Page 35 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

14
D A −A
2 Covalent Polar % Covalent % Polar
CHAPTER 1 CH 3 89 8 2 525
−F 38 0 3 938 1 189 63 90 27 35 70 30
Chemical Bonding
and Molecular Structure −OH 56 1 3 500 0 987 72 95 22 70 76 24
61 1 3 071 0 749 75 45 17 23 81 19
−NH 2
89 8 2 525 0 000 89 8 0 0 100 0
−CH 3
An important qualitative result emerges from these numbers. Bond strength is
increased by electronegativity differences. This is illustrated, for example, by the
strength of the bonds of fluorine with the other second-row elements.

F F
B C N O
F F F F
F F F F F F F F
B F C F N F O F F F
146.7 130.6 60.7 39.2 37.9
Bond strength in kcal/mol

1.1.6. Polarizability, Hardness, and Softness
The interaction of valence shell electrons with the nucleus and intervening filled
shells also affects the polarizability of the valence shell electrons. Polarizability can
be described in terms of hardness and softness. A relatively large atom or ion with a
small effective nuclear charge is relatively easily distorted (polarized) by an external
charge and is called soft. A more compact electron distribution resulting from a higher
net nuclear charge and less effective screening is called hard. The hard-soft-acid-base
22
(HSAB) theory of stability and reactivity, introduced by Pearson, has been extensively
applied to qualitative reactivity trends, 23 and has been theoretically justified. 24 The
qualitative expression of HSAB is that hard-hard and soft-soft reaction partners are
preferred to hard-soft combinations. As for electronegativity, numerical scales of
hardness and softness have been devised. One definition, like the Mulliken definition
of absolute electronegativity, is based on ionization potential and electron affinity:

Hardness = = ½ IP −EA and Softness = = 1/ ∼ 2 IP −EA (1.9)

Hardness increases with electronegativity and with positive charge. Thus, for the
−
−
−
−
halogens the order is F > Cl > Br > I , and for second-row anions, F > HO >
−
−
−
−
H N > H C . For cations, hardness decreases with size and increases with positive
2
3
+
+
charge, so that H > Li > Na > K . The proton, lacking any electrons, is infinitely
+
+
hard. In solution it does not exist as an independent entity but contributes to the
hardness of some protonated species. Metal ion hardness increases with oxidation state
as the electron cloud contracts with the removal of each successive electron. All these as
22
R. G. Pearson and J. Songstad, J. Am. Chem. Soc., 89, 1827 (1967); R. G. Pearson, J. Chem. Educ., 45,
581, 643 (1968).
23 R. G. Pearson, Inorg. Chim. Acta, 240, 93 (1995).
24
P. K. Chattaraj, H. Lee, and R. G. Parr, J. Am. Chem. Soc., 113, 1855 (1991).