Oxidation of benzyl alcohol by pyridinium dichromate in acetonitrile. Using the paralmeta ratio of substituent effects for mechanism elucidation

Article abstract of DOI:10.1039/b200742h

Rate constants were measured for the oxidation reaction of benzyl alcohol and twenty-five ortho-, recta- and para-monosubstituted derivatives in the temperature range 293-323 K at intervals of 10 K. The kinetics were followed spectrophotometrically in dry acetonitrile acidified with trichloroacetic acid (TCA) using pyridinium dichromate (PDC) as oxidising agent under pseudo-first-order conditions with respect to PDC. Benzaldehyde is the only oxidation product and no reaction takes place without TCA. From good linear Eyring plots activation enthalpies Δ^?H° and entropies Δ^?S° are calculated. For ortho-substituted benzyl alcohols high Δ^?H° values and small negative Δ^?S° values point to an ortho effect on the rate-determining step. Using the tetralinear approach to substituent effects, the average value λ = 1.09 ± 0.05 for the para/meta ratio of inductive or Electra effects is obtained and negative Hammett reaction constants decreasing in magnitude with increasing temperature are found. A mechanism implicating the prior acid-catalysed formation of neutral benzyl hydrogen dichromate ester followed by intramolecular proton transfer is proposed. Modelling of parameter λ in terms of the electrostatic theory showed its experimental value to be consistent with the ratio of electric potentials generated in the immediate vicinity of the nearest chromium atom by dipolar substituents introduced in the aromatic ring on para and meta positions. At a molecular level the oxidative, rate-determining step is suggested to be triggered by the retraction or shrinkage of electron pairs from sigma bonds in Cr₂^VI species to non-bonding orbitals in unstable Cr^IV-O-Cr^VI species. In contrast with past interpretations, an electrochemical approach is used to explain negative values for the Hammett reaction constant.

Full text of DOI:10.1039/b200742h

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Oxidation of benzyl alcohol by pyridinium dichromate in
acetonitrile. Using the para/meta ratio of substituent
effects for mechanism elucidation†
2
S. Kabilan,*a R. Girija,a João Carlos R. Reis,*b Manuel A. P. Seguradoc and
Jaime D. Gomes de Oliveirad
a Department of Chemistry, Annamalai University, Annamalai Nagar 608002, India.
E-mail: kabilan@satyam.net.in
b Departamento de Química e Bioquímica, Centro de Electroquímica e Cinética, Faculdade
de Ciências, Universidade de Lisboa, 1749-016 Lisboa, Portugal. E-mail: jreis@fc.ul.pt
c Faculdade de Farmácia and Centro de Electroquímica e Cinética, Universidade de Lisboa,
1649-003 Lisboa, Portugal. E-mail: asegurado@ff.ul.pt
d Instituto Superior de Engenharia de Lisboa, Centro de Electroquímica e Cinética,
1949-014 Lisboa, Portugal
Received (in Cambridge, UK) 21st January 2002, Accepted 3rd April 2002
First published as an Advance Article on the web 16th April 2002
Rate constants were measured for the oxidation reaction of benzyl alcohol and twenty-five ortho-, meta- and para-
monosubstituted derivatives in the temperature range 293–323 K at intervals of 10 K. The kinetics were followed
spectrophotometrically in dry acetonitrile acidified with trichloroacetic acid (TCA) using pyridinium dichromate
(PDC) as oxidising agent under pseudo-first-order conditions with respect to PDC. Benzaldehyde is the only
oxidation product and no reaction takes place without TCA. From good linear Eyring plots activation enthalpies
∆‡HЊ and entropies ∆‡SЊ are calculated. For ortho-substituted benzyl alcohols high ∆‡HЊ values and small negative
∆‡SЊ values point to an ortho effect on the rate-determining step. Using the tetralinear approach to substituent
¯
effects, the average value λ = 1.09 0.05 for the para/meta ratio of inductive or Electra effects is obtained and
negative Hammett reaction constants decreasing in magnitude with increasing temperature are found. A mechanism
implicating the prior acid-catalysed formation of neutral benzyl hydrogen dichromate ester followed by
intramolecular proton transfer is proposed. Modelling of parameter λ in terms of the electrostatic theory showed its
experimental value to be consistent with the ratio of electric potentials generated in the immediate vicinity of the
nearest chromium atom by dipolar substituents introduced in the aromatic ring on para and meta positions. At a
molecular level the oxidative, rate-determining step is suggested to be triggered by the retraction or shrinkage of
electron pairs from sigma bonds in Cr2VI species to non-bonding orbitals in unstable CrIV–O–CrVI species. In contrast
with past interpretations, an electrochemical approach is used to explain negative values for the Hammett reaction
constant.
