Chemistry Reference
In-Depth Information
right at the start by the photon, and the competition
between the various subsequent thermal pathways
can, to some extent, be controlled by varying the
temperature.
The relationship between the rate constant
k
for a
unimolecular reaction and the activation energy
E
a
is given by the well-known Arrhenius equation:
is activated thermally, the temperature has to be high
enough for the reactant molecules to surmount the
energy barrier of the first transition state. This tem-
perature, however, is much larger than is needed for
the reaction to proceed over the subsequent transi-
tion states to the products. Therefore, a higher pro-
portion of the minor product will be formed than if
the reaction could be carried out at a lower temper-
ature. If, on the other hand, the activation energy
required for the first step is supplied photochemically
(path (b) in Fig. 18.7), the reaction can be carried
out at a considerably lower temperature, thus reduc-
ing the proportion of the minor product.
Real reactions are likely to be more complicated
than the hypothetical case illustrated in Fig. 18.7. For
example, there may be several competing reactions,
not just two, and each pathway may involve more
than one intermediate. A product that can be iso-
lated at low temperatures may, when the tempera-
ture is raised, become only an intermediate on the
path to another product. In any case, studies of the
effects of temperature on product distributions in
photoreactions can yield quite dramatic results. An
example comes from the reactions of sulfuryl chlo-
ride (SO
2
Cl
2
) with aliphatic carboxylic acids, which
have been subjected to detailed studies in the
author's laboratory [10]. The thermal reaction of
propionic acid with sulfuryl chloride, initiated by
benzoyl peroxide, must be carried out at tempera-
tures above 80°C to achieve a reasonable rate of
homolysis of the initiator. Under these conditions, a
roughly equal mixture of 2- and 3-chloropropionic
acids is obtained (Fig. 18.8). In contrast, the photo-
kAe
ERT
-
=
a
where
A
is the pre-exponential factor,
R
is the
gas constant and
T
is the temperature. When there
are two competing reactions—let us say a major
pathway with Arrhenius parameters
k
(1) and
E
a
(1)
and a minor pathway with
k
(2) and
E
a
(2)—the ratio
of the yields of minor and major products is given by
k
k
()
()
=
2
1
A
A
()
()
2
1
-D
a
ERT
e
where D
E
a
=
E
a
(2) -
E
a
(1). Figure 18.6 shows how
the percentage of the minor product formed is
expected to vary with temperature in the range
-100 to +400°C for values of D
E
a
from 1.0 to
20.0 kJ mol
-1
, and on the assumption that the pre-
exponential factors for the two reactions,
A
(1) and
A
(2), are identical.
The point with regard to photochemistry is well
illustrated by the hypothetical reaction energy
profiles shown in Fig. 18.7. On the electronic
ground-state surface, the reaction proceeds via a
high-energy transition state to an intermediate from
which two reaction pathways lead to two products,
both via transition states with considerably lower
energy than the first transition state. If the reaction
Fig. 1
8
.6
Curves showing the
expected percentage of the minor
product from kinetically controlled
competition between two reaction
pathways as a function of temperature
(over the range
-
100 to
+
400°C) and
the difference in activation energies of
the two processes
D
E
a
(in the range
1-20 kJ mol
-
1
). It is assumed that the
two pathways have identical values of
the Arrhenius pre-exponential factor.
