LINEAR FREE-ENERGY RELATIONSHIPS IN Cr(VI) OXIDATION OF SUBSTITUTED BENZYLAMINES
363
products of oxidation, with the parent amine [7]. Usu-
ally, oxidation may be effected in many ways to yield
different products depending upon which reactant is
used in excess [7]. However, the scope, utility, and
mechanisms of such reactions have not been system-
atically investigated; hence, we undertook the present
study.
Further, the oxidation of organic compounds in non-
aqueous and aquo-organic solvent media has revealed
the important role of nonspecific and specific solvent
effects on the reactivity. It has been shown that the
reactivity is influenced by the preferential solvation of
the reactants and/or the transition state through nonspe-
cific and specific solvent–solvent–solute interactions.
Furthermore, it has been established that the technique
of correlation analysis may well be used to separate,
quantify, and rationalize such solvent–solvent–solute
interactions on reactivity [8–12].
The Hammett equation and its modified forms [13],
all known as linear free-energy relationships (LFER),
have been found useful for correlating reaction rates
and equilibrium constants for side chain reactions for
meta- and para-substituted derivatives. The isokinetic
relationship is also an important tool for deciding the
nature of a mechanism. Keeping this in view, a system-
atic study of various LFERs and the isokinetic relation-
ship has been made to establish the role of solvent and
substituents on reactivity and to decide the nature of
the mechanism being followed in the imidazolium fluo-
rochromate (IFC, a mild and selective oxidant [14]) ox-
idation of 11 meta- and para-substituted benzylamines
in nonaqueous media.
The progress of the reactions was followed by esti-
mating the unconsumed oxidant iodometrically at 26,
34, 42, and 49 ( 0.1)◦C. The rate constants were de-
termined by the least-squares method from the linear
plots (r > 0.97) of log [IFC] versus time. Replicate
runs showed that the rate constants were reproducible
to within 3%. The solution IR experiment was done
with a horizontal attenuated total reflectance ZnSe flat
prism plate, in a JASCO FT-IR 460 Plus spectrometer.
Data Analysis
Correlation analyses were carried out using Microcal
origin (version 6) computer software. The goodness of
the fit was discussed using the correlation coefficient (r
in the case of simple linear regression and R in the case
of multiple linear regression) and standard deviation
(SD). The percentage contribution (Px) of a parameter
to the total effect on the reactivity was computed using
the regression coefficient of each parameter as reported
earlier [15].
Product Analysis
The product analysis was carried out, under kinetic
conditions, by employing GC–MS. The results reveal
that the oxidation products were benzaldehyde (m/z
106, 78) and N-benzylidene(phenyl)methanamine
(m/z 195, 165, 118, 104, 89, 77, 63). In addition to
these, as reported earlier [16], two more fractions corre-
sponding to complex intermediates were also obtained.
RESULTS AND DISCUSSION
EXPERIMENTAL SECTION
Materials
The kinetic studies were carried out under pseudo-
first-order conditions with the [substrate] > [IFC].
The first-order dependence of the reaction on IFC is
obvious from the linearity of the plots of log [IFC]
versus time. Furthermore, the pseudo-first-order rate
constants, kobs, do not depend on the initial concen-
tration of IFC (Table I). The oxidation is zero order
in the substrate, both in the presence and absence of
acid; the pseudo-first-order rate constant remains con-
stant at different [substrate]o. The oxidation of ben-
zylamine by IFC in DMF is remarkably slow but is
catalyzed in the presence of p-toluenesulfonic acid,
and the reaction proceeds at a comfortable rate. Catal-
ysis by p-toluenesulfonic acid suggests protonation of
the IFC species rather than the benzylamine molecule,
which would have resulted in retardation. Parallel ob-
servations were made in the oxidation of substituted
anilines by the same oxidant [17]. The participation of
protonated chromium species in Cr(VI) oxidations is
All the chemicals and solvents used were of analytical
grade. The solvents tert-butanol (t-BuOH), dimethyl
formamide (DMF), dimethyl sulfoxide (DMSO),
acetonitrile (MeCN), chlorobenzene (CB), acetone
(Me2CO), and ethyl acetate (EA) are of analytical
grade and were purified by conventional methods. Ben-
zylamines (Aldrich or Merck, Bangalore, India) were
used with substituents H, p-Me, p-OMe, p-Ph, p-Cl,
p-F, p-COOH, m-Me, m-F, m-Cl, and m-OMe. Solid
benzylamines were used as such, and Liquid benzy-
lamines were used after vacuum distillation. IFC was
prepared by using the reported method [14], and its
purity was checked by the iodometric method.
Kinetic Measurements
The reactions were carried out under pseudo-first-order
conditions by keeping an excess of substrate over IFC.
International Journal of Chemical Kinetics DOI 10.1002/kin