OXIDATION OF SOME ␣-HYDROXY ACIDS BY 2,20-BIPYRIDINIUM CHLOROCHROMATE
253
cannot be discounted. The intermediate may well be
a chromate ester. It seems that the formation constant
of the intermediate is very small and is therefore not
reflected in the rate laws.
In all the three oxidations the polar reactions are nega-
tive. Essentially, similar mechanisms have been postu-
lated for the three reactions.
The role of Cr(II) in Cr(VI) oxidation of organic
compounds have been examined by Perez-Benito et
al. [29]. However, the reaction conditions are entirely
different and the results are not comparable with those
of the present investigation. Perez-Benito studied the
oxidation in aqueous perchloric acid in the presence of
Mn(II). Therefore, itisunlikelythatthepresentreaction
proceed through Cr(II). The formation and isolation
of Cr(IV) species has been well characterized in the
oxidation by PFC [30] and PCC [31].
The observed negative entropy of activation also
supports it. As the charge separation takes place, the
charged ends become highly solvated. This results in an
immobilization of a large number of solvent molecules,
reflected in the loss of entropy [32].
BIBLIOGRAPHY
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The mechanism depicted in Scheme 1 leads to the
following rate equation:
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1966.
Rate = k K[Hydroxy acid] [BPCC]
It can be shown that
(14)
[BPCC] = [BPCC]T/(1 + K[Hydroxy acid)] (15)
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J. E. J Phys Chem 1964, 29, 2199.
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1094.
Therefore,
d[BPCC]
kK[Hydroxy acid][BPCC]T
(1 + K[Hydroxy acid])
=
(16)
dt
Since the reaction is first order with respect to each
the hydroxy acid and BPCC, one can assume that
1
K [Hydroxy acid]. The Eq. (17) can therefore, be
written as
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d[BPCC]/dt = kK[Hydroxy acid] [BPCC] (17)
This rate equation is in accord with the experimental
results.
It is of interest to compare the mode of oxidation of
hydroxy acids by PCC [8], PFC [6], and BPCC. The
oxidation by PFC exhibited a Michaelis–Menten type
kinetics with respect to hydroxy acids, while the oxi-
dation by PCC and BPCC presented a similar kinetic
picture i.e. the first order with respect to hydroxy acids.
The rate laws, hydrogen ion dependence, and kinetic
isotope effect are similar in both the cases. In the oxi-
dations by PFC and BPCC excellent correlations were
obtained in terms of Swain’s equation with the cation-
solvating power of the solvents playing the major role.