Substituent effects can be used to probe the structure of
Introduction
activated complexes. An interesting approach is based on the
Substituent effects on the kinetics of organic reactions in
solution are useful tools to probe reaction mechanisms. These
determination of cross-interaction constants7 from which dis-
tances between substituents in different moieties of activated
tools have recently been applied to study the oxidation of
complexes may be estimated.8 However, we use a novel method9
different organic molecules in aqueous1 and non-aqueous2
for analysing substituent effects. In the latter method the para-
solvents.
meter λ giving para/meta ratios10 of substituent inductive or
An important transformation in organic synthesis is the
Electra11 effects is determined using a constrained tetralinear
formation of carbonyl groups using alcohol molecules as start-
equation.12 Experimental λ values are then modelled9 in terms
ing material.3 The kinetics of oxidation of benzyl alcohols
of the electrostatic theory of substituent effects.
In this work we measure the effect of eight different sub-
stituents from meta, para or ortho positions on the oxidation
rate of benzyl alcohol with pyridinium dichromate in
by various oxidants has also been studied,1b,4 but not using
pyridinium dichromate (PDC) as oxidising agent. The use of
PDC as a mild oxidising agent of alcohols in aprotic media was
established by Corey and Schmidt.5 Under these conditions the
anhydrous acetonitrile acidified with trichloroacetic acid. We
mechanism for converting primary and secondary alcohols to,
report interesting insights on the mode of interaction between
respectively, aldehydes and ketones is believed to involve the
polar substituents and reaction site in these oxidation reactions.
formation of an ester intermediate.6 In particular, Sharma and
We collect evidence showing that (i) this reaction is acid-
co-workers6b gathered compelling kinetic evidence on the form-
catalysed, (ii) the oxidative, rate-determining step is a concerted
ation of an inorganic ester in the oxidation of pentan-2-ol by
process triggered by the shrinkage of a pair of electrons from
different chlorochromates, which are chromium() reagents.
a sigma bond into a non-bonding chromium orbital, (iii)
this process involves an intramolecular proton transfer in the
dichromate ester with formation of benzaldehyde, and (iv) the
acceleration effect of dipolar electron-donor substituents from
the meta and para positions is due to increased electric potential
† Electronic supplementary information (ESI) available: Table S1 con-
taining kinetic data for establishing rate laws. See http://www.rsc.org/
suppdata/p2/b2/b200742h/
DOI: 10.1039/b200742h
J. Chem. Soc., Perkin Trans. 2, 2002, 1151–1157
This journal is © The Royal Society of Chemistry 2002
1151

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in the immediate vicinity of chromium atoms undergoing
reduction.
Eqn. (6) is written in a form that suggests chemical
kinetics of first order with respect to PDC, BnOH and TCA,
as experimentally found.
Results and discussion
HCr2O7Ϫ ϩ PhCH2OH ϩ CCl3CO2H
Kinetic analysis
PhCHO ϩ (H2Cr2O6) ϩ CCl3CO2Ϫ ϩ H2O (6)
The oxidation of twenty six benzyl alcohols (BnOH) by pyr-
idinium dichromate (PDC) in the presence of trichloroacetic
acid (TCA) is studied in acetonitrile at four temperatures in the
range 293–323 K. The rate of disappearance of Cr2VI species is
followed spectrophotometrically, from which experimental
first-order rate constants kobs are obtained.
Kinetic measurements for establishing rate laws were carried
out at 303 K. Rate constants were determined in pseudo-first-
order conditions with respect to PDC, using different initial
concentrations of PDC, BnOH and TCA. The results from
exploratory experiments, which are summarised in Table S1,
indicate the reaction to be of first order with respect to each of
these reagents. Chemical analysis shows that benzaldehyde is
the only product emerging from the oxidation of benzyl alco-
hol. No reaction is observed in the absence of TCA, pointing to
a catalytic action by TCA.
Since direct oxidation of BnOH by PDC is unlikely, on the
basis of the above chemical evidence we are led to postulate a
mechanism involving the acid-catalysed formation of an ester
intermediate. However, to avoid undue complication we before-
hand establish which is the main Cr2VI species formed from the
complete dissociation of PDC in acidic acetonitrile solution.
