rebuttal”, con-
tending that “In our opinion, there are no doubts that the acidity of carboxylic acids
is related to the low energy of the anion, not to a high energy of the acid molecule”,
although “the importance of resonance [in the anion] can only be estimated”, not
quantified; they conclude it is a minor factor. They also conclude, however, that “in
water” (all these publications focus on the gas phase, to pinpoint effects inherent to
the unencumbered acid) “resonance is the deciding factor.” They go on to say that
“The whole concept of resonance seems at present somewhat obsolete...”. It is
relevant to note that resonance/delocalization does not always stabilize a species
[ 53 ]. The resonance concept has been “philosophically” examined by Shaik [ 54 ].
From all this it appears that consensus has not been reached on the cause of the
enhanced acidity of carboxylic acids compared to alcohols, and one might almost
wonder if to some extent the role of electrostatics versus resonance is a metaphysi-
cal question.
9.1.3.2 Homoaromaticity
Aromaticity [ 55 ] is associated with the delocalization of (in the simplest version)p
electrons (the role of thesepelectrons in imposing symmetry on the prototypical
9.1 From the Literature 569