Enhanced reactivity in the quinolinium fluorochromate cooxidation of some cycloalkanones and oxalic acid

Article abstract of DOI:10.1246/bcsj.74.1043

Under identical experimental conditions, the kinetics of the oxidation of oxalic acid and cycloalkanones with quinolinium fluorochromate (QFC) was examined separately and the rate coefficients evaluated. Oxalic acid reacts ca. 70-times faster than cyclohexanone. It seems that the enol form of the cycloalkanone is the active substrate. π-Complex formation has been envisaged to explain the oxidation of cycloalkanones. The formation of a 2:1 oxalic acid-QFC complex has been assumed to be the slow rate-limiting step in the oxalic acid-QFC oxidation system. The cooxidation process has been characterized by the rate equation, k_obs = k′[Oxalic acid]² + k″[Oxalic][Ket][H⁺]. The presence of oxalic acid results in the suppression of a ring-cleavage reaction, yielding cyclohexane-1,2-dione as the main product. Cr(II) involvement in the reaction sequence is ruled out. Mn(II) catalyzes the cooxidation process at its higher concentrations. The order of reactivity C₆ > C₈ > C₅ > C₇, is explained.

Full text of DOI:10.1246/bcsj.74.1043

c. Jpn.
© 2001 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 74, 1043—1049 (2001) 1043
e Chemical
ciety of Ja-
n
e Chemical
ciety of Ja-
n
Enhanced Reactivity in the Quinolinium Fluorochromate Cooxidation of
Some Cycloalkanones and Oxalic Acid
Enhanced Reactivity in Cooxidation
S. Meenakshisundaram et al.
Subbiah Meenakshisundaram,* and Ramanathan Sockalingam
01
43
49
SJA8
09-2673
333
Department of Chemistry, Annamalai University, Annamalainagar-608 002, India
(Received September 25, 2000)
ptember
Under identical experimental conditions, the kinetics of the oxidation of oxalic acid and cycloalkanones with quin-
olinium fluorochromate (QFC) was examined separately and the rate coefficients evaluated. Oxalic acid reacts ca. 70-
times faster than cyclohexanone. It seems that the enol form of the cycloalkanone is the active substrate. π-Complex
formation has been envisaged to explain the oxidation of cycloalkanones. The formation of a 2:1 oxalic acid–QFC com-
plex has been assumed to be the slow rate-limiting step in the oxalic acid–QFC oxidation system. The cooxidation pro-
cess has been characterized by the rate equation,
00
00
.1
k
obs ꢀ k′[Oxalic acid]2 ꢁ k′′[Oxalic][Ket][Hꢁ].
The presence of oxalic acid results in the suppression of a ring-cleavage reaction, yielding cyclohexane-1,2-dione as the
main product. Cr(II) involvement in the reaction sequence is ruled out. Mn(II) catalyzes the cooxidation process at its
higher concentrations. The order of reactivity C6 > C8 > C5 > C7, is explained.
isolated by filtration and dried in a vacuum for 1 h, mp 160 ˚C.
All of the other chemicals used were of AnalaR grade. The re-
actions were carried out in 50% (v/v) aqueous acetic acid. All so-
lutions were thermostated to an accuracy of ꢂ0.1 ˚C, and mea-
surements were made at four different temperatures viz., 293, 303,
313 and 323 K. The required volumes of these solutions for each
run were mixed and 2 mL aliquots of the reaction mixture were pi-
petted out at convenient time intervals and quenched in 10 mL of a
2% potassium iodide solution; the liberated iodine was titrated
against standard thiosulfate to a starch end point.