The dichromic acid is a diprotic acid of general formula H2A,
its first and second acidity constants being denoted by K1 and
K2, respectively. Material balance for this acid is expressed by cT
= [H2A] ϩ [HAϪ] ϩ [A2Ϫ] so that the fraction of each species is
given by α0 = [H2A]/cT, α1 = [HAϪ]/cT and α2 = [A2Ϫ]/cT. Then, in
a solution having proton activity h, equilibrium considerations
lead to the well-known results in eqn. (1)–(3).
At this stage we observe that no detailed information is
available on the structure of the activated complex. In the
absence of a firm proposal on the mechanism and since in
the kinetic experiments reported here a large excess of substrate
over oxidant is used, we calculate experimental second-order
rate constants k2 using eqn. (7), where [BnOH]0 stands for the
initially fixed common concentration of each benzyl alcohol in
all kinetic runs.
k2 = kobs/[BnOH]0
(7)
Rate constants and activation parameters
Rate constants k2 for the present reactions are shown in Table 1,
where benzyl alcohols are classified according to the special10
or normal14 nature of the ring substituent. As substituents we
chose dipolar groups in which the atom linked to the benzene
nucleus has a full octet; but only in special substituents does it
bear a lone pair of electrons. We note a marked substituent
effect on oxidation rates, which are slowed by strong electron-
acceptor groups and by any kind of substituent in the ortho
position.
The kinetic data for each benzyl alcohol are fitted by a
least-squares method to the Eyring equation; eqn. (8).
ln (k2/T ) = a ϩ b/T
(8)
Good linearity is observed in all cases, individual correlation
coefficients being in the range 0.997–1.000. From the regression
coefficients in eqn. (8), enthalpies of activation ∆‡HЊ, entropies
of activation ∆‡SЊ and their standard deviations are calculated.
These quantities are also shown in Table 1, from which is
perceived a general trend associating low activation enthalpies
with large negative activation entropies. To explore this trend,
Exner’s plots15 for the possible verification of an isokinetic
relationship (IKR) in the meta and para series were drawn
for the kinetic data in Table 1. The corresponding statistical
analysis16 using the F-test failed to corroborate the IKR.
Although series of similar reactions sharing a common mech-
anism usually obey the IKR,15,16 there are instances in which
great structural variation within a given reaction series causes
IKR failure.17 However, changing the substituent in the meta or
para position cannot be regarded as an important structural
change. We are thus led to ascribe the poor validation of IKR
to the consequences of a complex, multi-step reaction mechan-
ism. Indeed, experimental rate constants k2 are likely to include
contributions from equilibrium quotient K for eqn.(4), which in
turn also vary with temperature and substituent. To this end we
examine the relation between k in eqn. (5) and k2 in eqn. (7).
Given the meaning of kobs, eqn. (9) is obtained from eqn. (7).
α0 = h2/(h2 ϩ K1h ϩ K1K2)
α1 = K1h/(h2 ϩ K1h ϩ K1K2)
α2 = K1K2/(h2 ϩ K1h ϩ K1K2)
(1)
(2)
(3)
Using the similar aqueous chromic acid as a rough guide, for
which pK1 = Ϫ0.2 and pK2 = 6.51 at 25 ЊC, at a relatively high
experimental concentration of trichloroacetic acid (68 mmol
dmϪ3) we estimate that hydrogen dichromate anions account
for over 95% of the Cr2VI species in acetonitrile solution. There-
fore, in the mechanistic analysis developed below, the species
Ϫ
HCr2O7 is assumed to be the only oxidising agent present in
the reacting solution.
For the first step of the oxidation mechanism we take the
acid-catalysed formation of the neutral ester intermediate;
eqn. (4).
(4)
In the present reacting mixture, no fairly strong external
nucleophile can be envisaged in appreciable concentration.
Hence, for the second, oxidative step, which is also the rate-
determining step, we tentatively propose an intramolecular pro-
ton transfer in the dichromate ester, leading to the formation of
benzaldehyde and unstable CrIV–O–CrVI species; eqn. (5).
(9)
Since Cr2VI species are exclusively consumed in the oxidative
step [cf. eqn. (5)], then eqn. (10) holds and eqn. (11) is also
obtained.
(5)
Ϫd[Cr2VI]/dt = k[PhCH2OCr2O6H]
(10)
(11)
Then the chemical equation for the overall oxidation reac-
tion is given by eqn. (6), which results from summing eqn. (4)
and (5).
