Page 37 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

16 H CH 3 NH 2 OH CN F Cl Br I

CHAPTER 1
Chemical Bonding –14.0 –17.4
and Molecular Structure
–13.6
–13.2 –13.0
–11.8
–11.4
–9.8 –10.5
14.0
10.2
12.9 11.4 9.4
10.7 8.4
7.4
9.7 –3.6
–3.8
–3.4 –3.4
–3.1
–1.8
–0.7 –0.7
–0.1
Fig. 1.3. Ionization (IP) and electron affinity (EA) gaps in eV for neutral atoms and
radicals. Adapted from R. G. Pearson, J. Am. Chem. Soc., 110, 7684 (1988).

be as hard as possible.” Expressed in terms of reaction energetics,
E is usually
negative for
h−s+h−s h−h +s−s

The hard-hard interactions are dominated by electrostatic attraction, whereas soft-soft
interactions are dominated by mutual polarization. 26 Electronegativity and hardness
determine the extent of electron transfer between two molecular fragments in a reaction.
This can be approximated numerically by the expression

x− y

N = (1.10)
2 +
x
y
where is absolute electronegativity and is hardness for the reacting species. For
example, we can calculate the degree of electron transfer for each of the four halogen
atoms reacting with the methyl radical to form the corresponding methyl halide.
X ·+CH ·→ CH X
3
3

X x x
N CH3X
F 10 4 7 0 0 23 9 4
Cl 8 3 4 7 0 17 7 5
Br 7 6 4 2 0 14 5 8
I 6 8 3 7 0 10 4 7

26
R. G. Pearson, J. Am. Chem. Soc., 85, 3533 (l963); T. L. Ho, Hard and Soft Acids and Bases in
Organic Chemistry, Academic Press, New York, 1977; W. B. Jensen, The Lewis Acid-Base Concept,
Wiley-Interscience, New York, 1980, Chap. 8.