Under the maintained experimental conditions, the exact sto-
ichiometry was difficult to predict. The reaction product cyclo-
hexane-1,2-dione was identified by the formation of cyclohexane-
1,2-dione dioxime (white crystals), (yield 75–80%, mp 187 ˚C).9
The liberation of carbon dioxide during the oxidation was detected
as described by Vogel.10
Depending on the substrate and experimental conditions,
Cr(VI) functions both as a one- and a two-electron oxidant.1–5
These processes involve intermediate valence states viz., Cr(V)
and Cr(IV). The reduction of Cr(VI) directly to Cr(III) in a
single step by a “three-electron transfer” process, involving the
participation of an oxidizable bidentate ligand, has been re-
ported by Hasan and Rocek6 in the oxidation of isopropyl alco-
hol by Cr(VI) in the presence of oxalic acid. The formation of
energetically unfavorable Cr(IV) is avoided. A significant rate
acceleration was observed in the above process wherein both
substrates underwent oxidation simultaneously; this phenome-
non is called cooxidation. The general feature of Rocek’s
cooxidation mechanism involves a complex containing both
substrates, which offers the reaction a more favorable pathway
than the oxidation of a single substrate. The cooxidation pro-
cess offers the possibility of useful synthetic applications. The
chromic acid oxidation of cyclobutanol7 in the presence of ox-
alic acid yields mainly cyclobutanone with complete suppres-
sion of ring-cleavage side reactions. These interesting features
led us to examine the cooxidation process involving some cy-
cloalkanones in the presence of oxalic acid with a newly devel-
oped Cr(VI) reagent, quinolinium fluorochromate.
Results and Discussion
All oxidation experiments were carried out under pseudo-
first order conditions with at least a ten-fold excess of sub-
strates over [QFC]. The kobs values did not vary appreciably
with changing the QFC concentrations, which indicates a first-
order dependence on [QFC] (Table 1). Good straight-line log
titre vs time plots are shown in Fig. 1 (r > 0.955; s < 0.025).
The effects of cyclic ketones on the cooxidation rate at 303 K
are listed in Table 2. The Michaelis–Menten dependence on
[cyclohexanone] is depicted in Fig. 2 (r ꢀ 0.995; s ꢀ 22.07).
The same fractional order dependence on [ketone] was ob-
served for the other cycloalkanones studied. It can be seen that
the rate increases with increasing the oxalic acid concentration
(Table 3). The order dependence on [oxalic acid] was > 1 and
< 2 throughout the entire range of concentrations studied. The
Experimental
After chromium(VI) oxide (1.5 g, 0.15 mol) was dissolved in
water (25 mL) in a polyethylene beaker, 40% hydrofluoric acid
(11.3 mL, 0.23 mol) was added to it with stirring at room tempera-
ture. Within 5 min, a clear solution resulted. To this solution,
quinoline (17.7 mL, 0.15 mol) was added slowly with stirring.
The mixture was heated on a steam-bath for half an hour; then
cooled to room temperature and allowed to stand for one hour.
Bright red-orange crystalline quinolinium fluorochromate8 was

1044 Bull. Chem. Soc. Jpn., 74, No. 6 (2001)
Enhanced Reactivity in Cooxidation
Table 1.
Effect of Oxidant on the Reaction Rate at 303 K
103[QFC]
mol dmꢃ3
103 kobs
sꢃ1
Cyclopentanone
Cyclohexanone
Cycloheptanone
Cyclooctanone
0.50
1.00
1.25
1.50
2.00
2.50
—
1.02
1.19
—
1.24
1.09
1.04
—
—
0.70
0.73
0.74
0.74
—
0.66
0.65
0.63
0.64
—
0.89
0.89
0.83
0.90
—
[Cycloalkanone] ꢀ 5.01 ꢄ 10ꢃ2 mol dmꢃ3
;
[Oxalic acid] ꢀ 4.77 ꢄ 10ꢃ2 mol dmꢃ3
;
[HClO4] ꢀ 5.04 ꢄ 10ꢃ2 mol dmꢃ3; AcOH:Water ꢀ 50:50 (v/v).
Table 2.
Effect of Substrate on the Reaction Rate at 303 K
102[S]
mol dmꢃ3
103 kobs
sꢃ1
Cyclopentanone
Cyclohexanone
Cycloheptanone
Cyclooctanone
2.50
3.75
5.01
10.01
15.02
20.02
0.64
—
0.74
0.94
0.98
1.04
0.77
1.07
1.24
1.63
1.90
2.19
0.59
—
0.63
0.77
0.87
1.01
0.71
—
0.83
1.36
1.66
2.04
[Oxalic acid] ꢀ 4.77 ꢄ 10ꢃ2 mol dmꢃ3; [QFC] ꢀ 1.50 ꢄ 10ꢃ3 mol dmꢃ3
[HClO4] ꢀ 5.04 ꢄ 10ꢃ2 mol dmꢃ3; AcOH:Water ꢀ 50:50 (v/v).