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J. Chem. Soc., Perkin Trans. 2, 2002, 1151–1157

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Table 1 Second-order rate constants k2 at different temperatures, activation enthalpies ∆‡HЊ and activation entropies ∆‡SЊ for the oxidation of
twenty-six benzyl alcohols XPhCH2OH (BnOH) using pyridinium dichromate (PDC) in acetonitrile acidified with trichloroacetic acid (TCA)a
k2/10Ϫ3 dm3 molϪ1 sϪ1
Substituent
σЊb
293.15 K
303.15 K
313.15 K
323.15 K
∆‡HЊ/kJ molϪ1
Ϫ∆‡SЊ/J KϪ1 molϪ1
Special
m-OMe
m-F
m-Cl
m-Br
m-I
p-OMe
p-F
p-Cl
p-Br
p-I
o-OMe
o-F
o-Cl
o-Br
o-I
0.102
0.335
0.365
0.369
0.343
Ϫ0.120
0.151
0.242
0.265
0.277
—
86.7
10.3
11.7
12.7
14.0
263
13.7
10.3
11.0
26.0
50.0
7.50
6.20
5.90
7.00
127
187
280
28.2 0.9
55.9 1.0
49.0 0.9
46.5 1.0
43.1 0.7
24.0 1.3
41.7 0.4
49.9 0.5
47.3 2.4
32.2 0.5
52.7 2.6
64.7 1.8
66.9 0.3
66.2 1.4
65.1 1.1
169
92
3
3
3
3
2
4
1
2
8
2
8
6
1
5
4
21.7
22.7
23.7
25.3
367
24.3
20.7
19.4
41.7
100
19.0
16.0
14.0
18.0
46.8
43.3
43.9
45.0
500
43.3
40.0
37.2
63.3
190
41.0
38.0
36.0
40.0
95.1
83.4
82.3
80.0
733
73.3
76.7
73.3
98.4
420
115
123
133
174
138
113
121
165
90
65
59
62
64
—
—
—
—
100
88.0
79.0
94.0
Normal
H
m-Me
m-CO2H
m-NO2
p-Me
p-CO2H
p-NO2
o-Me
0
70.7
107
12.7
0.225
113
3.67
0.150
43.0
37.0
2.80
0.140
125
146
273
201
40.0
2.17
237
18.0
1.06
180
170
15.0
1.20
497
274
76.7
6.37
367
43.3
3.28
370
340
39.0
2.78
49.6 2.6
22.2 0.4
44.4 1.9
85.3 1.2
27.9 1.4
61.8 2.2
78.3 4.1
53.9 1.1
56.0 0.9
66.4 3.2
77.7 4.0
98
188
130
24
168
81
52 13
87
81
9
1
6
4
5
7
Ϫ0.062
0.356
0.713
Ϫ0.135
0.440
0.814
—
22.6
0.697
167
8.33
0.380
88.4
78.0
6.30
0.350
4
3
o-Et
o-CO2H
o-NO2
—
—
—
68 10
54 13
a [BnOH]0 = 30 mmol dmϪ3, [PDC]0 = 1.0 mmol dmϪ3 and [TCA]0 = 68 mmol dmϪ3 b Unified sigma-zero substituent constants taken from ref. 13.
.
Next we define quotient Q = [CCl3CO2Ϫ]/[CCl3CO2H], which
is a constant at fixed temperature. In these terms from the
equilibrium quotient K for eqn. (4) follows an expression for the
ester concentration; eqn. (12).
the attack on the substrate is sterically hindered. However, on
close inspection we observe that these activation enthalpies
span over a small range of energy values. Most noteworthy are
the values for the halogen family, which are almost insensitive
to the wide difference in size of their elements. No doubt the
ortho substitution will entail a less energetically favourable
route to be followed, but will still be consistent with the pro-
posed unimolecular decomposition of the ester in the oxidative
step.
(12)
From stoichiometric analysis it is clear that [H2O] = [BnOH]0
Ϫ [BnOH], and from our initial simplifying assumption it is
also clear that [Cr2VI] = [PhCH2OCr2O6H] ϩ [HCr2O7Ϫ]. Hence
combining these results with eqn. (11) and (12) yields the inter-
esting equation, eqn. (13).
Analysis of meta and para substituent effects
Preliminary analysis of rate constants in Table 1 revealed that
substituent effects in the meta and para series are best described
using σЊ constants. The suggestion is that in activated com-
plexes the reaction centre be effectively insulated against
through-resonance interactions from substituents.
(13)
We apply the constrained tetralinear approach12 to substit-
uent effects in benzene derivatives. In the correlation analysis
of a given reaction series, this method uses four regression
straight lines, each assigned to one of the following subsets:
meta normal, para normal, meta special and para special sub-
stituents. We note that a total of eight independent parameters
would be required for an unconstrained fit of the data.
