Page 179 - Mechanism and Theory in Organic Chemistry

principle. As we have emphasized, the principle is a statement summarizing
experimental facts and cannot at present be explained in detail at the molecular
level. Yet there are general trends in properties of acids and bases that correspond
to their classification as hard and soft, and we might therefore look for a qualita-
tive theoretical explanation.
Although complete understanding can come only with full comprehension
of chemical bonding itself, it is possible to identify various factors that appear to
be of particular importance to the stability of Lewis acid-base complexes. These
factors are discussed in a number of places in the chemical literature; we shall
summarize them briefly here and refer the reader to the original papers for more
detailed discussion.124 The central theme of current thinking about the nature of
the bonding may be referred to as the ionic-covalent theory; it maintains that
hard-hard interactions involve strong ionic bonding, whereas soft-soft inter-
actions occur mainly through covalent bonding. Speaking more specifically, one
may say that the sites of interaction between a hard acid and a hard base combine
relatively large charges with small size; the result is that electrostatic (coulombic)
forces are large. A strong, highly ionic bond results. In a soft-soft interaction, on
the other hand, the easily polarized orbitals of the acid and base interact strongly
to produce a bonding orbital extending over both atoms, the electron pair is
effectively shared, and good covalent bonding results. The strengths of soft-soft
interactions are enhanced when, in addition to those electrons directly involved
in the formation of the o bond, the acceptor has unshared electrons an-
donor has low-lying vacant orbitals. These features allow covalent .rr bonding by
donation of electrons by the acid back to the base, with resulting increase in
stability of the complex. This factor is most important in compounds in which the
acid is a transition metal ion.lZ5

PROBLEMS
1. Using the pKa values in the table below, find, with the aid of Figure 3.1, (a) the
fraction of each compound protonated in 60 percent H2S04; (b) the H2S04-water
mixture required to protonate 40 percent of 4,4'-dinitrobenzophenone.
Compound pK, of Conjugate ~cid&
Diethyl ether - 3.6
Acetophenone -6.1
4,4'-Dinitrobenzophenone - 10.1
E. M. Arnett, Prog. Phys. Org. Chem., 1, 223 (1963).
A 0.01 rnolal solution of $&OH in H2S04 freezes at 10.09OC. The freezing
point of pure H2S04 is 10.36OC, and the molal freezing-point depression constant is
6.81 OC. Explain.
3. What would be the pKa of a base that was 25 percent protonated in HS0,F
containing 10 percent by weight SbF, ?

lZ4 See (a) note 110, p. 165, (b) K. S. Pitzer, J. Chem. Phys., 23, 1735 (1955); (c) K. S. Pitzer and
E. Catalano, J. Amer. Chem. Soc., 78, 4844 (1956); (d) R. S. Mulliken, J. Amer. Chem. Soc., 77, 884
( 1955). For a summary see Pearson, notes 1 17 (a) and 1 17 (c), p. 166.
12s S. Ahrland, Structure and Bonding, Vol. 1, Springer-Verlag, Berlin, 1966, p. 207.