;
Fig. 1.
Typical first order plots for the cooxidation of cy-
clohexanone and oxalic acid with QFC at 303 K.
102[cyclohexanone] ꢀ 5.01 mol dmꢃ3; 102[oxalic acid] ꢀ
4.77 mol dmꢃ3; 102[HClO4] ꢀ 5.04 mol dmꢃ3; AcOH:
Water ꢀ 50:50 (v/v).
Fig. 2.
Michaelis–Menten plot for the oxidation of the cy-
clohexanone on the Cooxidation Reaction at 303 K.
103[QFC] ꢀ 1.50 mol dmꢃ3; 102[oxalic acid] ꢀ 4.77 mol
dmꢃ3; 102[HClO4] ꢀ 5.04 mol dmꢃ3; AcOH:Water ꢀ
50:50 (v/v).
a, 103[QFC] ꢀ 0.5 mol dmꢃ3; b, 103[QFC] ꢀ 1.5 mol
dmꢃ3; c, 103[QFC] ꢀ 2.0 mol dmꢃ3; d, 103[QFC] ꢀ 2.5
mol dmꢃ3
.
acidity dependence in this cooxidation system was marginal,
as shown in Table 4. The ionic strength had no significant ef-
fect on the reactivity. Acrylonitrile had no perceptible effect
on the rate (Table 5). The rate increased with an increase in the
dielectric constant of the medium. A plot of log kobs vs. Dꢃ1
exhibits curvature (Fig. 3). The data in Table 6 shows that D2O
enhanced the reactivity, as expected, since D2O and D3Oꢁ are
stronger acids than H2O and H3Oꢁ, respectively.11 Quinoline
had no effect on the reactivity. Studies were carried out in so-
lutions containing an excess of [CoCl(NH3)5]2ꢁ, a specific
trapping agent for Cr(II). The rates were almost independent
of [CoIII].
The pseudo-first order rate constants for the cooxidation of
cyclohexanone in the absence of Al3ꢁ and in the presence of
0.18 mol dmꢃ3 Al3ꢁ were 1.20 and 0.20 ꢄ 10ꢃ3 ꢃ1, respec-
s
tively. Detailed kinetic studies were carried out for the cooxi-
dation process in the presence of Mn(II). The results are sum-
marized in Table 7.
The temperature dependence of the cooxidation of cycloal-
kanones is given in Table 8. The activation parameters were
evaluated using an Eyring’s plot of ln kobs/T vs. l/T (r ≈ 0.998; s
≈ 0.054).

S. Meenakshisundaram et al.
Bull. Chem. Soc. Jpn., 74, No. 6 (2001) 1045
Table 3.
Effect of Oxalic Acid on the Reaction Rate at 303 K
103 kobs
sꢃ1
102[Oxalic acid]
mol dmꢃ3
Cyclopentanone
Cyclohexanone
Cycloheptanone
Cyclooctanone
0
—
0.24
0.43
0.74
—
1.04
—
1.40
—
0.007
0.42
0.74
1.09
1.35
—
1.70
—
2.13
—
0.22
0.41
0.64
—
0.93
—
1.23
—
—
0.33
0.64
0.90
—
1.37
—
1.85
—
2.39
3.58
4.77
5.73
5.96
6.68
7.16
7.63
[Cycloalkanone] ꢀ 5.01 ꢄ 10ꢃ2 mol dmꢃ3; [QFC] ꢀ 2.00 ꢄ 10ꢃ3 mol dmꢃ3
[HClO4] ꢀ 5.04 ꢄ 10ꢃ2 mol dmꢃ3; AcOH:Water ꢀ 50:50 (v/v).
;
Table 4.