However, the use of a hyperbolic model for the meta–para inter-
relationship10 has allowed reduction of the number of adjust-
able parameters to four in the case of the Hammett equation12a
and to five in the case of the Yukawa–Tsuno equation.12b In
the context of the tetralinear method reaction series are con-
veniently described using the parameters λ and γ. The former
yields experimental values for the para/meta ratio of the
substituent inductive or non-mesomeric effect, which has been
proposed9,11 to be named the Electra effect. The parameter γ is
a sort of absolute Hammett reaction constant.12 The observ-
ation is made that the present analysis is based on a large
number of different substituents distributed over the four
This equation nicely shows that k2 is not a pure rate constant.
Indeed, it contains contributions from the temperature depend-
ent quantity Q and from the temperature and substituent
dependent quantity K. On the other hand, it is also true that in
kinetic experiments under pseudo-first-order conditions with
respect to PDC the last term in eqn. (13) becomes close to zero.
Under these circumstances the pure first-order rate constant
k for the oxidative step is approximately proportional to the
measured k2, the proportionality constant being then independ-
ent of the substituent nature.
We turn attention to the analysis of activation enthalpies in
the ortho series (Table 1). Except for the nitro group, in each of
the other seven cases the highest activation enthalpy is associ-
ated with ortho substitution. Since the chemistry of the nitro
group as a substituent is reviewed by Exner and Krygowski,18
we do not discuss this substituent further. The trend noted
above for activation enthalpies is normally regarded as support-
ing evidence for a bimolecular rate-determining step in which
J. Chem. Soc., Perkin Trans. 2, 2002, 1151–1157
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Table 2 Tetralinear analysis of the oxidation rates of meta and
para monosubstituted benzyl alcohols in acetonitrile at different
temperaturesa
Indeed, this would lie on the outskirts of the ester molecule.
Further, this distance increases with increasing length of the
dipolar reaction centre as well as for most other angular orient-
ations. Next we examined the case of dipole–charge inter-
actions. We found that the value of 1.09 for the para/meta ratio
of electrostatic interaction energies is obtained when an electric
charge is placed at 0.36 nm from the aromatic ring on its 1,4
axis (Fig. 1). In this case the calculated distance is encourag-
ing because it corresponds to the immediate vicinity of the
chromium atom undergoing reduction. However, it also poses
the embarrassing question of the origin and sign of that charge.
To find the way out of this dilemma we need to break with
common wisdom. We refer here to the well-established theory
of polar effects on the rates of reactions for which the tran-
sition state formation is accompanied by great redistribution of
electric charges.
Now we are dealing with a redox process in which a trans-
ition element changes its oxidation state. Therefore, by drawing
an analogy with the influence of the electric potential on the
rates of electrode reactions,19 we can develop a different
interpretation of polar effects. To this end we return to basic
electrostatic theory9 and note that the electric potential V
created at a given point by a dipole composed of symmetrical
charges is the sum of the potentials originated separately by the
charges Q and ϪQ, being described by eqn. (14).
Temperature/K
Parameter
293.15
303.15
313.15
323.15
λ
γ
1.10 0.14
0.62 0.15
1.12 0.14
0.58 0.09
1.09 0.13
0.55 0.08
1.05 0.17
0.51 0.09
γ/γЊ
Ϫ2.76
0.28
Ϫ2.56
0.25
Ϫ2.45
0.27
Ϫ2.25
0.29
sb
a Data from Table 1 in the form log (k2, X/k2, H) fitted to the constrained
tetralinear equation12 using σЊ constants.13 b Standard deviation of the fit.
subsets described above. Indeed, this is the cost for obtaining
reliable experimental estimates of parameters λ and γ.
Using a previously described non-linear least-squares com-
puting program,12 values of log (k2,X/k2,H) are correlated with σЊ
substituent constants13 in terms of the constrained tetralinear
equation.12 This equation, in the version with four fitting
parameters, yields the values shown in Table 2, the associated
standard deviations being given by a Monte Carlo procedure.9,12b
For the reference ionisation process in solution used to build
the σЊ scale,10 γЊ = Ϫ0.225. Therefore, in the framework of the
tetralinear approach γ/γЊ is the equivalent of Hammett reaction
constant ρ.12 From Table 2 we observe that the negative γ/γЊ
decreases in magnitude with increasing temperature.
(14)
Parameter ꢀ and mechanism
In this equation ε is the medium electric permittivity and
the interaction distances rϩ and rϪ refer to the corresponding
point charges in the dipole. In these terms the ratio of electric
potentials generated by a given substituent from the para and
from the meta positions is expressed by eqn. (15).
