NUCLEOPHILIC SUBSTITUTION IN RADICALS
1051
1
The value 2k10 is equal to 4 108 l mol 1 s [5],
i.e., it is lesser than 2k5 by a factor of 3.5. Therefore,
the yield of 2-propanol decreases at an alkali concen-
tration of 0.1 M and greater. Acetone is formed
mainly by concurrent reaction (11), and the yield of
acetone considerably exceeds the yield of 2-propanol.
is reaction (4). In this case, the rate of formation of
acetone and isopropyl alcohol is equal to the rate of
reaction (4), and
k4 [I]st [H2O] = (Ga + Gi)I/N 100,
(15)
where Ga and Gi are the radiation chemical yields of
acetone and 2-propanol, which are equal, respectively,
to 1.7 and 0.6 molecule/100 eV. Hence k4 0.12 l
(11)
1
mol 1 s . In alkaline medium, at a low concentration
of NaOH (0.05 M), the formation of acetone and
isopropyl alcohol will be limited by reaction (7), and
Reaction (11) is responsible for increase in the
yield of chloride ions as the alkali concentration rises.
The radiation chemical yield of Cl ions is approxi-
mately twice as high as the overall yield of active
species formed by radiolysis of water. This suggests
chain character of the process leading to formation of
chloride ions, which includes reactions (2), (7), (8),
(11), and (12).
k7 [I]st [NaOH] = (Ga + Gi)I/N 100.
(16)
From equation (16), k71is estimated at greater than
or equal to 249 l mol 1 s . According to the calcula-
tions, the nucleophilicity of hydroxide ions is by
a factor of 2000 higher than the nucleophilicity of
water molecules.
(12)
The formation of 2-propanol was also observed
in alkaline solution without irradiation [reaction (17)],
but the efficiency of this process was much lower.
Ionizing radiation gives rise to establishment of
a stationary concentration of radicals in solution,
which depends on the radiation dose. Using the
stationary concentration technique, the rate constants
for nucleophilic substitution reactions (4) and (7) can
be estimated. The stationarity condition implies that
the rates of formation (vf) and disappearance (vd) of
radicals be equal. In the case of radiation generation,
vf = GR I/N 100, where GR is the overall radiation
chemical yield of radical species formed by radiolysis
of water (it is equal to 6.1 molecules/100 eV); I is
the power of the absorbed radiation dose, equal to
(17)
The rate constant of reaction (17) (k17), calculated
from the time dependence of 2-propanol concentra-
7
1
tion, was 3.9 10 l mol 1 s . The ratio k7/k17
indicates that the efficiency of nucleophilic substitu-
tion in the radical derived from isopropyl chloride is
greater by a factor of 6.4 108 than in the isopropyl
chloride molecule.
1
2.1 1018 eV l 1 s ; and N is the Avogadro constant.
The rate of decay of radicals I is determined at
least by their recombination (13), vd = 2k13 [I]2st.
Thus, secondary radicals formed from isopropyl
chloride, as well as primary ClCH2 radicals, are
much more reactive (approximately by 9 orders of
magnitude) in nucleophilic substitution than the parent
molecules. Comparison of the obtained results with
those reported in [1, 2] suggests the absence of steric
inhibitory effect of methyl groups on nucleophilic
substitution in radicals derived from 2-chloropropane.
(13)
Then, GR I/N 100 2k13 [I]2st, and the stationary
concentration of radicals I can be calculated by the
following equation:
The experimental conditions were similar to those
described in [1]. Acetone and 2-propanol were deter-
mined by GLC using a 3-m column packed with 10%
of PEG-400 on Chromosorb W (flame-ionization
detector). The average errors in the determination of
of acetone, isopropyl alcohol, and chloride ions were
11, 9, and 12%, respectively.
8
[I]st
GR I/(N 100 2k13)
1.2 10 M. (14)
There are no data on k13; however, as a rough
approximation, 2k13 = 2k5 may be assumed. Most
probably, the rate-determining stage in the formation
of acetone and isopropyl alcohol in neutral medium
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 39 No. 7 2003
Kosobutskii