Effect of Acidity on the Reaction Rate at 303 K
102[HClO4]
mol dmꢃ3
103 kobs
sꢃ1
Cyclopentanone
Cyclohexanone
Cycloheptanone
Cyclooctanone
0
0.5
—
—
0.71
0.74
—
0.86
—
0.91
0.93
0.76
0.82
1.02
1.24
1.26
—
1.59
1.62
—
—
—
0.60
0.63
—
0.77
—
0.78
0.82
—
—
0.80
0.83
—
1.17
—
1.44
1.55
2.52
5.04
7.56
12.60
15.12
20.16
30.24
[Cycloalkanone] ꢀ 5.01 ꢄ 10ꢃ2 mol dmꢃ3
; ;
[Oxalic acid] ꢀ 4.77 ꢄ 10ꢃ2 mol dmꢃ3
[QFC] ꢀ 1.50 ꢄ 10ꢃ3 mol dmꢃ3; AcOH:Water ꢀ 50:50 (v/v).
Table 5.
Dependence of the Oxidation Rate on
Table 6.
Effect of D2O on the Reaction Rate at 303 K
[NaClO4], Solvent Composition and [Acrylonitrile] at
303 K
AcOH:Water:D2O
103 kobs
sꢃ1
1.24
1.25
1.54
1.93
2.45
%
102[NaClO4]
mol dmꢃ3
—
Da)
104[Acrylo]
mol dmꢃ3
103 kobs
sꢃ1
50:50:0
50:47.5:2.5
50:42.5:7.5
50:35:15
50:25:25
42.37
—
1.24
1.20
1.24
1.35
1.33
1.00
1.16
1.24
1.28
1.67
1.95
1.17
1.20
1.21
5.00
10.01
15.02
20.02
[Cyclohexanone] ꢀ 5.01 ꢄ 10ꢃ2 mol dmꢃ3
;
[HClO4] ꢀ 5.04 ꢄ 10ꢃ2 mol dmꢃ3; [Oxalic acid]
ꢀ 4.77 ꢄ 10ꢃ2 mol dmꢃ3; [QFC] ꢀ 1.50 ꢄ 10ꢃ3
24.29
35.14
42.37
49.60
60.46
71.31
mol dmꢃ3
.
Under identical experimental conditions, the kinetics of the
oxidation of oxalic acid and cycloalkanones with QFC was ex-
amined separately, and the rate coefficients were evaluated. At
5.04 ꢄ 10ꢃ2 mol dmꢃ3 HClO4, the values of single-substrate
oxidation rates clearly indicate that oxalic acid reacted ca. 70-
times faster than cyclohexanone (Table 9).
Cycloalkanone Oxidation. The log titre-time profile for a
typical cyclohexanone oxidation kinetic run is presented in
Fig. 4 (r > 0.991; s < 0.021). In the presence of excess [cyclo-
hexanone] the reaction showed a first-order dependence on the
concentration of the substrate. The acidity dependence was a
fractional order throughout the entire range of acidities. The
6.15
12.30
24.60
[Cyclohexanone] ꢀ 5.01 ꢄ 10ꢃ2 mol dmꢃ3; [HClO4]
ꢀ 5.04 ꢄ 10ꢃ2 mol dmꢃ3; [Oxalic acid] ꢀ 4.77 ꢄ
10ꢃ2 mol dmꢃ3
; ;
[QFC] ꢀ 1.50 ꢄ 10ꢃ3 mol dmꢃ3
AcOH:Water ꢀ 50:50 (v/v).
a) Dielectric constant values are calculated from values
of pure solvents.

1046 Bull. Chem. Soc. Jpn., 74, No. 6 (2001)
Enhanced Reactivity in Cooxidation
(2)
(3)
The conversion has been characterised by the rate equation,
ꢃd[QFC]
dt
k1KK′[Ket][Hꢁ][CrVI
1ꢁK[Hꢁ]
]
Fig. 3.
A plot of log kobs vs. 1/D at 303 K.
(4)
ꢀ
.
102[cyclohexanone] ꢀ 5.01 mol dmꢃ3; 103[QFC] ꢀ 1.50
mol dmꢃ3; 102[oxalic acid] ꢀ 4.77 mol dmꢃ3; 102[HClO4]
ꢀ 5.04 mol dmꢃ3
.
Oxalic Acid Oxidation. Semilogarithmic plots of titre vs.
time were linear, over three half-lives (r > 0.998; s < 0.012).