The Electra effect does not depend on temperature, at least over
a modest range of 40 K. Likewise no temperature effect on
experimental λ values can be inferred from Table 2. Hence our
discussion will be based on the average value for the experi-
mental para/meta ratio of the Electra substituent effect, which
¯
is found to be λ = 1.09 0.05.
The parameter λ has been successfully modelled9,12 in terms
of the electrostatic theory. Here we use this modelling in
an attempt to elucidate the activated complex (AC) struc-
ture, which is the key to the characterisation of the reaction
mechanism.
(15)
Most appropriately, in the context of actions mediated by
changes in the electric potential we have arrived at a theoretical
expression for the parameter λ which is exactly the same as
that holding in the case of dipole–charge interactions.9 This
illuminating result puts us again on the right path.
In line with previous research on the ester mechanism for
similar oxidation reactions,4e,20,21 we favour the formation of
planar five-member cyclic ground and transition states
stabilised by intramolecular hydrogen bonds linking an oxygen
atom from the inorganic moiety to an α-hydrogen in the organic
counterpart (Fig. 2). Using structural data for the molecular
Substituent effects on reaction rates measure the changes
operated at molecular level on going from the ground state to
AC along the oxidative pathway. We are faced with the task of
finding the AC structure and mode of action for dipolar sub-
stituent effects that are simultaneously compatible with the
relatively high average value of 1.09 for the parameter λ. Using
a recent accurate procedure,9 we performed systematic calcu-
¯
lations in order to model the value λ = 1.09 for the benzyl
hydrogen dichromate molecule. We started with the case of a
dipolar reaction site being 0.1 nm in length and being localised
along the phenyl 1,4 axis. If that λ value were for dipole–dipole
interactions, then the reaction-centre middle-point would be
0.72 nm from the aromatic ring (Fig. 1), which is unrealistic.
Fig. 2 Proposed cyclic transition state in the oxidation of benzyl
alcohols by hydrogen dichromate ions.
ester (see Experimental section), in the corresponding ground
state the hydrogen bond CH ؒ ؒ ؒ O᎐Cr is estimated to be 0.20
᎐
nm in length, which is typical for weak hydrogen bonds22 and is
to be compared with the value 0.185 nm in pure liquid water.
For this molecular framework we assume free rotation about
the C–C single bond. Then in relation to the aromatic ring 1,4
axis the nearest chromium atom describes a circle on a plane
0.234 nm distant from the ring, as depicted in Fig. 3. The circle
radius is calculated to be 0.225 nm. Because the theoretical
parameter λ depends on the rotational angle ꢀ in the ester, we
Fig. 1 Models for dipole–dipole and dipole–charge interactions in
benzene derivatives yielding λ = 1.09.
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J. Chem. Soc., Perkin Trans. 2, 2002, 1151–1157

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use a previously described procedure9 to estimate an average
λe(ꢀ) value; eqn. (16).
Negative reaction constants are traditionally associated with an
¯
electron-deficient centre in transition states; a convention
originally developed from the analysis of substituent effects in
nucleophilic displacement reactions. Negative reaction con-
stants have been used by Banerji4a,f,g,23 as supporting evidence
for oxidation mechanisms involving a hydride-ion transfer in
the rate-determining step. It has also been linked to the oxid-
ative formation of a carbonyl group, which would seemingly be
favoured by electron-donating substituents.4f,21 Next we develop
further our electrochemical approach to explain the acceler-
ation effect of meta and para electron-donor substituents on the
rates of oxidation reactions occurring by the ester mechanism.
We need, however, to clarify the electronic structure of the ester.
For dichromate compounds we did not find evidence support-
ing an octahedral structure around chromium atoms, and hence
the d2sp3 hybridisation state in the latter, as assumed by Kwart
and Nickle24 for chromate esters. Instead, oxochromium
compounds should have a tetrahedral structure resulting from
the sp3 hybridisation state in chromium atoms. In addition
to forming four sigma bonds, this state leaves two unpaired d
electrons for establishing d–p pi bonds with two oxygen atoms.
We conclude that one sp3 chromium orbital is engaged in the
O–Cr sigma bond being broken in the oxidative step.
(16)
Individual λe values are calculated at different rotations ꢀ by
inserting analytically determined distances in eqn. (15). These
values, which range from 0.995 to 1.452, lead to the smooth
curve shown in Fig. 4. Their numerical integration yields the
Then the actual redox process in dichromate ester molecules
is triggered by the two electrons in the O–Cr sigma bond
retracting into a non-bonding sp3 orbital in chromium atoms.
