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Encyclopedia of Physical Science and Technology EN012G-576 July 28, 2001 12:44

222 Physical Organic Chemistry

+
−
K a = [H ][A ]/[HA], (16)
+
−
HA + H 2 O H 3 O + A , (17)

pK a =−log K a . (18)
10
Acidity increases from left to right across the periodic
table. For example, the pK a values of CH 3 H, H 2 N H,
HO H, and F H are ∼50, 32.5, 15.7, and 3.2, respec-
tively. This is a consequence of increasing electronegativ-
−
ity, which stabilizes the anions F > OH > NH > CH .
−
−
−
2 3
Similarly, the acidity of hydrocarbons increases with
3
carbon electronegativity from ethane (CH 3 CH 3 ,sp )to
2
FIGURE 10 Effects of energy modifications on position of equi- ethene (CH 2 CH 2 ,sp ) to ethyne (HC CH, sp). Of

librium A B relative to A B: (a) A destabilized or B stabilized

course electronegativity and acidity also increase with
(b) A stabilized or B destabilized.
increasing positive charge, as for H 2 O < H 3 O + and
+
NH 3 < NH .
4
(84 + 103). Therefore more energy is released when the
products on the right are formed, meaning that this reac- B. Chemical Kinetics
tion is exothermic by 25 kcal/mole. In contrast, the corre-
Just because a reaction is exothermic or has a favorable
sponding reaction with iodine, Eq. (15 ), is endothermic

equilibrium constant does not mean that it will occur. The
by 13 kcal/mole (104 + 36 − 56 − 71):
reaction of Eq. (15) is exothermic by 25 kcal/mole and
has a very large equilibrium constant, but if the reactants
CH 3 H + Cl Cl (15) are mixed in the dark at room temperature, no reaction
CH 3 Cl + H Cl,
occurs. To understand such phenomena it is necessary to
CH 3 H + I I (15 ) study chemical kinetics, the rates of chemical reactions.

CH 3 I + H I.
For a general reaction [Eq. (19), slightly more elabo-
The modification of changing chlorine to iodine shifts the rate than Eq. (13)] the rate of reaction can be defined by
equilibrium to the left because the C I and H I bonds
any of the forms of Eq. (20). The derivatives represent
of the modified product are less stable than the C Cl and
the decreasing concentration of reactants (hence the mi-
H Cl bonds of the unmodified product.
nus signs) or the increasing concentration of products as
Other bond-dissociation data that are widely tabulated
time passes. It is often found that the rate of reaction is
are acid-dissociation constants [Eq. (16), where HA is an proportional to some power (n AB or n C ) of the concen-
acid in the reaction of Eq. (17)]. In contrast to the (ho- trations of reactants (and perhaps to the concentrations of
molytic)bonddissociationsinTableI,theseareheterolytic other species, such as catalysts), as in Eq. (21):
because when the H A bond is broken, both electrons
remain with A. Because acid-dissociation constants can A B + C → A + B C, (19)
range over many orders of magnitude, from the strongest d[AB] d[C]
20
acids, where K a ≈10 , to the weakest, where K a ≈10 −50 , rate =− dt =− dt
the data are always tabulated in terms of pK a [Eq. (18)]:
d[A] d[BC]
= = , (20)
dt dt
rate = k[A B] n AB [C] ··· . (21)
n C
TABLE I Some Important A B Bond-Dissociation Energies
(kcal/mole)
The constant of proportionality k is called the rate con-
H CH 3 OR F Cl Br I stant. It is a measure of how fast the reaction is.
H 104 104 110 135 103 87 71
CH 3 104 88 85 108 84 68 56 C. The Transition State
OR 35
In order to understand rate constants, it is necessary to
F 38
specify a measure of the progress from reactants to prod-
Cl 58
ucts. This measure is a geometric parameter called the
Br 46
reaction coordinate. It is a composite of bond distances,
I 36
chosen to increase in value from reactants to products. For