Nearly a second-order dependence on [oxalic acid] was very
much evidenced by a linear kobs vs. [oxalic acid]2 plot with a
zero intercept, as shown in Fig. 5 (r ꢀ 0.998; s ꢀ 0.27). No
levelling off the rate constants took place at higher substrate
concentrations. HClO4 had no effect on the reactivity. The ki-
netic data can be described by the following mechanism.
The formation of a neutral 1:1 complex (C2) between QFC
and the oxalic acid can be envisaged. The second-order depen-
dence on oxalic acid can be explained by the formation of a
2:1 oxalic acid–QFC complex (C3). An activated complex
containing two substrate molecules has already been pro-
posed.6,7,14–20 The acid decomposes C3 in a fast step to give the
products.
Fig. 4.
Typical first order plots for the oxidation of cyclo-
hexanone with QFC at 303 K.
102[cyclohexanone] ꢀ 5.01 mol dmꢃ3; 102[HClO4] ꢀ 5.04
mol dmꢃ3; AcOH:Water ꢀ 50:50 (v/v); a, 103[QFC] ꢀ
0.5 mol dmꢃ3; b, 103[QFC] ꢀ 1.0 mol dmꢃ3; c, 103[QFC]
ꢀ 1.5 mol dmꢃ3; d, 103[QFC] ꢀ 2.0 mol dmꢃ3; e
(5)
103[QFC] ꢀ 2.5 mol dmꢃ3
.
same kinetic behavior was observed in all of the cycloal-
kanones.
Protonation of the oxidant in acidic medium has already
been postulated.12 Protonation enhances the electrophilic ac-
tivity of the oxidant. It reacts with the cyclic ketone in a slow
rate-limiting step to give the products. We believe that the enol
form of the cycloalkanone would be the active substrate.
Clearly, an enol would be extremely susceptible to an attack by
the electrophilic oxidant. The formation of a π-complex has
been envisaged to explain the kinetic observations. A similar
type of π-complexes has been proposed concerning the oxida-
tion of alcohols by Heasley et al.13
(6)
k4
C3ꢁHꢁ → products,
(7)
fast
ꢃd[QFC]
ꢀ k3 [Oxalic][C2 ],
(8)
(9)
dt
[CrVI]t ꢀ [CrVI] ꢁ [C2] ꢁ [C3],
(1)
where [CrVI]t represents the total CrVI concentration.

S. Meenakshisundaram et al.
Bull. Chem. Soc. Jpn., 74, No. 6 (2001) 1047
Table 7.
Effect of Mn(II) on Reactivity at 303 K
102[Cyclohex]
mol dmꢃ3
102[Oxalic acid]
102[HClO4]
mol dmꢃ3
5.04
104[Mn2ꢁ
mol dmꢃ3
—
]
103kobs
mol dmꢃ3
4.77
sꢃ1
1.24
1.06
1.20
1.21
1.82
2.53
3.54
2.13
4.65
10.21
2.19
4.01
8.64
5.01
0.55
1.84
3.68
7.36
14.72
29.4
—
15.0
30.0
—
5.01
7.63
4.77
5.04
5.04
20.02
15.0
30.0
[QFC] ꢀ 1.50 ꢄ 10ꢃ3 mol dmꢃ3; AcOH:Water 50:50 (v/v).
Fig. 5.
Second order dependence of rate on [Oxalic acid]
Fig. 6.
A plot of kobs/[Oxalic acid] vs. [Oxalic acid] in the
cooxidation reaction at 303 K.
for the reaction of oxalic acid with QFC at 303 K.
103[QFC] ꢀ 1.50 mol dmꢃ3; 102[HClO4] ꢀ 5.04 mol
dmꢃ3; AcOH:Water ꢀ 50:50 (v/v).
102[cyclohexanone] ꢀ 5.01 mol dmꢃ3; 103[QFC] ꢀ 1.50
mol dmꢃ3; 102[HClO4] ꢀ 5.04 mol dmꢃ3; AcOH:Water ꢀ
50:50 (v/v).
[CrVI]t ꢀ [CrVI] ꢁ K2[CrVI][Oxalic]
ꢁ K3[Oxalic]2K2[CrVI]
(10)
The middle term represents the cooxidation process. The last
term is related to the oxidation of cycloalkanone alone, which
can be neglected in comparison with the other two terms.