In this process there is a change in oxidation state from  to ,
together with the corresponding loss of valence, but the hybrid-
isation state of chromium atoms remains sp3. According to our
approach, reactions initiated by the shrinkage of electron pairs
are promoted by increasing the electric potential on the sigma
bonds to be broken. Conversely, the electric potential lowering
due to electron-acceptor substituents in the organic moiety will
slow down the valence loss in the atom being reduced and hence
the oxidation rate. Therefore, the Electra effect on these oxid-
ation rates seems to be primarily an effect on the ground state,
rather than on the transition state. The mode of substituent
action described above entails the observation of negative
Hammett reaction constants, in agreement with experimental
evidence. Further support for this interpretation is obtained
from electrode kinetics. Thus the reduction of substituted benz-
aldehydes on a mercury electrode is faster the more electron
accepting the substituent.25 However, our proposal represents
a break with conventional interpretations that simply extend
to redox reactions the concepts of charge stabilisation and
destabilisation in transition states, the former leading to
increased reaction rates and the converse for the latter.
Another controversial aspect is related to the electron flow in
electrocyclic mechanisms for the unimolecular decomposition
of the ester.4a,e,6b,20,21,23,24 Stewart and Lee21 refer to this problem
as a vexing question. We can now offer a clear answer (see
Fig. 2). If the retraction of sigma electrons towards chromium
atoms initiates the electron flow, then the ensuing electron
movements are clarified. Thus, electrons forming the d–p pi
bond retract to the oxygen atom engaged in the hydrogen bond,
which in turn becomes a covalent bond. Ultimately, the
electrons liberated by this proton transfer are used to form the
carbonyl pi bond. We comment that the only long undisputed
feature in ester mechanisms has been the heterolysis of the C–H
bond.26
Fig.
3
In molecules of benzyl hydrogen dichromate, changing
conformation will affect interaction distances from meta and para
substituents to the nearest chromium atom.
Fig. 4 The para/meta ratio of electric potential effects on the nearest
chromium atom, λe, for dipolar substituents in the cyclic ester benzyl
hydrogen dichromate (see Fig. 3) as a function of the rotation ꢀ about
the benzene nucleus 1,4 axis.
¯
average value λe(ꢀ) = 1.139. In another modelling experiment,
the reaction centre was displaced to the O–Cr ester-bond
middle point. For this geometry the circle (radius, 0.187 nm) lies
on a plane at a distance of 0.218 nm, the individual λe(ꢀ) values
are in the range 1.001–1.173 and the average value λe(ꢀ) = 1.058
is obtained. Rather suitably, the experimental average value of
1.09 for the parameter λ is between these marks.
Therefore, from the modelling of para/meta ratios for
substituent Electra effects on rate constants we have confirmed
the formation of an ester intermediate. More importantly, we
obtain strong evidence supporting the view that the rate of this
oxidation reaction is influenced by changes in the electric
potential in the vicinity of the chromium atom to be reduced.
¯
Activation parameters and mechanism
In the cases of meta and para monosubstituted benzyl alcohols,
we note from Table 1 that low activation enthalpies are associ-
ated with electron-donating substituents which raise the electric
potential on the reaction site. Then the extent of bond breaking
should be small and earlier transition states are formed. If so,
incipient bond polarisation in the internal cyclic structure
would account for a lesser degree in the loss of solvation
Parameter ꢁ and substituent effect
We still need to conciliate the observed negative γ/γЊ values
(Table 2) with the likely absence of positive charge development
in the transition state.
The meaning of negative values for the Hammett reaction
constant in oxidation reactions has been a difficult matter.21
J. Chem. Soc., Perkin Trans. 2, 2002, 1151–1157
1155

View Article Online
entropy and thus for smaller negative activation entropies, as
experimentally found (Table 1). In a similar manner, electric-
potential lowering substituents raise enthalpic activation
barriers, late transition states are formed, and more significant
bond polarisation and entropy loss due to extra solvation occur.
