Page 163 - Mechanism and Theory in Organic Chemistry

Table 3.5 BOND DISSOCIATION ENERGIES (KCAL MOLE-^)^
Group IV Group V Group VI Group VII
NH2-H HO-H H-F
100 116 134
pH3 HS-H H-Cl
- 77b - 90 102
H,Se H-Br
- 66b 86
H,Te H-I
- 57b 7 1
a T. L. Cottrell, The Strengths of Chemical Bonds, Butterworths, London, 1954.
Average bond energy.

number, more and more strongly, so twivee_ims_ hecome more and
m compare &dhithGwithTGJZ&tkbnndedonized compounds. The
electronegativity analogy, however, clearly fails in comparisons among members
of a given group. The atoms become less electronegative as one goes down a
column of the table, but the hydrides become stronger acids. Carbon and
iodine have the same electronegativity on the Pauling scale, but the acidities of
CH, and HI differ by something approaching 60 powers of ten. The-!wing
o~laation an he&& m the bwd disakba energies& the hydrides
.. .
(Table 3.5). Thed9sreasing ele~apps~dy-~e~b~p-en-
sz&&bja-aker - bond to hydrogen.85 One may rationalize the observations in a
rough way by saying &it-=going to larger atoms with valence electrons in
higher principal quantum levels and hence farther from the nucleus, the overlap
with the orbital on the small hydrogen atom becomes less favorable and the bonds
become weaker.

It can be seen from the foregoing discussion that the interpretations of the ob-
served acidities leave something to be desired even for such a fundamental series
of compounds as the simple hydrides. The matter has been reopened in recent,
years by the development of techniques for measuring acidities in the gas phase.86
The available results reemphasize the fact, already well known from previous
work, that solvation factors have a profound influence on the course of acid-base
reactions. But the gas-phase experiments do more than this; they call into
question some of the fundamental assumptions and interpretations that haire
long been used to account for observed acidities in terms of molecular structure.
As an example, let us consider the effect on acidity of substituting one hydro-
gen of HzO by various organic groups. Table 3.6 presents the available data for
relative acidities of the simple alcohols in solution, whereas Table 3.7 shows the
relationships in the gas phase. On the basis of the solution data alone, one
would conclude that substitution by successively more bulky groups causes a
steady lowering of acidity, although the relative positions of water and methanol
..

85 For a more complete analysis, see R. P. Bell, The Proton in Chemistry, Cornell University Press,
Ithaca, N.Y., 1959, p. 90.
88 J. I. Brauman and L. K. Blair, J. Amer. Chem. Soc., 92, 5986 (1970).