Hence,
Neglecting the higher terms,
ꢃd[QFC]
dt
k3K2[Oxalic]2[CrVI
1ꢁK2 [Oxalic]
]
t
k
obs ꢀ k′[Oxalic]2 ꢁ k′′[Oxalic][Ket][Hꢁ].
(13)
ꢀ
(11)
Eq. 13 demands a linear relationship between kobs/[Oxalic]
vs. [Oxalic] A reasonably good line (Fig. 6) (r ꢀ 0.989; s ꢀ
0.060) supports the validity of the above assumptions. This
rate equation is consistent with the available kinetic data for
the cooxidation process.
The cooxidation mechanistic pathway closely resembles
Rocek’s mechanism proposed for the Cr(VI) cooxidation of
other substrates.6,7,18,21 A highly negative activation entropy
(Table 8) very much suggests that there is a highly ordered
transition state. Decomposition of the termolecular complex in
a slow rate-limiting step yields the products. The product anal-
ysis very much supports the three-electron oxidation–reduc-
The deduced rate equation is consistent with the experimen-
tal observations.
Cooxidation of Cycloalkanone and Oxalic Acid. The
cooxidation process can be described by the composite rate
law,
k′′′[Ket][Hꢁ]
k
obs ꢀ k′[Oxalic]2 ꢁ k′′[Oxalic][Ket][Hꢁ]ꢁ
.
(12)
1ꢁK1[Hꢁ]
The rate Eq. 12 is valid at low concentrations of oxalic acid.
The first term represents the oxidation of oxalic acid alone.

1048 Bull. Chem. Soc. Jpn., 74, No. 6 (2001)
Enhanced Reactivity in Cooxidation
Table 8.
No.
Rate Constants and Activation Parameters for the Cooxidation of Cycloalkanones and Oxalic Acid with QFC
Substrate
104 kobs/sꢃ1
∆H#
ꢃ∆S#
∆G# (303 K)
r
s
293 K
3.41
5.90
3.04
4.10
303 K
313 K
15.32
23.04
12.90
16.49
323 K
28.28
40.03
21.63
33.36
kJ molꢃ1
53.2
J Kꢃ1 molꢃ1
129.6
kJ molꢃ1
92.5
1
2
3
4
Cyclopentanone
Cyclohexanone
Cycloheptanone
Cyclooctanone
7.37
12.40
6.25
0.999
0.999
0.998
0.999
0.026
0.039
0.055
0.033
47.8
49.6
52.4
143.6
142.8
131.2
91.3
92.9
92.1
8.32
[Cycloalkanone] ꢀ 5.01 ꢄ 10ꢃ2 mol dmꢃ3; [HClO4] ꢀ 5.04 ꢄ 10ꢃ2 mol dmꢃ3; [Oxalic acid] ꢀ 4.77 ꢄ 10ꢃ2 mol dmꢃ3; [QFC]
ꢀ 1.50 ꢄ 10ꢃ3 mol dmꢃ3; AcOH:Water ꢀ 50:50 (v/v).
Table 9.
Cooxidation Rate
Comparison of Individual Substrate Rates with
tion. Cyclohexanone undergoes two-electron oxidation, yield-
ing cyclohexane-1,2-dione, and oxalic acid undergoes one-
electron oxidation. It has been reported22 that the oxidation of
cyclohexanone by pyridinium dichromate as well as quinolini-
um dichromate yields adipic acid as the main product. We
have examined23 in our laboratories the EDTA-catalyzed oxi-
dation of some cycloalkanones with QFC; the solid product
was found to be adipic acid (mp 155 ˚C). In the present inves-
tigation, the presence of oxalic acid resulted in the suppression
of a ring-cleavage reaction, yielding cyclohexane-1,2-dione as
the main product.
103 kobs/sꢃ1
Oxalic acid Cyclohexanone Total
Cooxidation of
cyclohexanone
and oxalic acid
1.24
0.506
0.007
0.513
[Cyclohexanone] ꢀ 5.01 ꢄ 10ꢃ2 mol dmꢃ3; [HClO4] ꢀ
5.04 ꢄ 10ꢃ2 mol dmꢃ3; [Oxalic acid] ꢀ 4.77 ꢄ 10ꢃ2 mol
dmꢃ3; [QFC] ꢀ 1.50 ꢄ 10ꢃ3 mol dmꢃ3; AcOH:Water ꢀ
50:50 (v/v).