It remains to conciliate a unimolecular rate-determining step
with the observed ortho retarding effect, which arises from the
conjunction of high activation enthalpies with small negative
activation entropies. In ground and transition states stabil-
ised by intramolecular hydrogen bonds, the ortho substitution
sterically hinders the free rotation of internal cycles in ester
molecules and activated complexes. This hindrance has been
confirmed using space-filling molecular models. Since rotation
between certain angles will jeopardise stabilisation by intra-
molecular bonding, ortho derivatives are destabilised. Interest-
ingly, this unusual ortho effect is only slightly dependent on the
substituent bulkiness. Its effect on activation entropies will be
common to each substituent. Excluding the anomalous nitro
group, for the other seven different substituents the loss in
activation entropy amounts, on average, to 46% when a given
group is moved from either para or meta positions to the ortho
position. We attribute low entropy losses on activation to
diminished solvation of both ground and transition states
resulting from bulky ortho groups.
ing procedure by being repeatedly distilled over phosphorous
pentaoxide (P2O5) until the drying agent no longer became
coloured. To remove traces of P2O5, the dried acetonitrile
was distilled over anhydrous potassium carbonate followed by
distillation without a drying agent.
Kinetic measurements
All kinetic runs were carried out in glass stoppered iodine flasks
protected from light. The temperature was kept constant to
within 0.01 K. Unless otherwise stated, reacting solutions
prepared with 1.0, 30 and 68 mmol dmϪ3 of, respectively,
pyridinium dichromate (PDC), monosubstituted benzyl
alcohol and trichloroacetic acid in acetonitrile were sampled
at different time intervals. The rate of disappearance of PDC
under pseudo-first-order conditions was followed spectro-
photometrically by monitoring the decrease in absorption of
PDC at 350 nm employing a JASCO UDIVEC 340 UV-V
spectrophotometer fitted with variable temperature control.
In terms of the Beer–Lambert law, measured absorvances At
VI
are directly proportional to concentrations in Cr2 species.
Experimental first-order rate constants kobs were calculated
from the negative slope of linear plots of ln At vs. time.
Duplicate kinetic runs were made in every case. Reported rate
constants are reproducible to within 4%.
Product analysis
Concluding remarks
After at least 70% conversion, reaction mixtures from actual
kinetic runs for benzyl alcohol and each of their twenty-five
monosubstituted derivatives were neutralised with sodium
hydrogen carbonate and extracted with chloroform. Using
IR and UV spectrophotometers, analysis of the extracts
showed the presence of benzaldehyde or the corresponding
X-substituted derivative. Analysis by TLC and HPLC con-
firmed the formation of aldehydes in quantitative yield and
did not detect the presence of corresponding carboxylic
acids. Finally, semicarbazones of the produced aldehydes were
prepared and isolated, their melting points being determined.
In summary, from the analysis of chemical data for oxidation
rates of benzyl alcohol (BnOH) using pyridinium dichromate
(PDC) in acetronitrile solution acidified with trichloroacetic
acid (TCA), we have gathered evidence that strongly supports
the following detailed mechanism proposal.
(i) The main oxidising species are hydrogen dichromate
ions (HCr2O7Ϫ), the kinetics being first order with respect to
HCr2O7Ϫ (as a representative for PDC), BnOH and TCA.
(ii) TCA is a catalyst in the formation of intermediate benzyl
hydrogen dichromate molecules, which are stabilised by intra-
molecular hydrogen bonds.
(iii) The oxidative, rate-determining step is initiated by the
change in oxidation state from  to  taking place in
chromium atoms. During this reduction chromium sp3 orbitals
engage incoming electron pairs from O–Cr sigma bonds. In the
Structural data
In X-substituted benzyl hydrogen dichromate molecules, the
average interatomic distance C–X was taken to be12a 0.153 nm.
All the other distances (nm) and angles ꢀ (Њ) are from Sutton’s
tables,27 as follows: C–C (benzene), 0.139; C1–C2, 0.152; C1–H,
ensuing electron cyclic transfer, heterolysis of Cr᎐O d–p pi
᎐
bonds occurs, protons are transferred along hydrogen bonds
from carbon to oxygen atoms, and disengaged electrons in
carbon atoms are used to form the new carbonyl pi bond from
which benzaldehyde molecules are produced. Chemical bonds
across the five-member cyclic transition states are expected to
show varying polarity differences.
(iv) Oxidation rates are increased by meta and para electron-
donor substituents because these polar groups increase the
electric potential near chromium atoms, thus facilitating their
reduction in oxidation state. Since the converse mode of action
applies to electron-acceptor substituents, this mechanism pro-
posal is consistent with the experimentally observed negative
Hammett reaction constants.
0.109; C –O, 0.147; O–Cr, 0.177; Cr᎐O, 0.163; ꢀ (benzene ring),
᎐
1
120; ꢀ (tetrahedral carbon and chromium atoms), 109.5;
ꢀ (C–O–Cr), 112; ꢀ (Cr–O–Cr), 115.
Acknowledgements
This work was supported in part by Fundação para a Ciência e
a Tecnologia (Portugal).
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