There was a striking decrease in the rate of the reaction
upon the introduction of Al3ꢁ ions. This is obviously due to
the formation of a complex between Al3ꢁ ions and oxalic ac-
id.24 This observation is consistent with the assumption that
the ternary complex involving oxalic acid is an intermediate in
this type of cooxidation reaction. Co(III) will oxidize Cr(II) to
Cr(III):
(14)
Co(III) ꢁ Cr(II) → Cr(III) ꢁ Co(II).
(18)
It has been reported25 that none of the other oxidation states
of chromium will reduce ammonia-bound Co(III). In the
present study, the rates were almost independent of [CoIII].
This rules out the involvement of Cr(II) in the reaction se-
quence.
Mn(II) is known by an inhibiting effect on Cr(VI) oxida-
tions with many reductants. An inhibition effect of Mn(II) on
Cr(VI) oxidations is due to trapping of the formed Cr(IV).26
Studies carried out at low concentrations of Mn(II) (< 4.0 ꢄ
10ꢃ4 mol dmꢃ3) indicate that there is no appreciable change in
the reactivity. An enhanced reactivity was observed at higher
concentrations of Mn(II) (> 7.0 ꢄ 10ꢃ4 mol dmꢃ3). Huber and
Haight27 showed that the oxidation of oxalic acid is consider-
ably enhanced by the addition of Mn(II). In contrast to the
present study, the cooxidation of secondary alcohols by
Cr(VI)28 reveals the absence of any enhanced reactivity with
Mn(II). Catalysis of the cooxidation process by Mn(II) (Table
7) is quite likely due to the more reactive Mn(II)–oxalate spe-
cies. We examined29 the cooxidation of some substituted phe-
nols with quinolinium chlorochromate (QCC) in an acidic me-
dium. The results reveal that there is a rate acceleration of the
cooxidation process in the presence of Mn(II). This aspect is
to be probed further.
(15)
(16)
•ꢃCO2 ꢁ Cr(VI) → CO2 ꢁ Cr(V)
(17)
No polymerisation was observed with acrylonitrile. This
observation need not be taken as concrete evidence for the ab-
sence of free radicals. Also, under acidic conditions it was not
possible to detect free radicals, because they may be oxidized,
catalyzed by acid.15
There was a change in the coordination number from four in
Cr(VI) to six in stable Cr(III) product. It has been reported by

S. Meenakshisundaram et al.
Bull. Chem. Soc. Jpn., 74, No. 6 (2001) 1049
4
G. Mangalam and S. Meenakshisundaram, J. Indian Chem.
Soc., 6B(2), 77 (1991).
5
R. Asopa, A. Mathur, and K. K. Banerji, J. Chem. Res.
Symp., 1992, 152.
6
7
8
F. Hasan and J. Rocek, J. Am. Chem. Soc., 94, 3181 (1972).
F. Hasan and J. Rocek, J. Am. Chem. Soc., 96, 534 (1974).
a) V. Murugesan and A. Pandurangan, Indian J. Chem.,
31B, 377 (1992). b) Mihir K. Chaudhuri, Shiv. K. Chettri,
Synjukta Lyndem, Pradip C. Paul, and Pendyala Srinivas, Bull.
Chem. Soc. Jpn., 67, 1894 (1994).
9
A. I. Vogel, “A Text Book of Practical Organic Chemistry,”
4th ed, Longmans, London (1978), p. 441.
10 A. I. Vogel, “A Text Book of Micro and Semimicro Inor-
ganic Analysis,” Orient Longmans, London, (1967), p. 326.
11 Francis A. Carby and Richard J. Sundberg, Adv. Org.
Chem., 1977, 157.
12 Subbiah Meenakshisundaram and V. Sathiyendiran, Pol. J.
Chem., 72, 2261 (1998).
13 V. L. Heasley, T. J. Louie, D. K. Luttrull, M. D. Miller, H.
B. Moore, D. F. Nogales, A. M. Sauerbrey, A. B. Shevel, T. Y.
Shibuya, M. S. Stanley, D. F. Shellhamer, and G. E. Heasley, J.
Org. Chem., 53, 2199 (1988).
14 I. P. Dominic and J. Rocek, J. Am. Chem. Soc., 101, 6311
(1979).
15 F. Hasan and J. Rocek, J. Am. Chem. Soc., 94, 9073 (1972).
16 F. Hasan and J. Rocek, J. Org. Chem., 39, 2612 (1974).
17 M. Krumpolc and J. Rocek, J. Am. Chem. Soc., 99, 137
(1977).
18 F, Hasan and J. Rocek, J. Am. Chem. Soc., 94, 8946 (1972).
19 F. Hasan and J. Rocek, J. Am. Chem. Soc., 95, 5421 (1973).
20 F. Hasan and J. Rocek, J. Am. Chem. Soc., 97, 3762 (1975).
21 F. Hasan and J. Rocek, J. Org. Chem., 38, 3812 (1973).
22 M. Ravishankar, Ph. D. Thesis, Annamalai University
(1995).
23 V. Sathiyendiran, Ph. D. Thesis, Annamalai University
(1999).
24 S. Chandra, S. N. Shukla, and A. C. Chatterjee, Phys.
Chem. (Leipzig), 237, 137 (1968).
25 K. Swapan Chandra, E. Gelerinter, and E. S. Gould, Inorg.
Chem., 34, 4057 (1995).
26 G. P. Haight, T. J. Huang, and B. Z. Shakhashiri, J. Inorg.
Nucl. Chem., 33, 2169 (1971).
27 C. F. Huber and G. P. Haight, Jr., J. Am. Chem. Soc., 98,
4128 (1976).
28 K. Nagarajan, S. Sundaram, and N. Venkatasubramanian,
Indian J. Chem., 18A, 335 (1979).
29 Subbiah Meenakshisundaram and M. Selvaraju, Annamalai
University, to be submitted for publication.
Fig. 7.
The Exner plot (Numbered as in Table 8).
a, log kobs (303 K) vs log kobs (293 K); b, log kobs (313 K)
vs. log kobs (293 K)
102[cycloalkanone] ꢀ 5.01 mol dmꢃ3; 103[QFC] ꢀ 1.50
mol dmꢃ3; 102[oxalic acid] ꢀ 4.77 mol dmꢃ3; 102[HClO4]
ꢀ 5.04 mol dmꢃ3; AcOH:Water ꢀ 50:50 (v/v).
Adegobyega and co-workers30 concerning the oxidation of me-
thionine by Cr(VI) that the coordination of a second substrate
favors Cr(VI) shell expansion. The linear Exner plot (Fig. 7) (r
≈ 0.99; s ≈ 0.019) favors a similar mechanism in all of the cy-
cloalkanones listed.
The order of reactivity is C6 > C8 > C5 > C7. The reactivity
order clearly shows that even-numbered (C6, C8) cyclic ke-
tones react much faster than odd-numbered (C5, C7) ketones in
this cooxidation process. The higher enolisation constant of
the cyclohexanone is one of the most important factors con-
tributing towards greater reactivity of this ketone. This would
suggest that the enol form of cyclic ketone is involved in the
reaction, as the active substrate. The reactivity order of the
acid-catalyzed enolisation of cycloalkanones31 was primarily
interpreted on the basis of the difference in the steric require-
ment in the conversion of ketones to transition states having an
endocyclic unsaturated character which is highly developed.
This is highly favored for 8- and 6-membered rings compared
with 5- and 7-membered rings.
The observed reactivity of a series of common ring com-
pounds depends mainly on their conformations. An increase
in the symmetry order lowers the reactivity. Cycloheptanone
exists in the twist-chair form32 and cyclopentanone in the half
chair form (stable conformations).33 The higher reactivity of
cyclooctanone is attributed to its existence in the lower sym-
metry crown form.
30 M. Adegobyega Olatunji and G. Adefikayo Ayoka, Tetrahe-
dron, 7, 14 (1998).
31 H. Shetcher, J. Michael Collis, and R. Desby, J. Am. Chem.
Soc., 84, 2905 (1962).
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