Ruthenium(III)-catalyzed mechanistic studies of oxidation of benzhydrols by sodium N-chloro-p-toluenesulfonamide in HCl medium

Article abstract of DOI:10.1002/(SICI)1097-4601(1997)29:10<773::AID-KIN6>3.0.CO;2-J

The reaction rate of the ruthenium(III)-catalyzed oxidation of benzyhydrol and its derivatives by sodium N-chloro-p-toluenesulfonamide or chloramine-T (CAT) in the presence of hydrochloric acid in 30% (v/v) MeOH medium was determined at 35 °C. The reaction rate showed a first-order dependence on [CAT]_o and a fractional-order each on [YBH]_o [Ru(III)], and [H⁺]. The reaction rate also has a negative fractional-order behavior in the reduction product of CAT and p-toluenesulfonamide.

Full text of DOI:10.1002/(SICI)1097-4601(1997)29:10<773::AID-KIN6>3.0.CO;2-J

Ruthenium(III)-Catalyzed
Mechanistic Studies of
Oxidation of Benzhydrols
by Sodium N-Chloro-p-
Toluenesulfonamide in
HCl Medium
H. RAMACHANDRA,1 D. S. MAHADEVAPPA,1 K. S. RANGAPPA,1 N. M. MADE GOWDA2
1Department of Studies in Chemistry, University of Mysore, Manasagangothri, Mysore 570 006, India
2Department of Chemistry, Western Illinois University, Macomb, Illinois 61455
Received 30 September 1996; accepted 13 March 1997
ABSTRACT: The kinetics of oxidation of benzhydrol and its p-substituted derivatives (YBH,
where Y ϭ H, Cl, Br, NO2, CH3, and OCH3) by sodium N-chloro-p-toluenesulfonamide or chlor-
amine-T (CAT), catalyzed by ruthenium(III) chloride, in the presence of hydrochloric acid in
30% (v/v) MeOH medium has been studied at 35ЊC. The reaction rate shows a first-order
dependence on [CAT]O and a fractional-order each on [YBH]O, [Ru(III)], and [Hϩ]. The reaction
also has a negative fractional-order (Ϫ0.35) behavior in the reduction product of CAT, p-
toluenesulfonamide (PTS). The increase in MeOH content of the solvent medium retards the
rate. The variation of ionic strength of the medium has negligible effect on the rate. Rate
studies in D2O medium show that the solvent isotope effect, kЈH2O/kЈD2O, is equal to 0.60.
Proton inventory studies have been made in H2O9D2O mixtures. The rates correlate satis-
factorily with Hammett ␴ relationship. The LFE relationship plot is biphasic and the reaction
constant ␳ ϭ Ϫ2.3 for electron donating groups and ␳ ϭ Ϫ 0.32 for electron withdrawing
Þ
Þ
Þ
groups at 35ЊC. Activation parameters ⌬H , ⌬S , and ⌬G have been calculated. The param-
Þ
Þ
eters, ⌬H and ⌬S , are linearly related with an isokinetic temperature ␤ ϭ 334 K indicating
enthalpy as a controlling factor. A mechanism consistent with the observed kinetics has been
proposed. ᭧ 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 773–780, 1997.
INTRODUCTION
their versatile ability to act as mild oxidants, haloge-
nating agents, and N-anions. They can behave as both
electrophiles and nucleophiles depending on the re-
action conditions. Chloramine-T or CAT ( p-Me9
C6H4SO2NClNa 1.5 H2O), which is a prominent
chlorine derivative of this class of organic haloamines
and a byproduct in saccharin manufacture, is a well-
known analytical reagent. Mechanistic aspects of
many of its reactions have been reported [1-4]. Chlor-
amine-T has been found to specifically oxidize pri-
Considerable attention has centered around the chem-
istry of N-metallo-N-haloarylsulfonamides because of
Correspondence to: N. M. Made Gowda or K. S. Rangappa
Contract grant Sponsor: University Grant’s Commission, New
Delhi
Contract grant Sponsor: Principal and the Management, the PES
College of Engineering, Mandya, Karnataka
International Journal of Chemical Kinetics, Vol. 29, 773–780 (1997)
᭧ 1997 John Wiley & Sons, Inc.
CCC 0538-8066/97/100773-08

774
RAMACHANDRA ET AL.
mary and secondary alcohols to corresponding alde-
hydes and ketones in good yields in aqueous solutions
under mild conditions [5,6]. Mild oxidation of alco-
hols to carbonyl compounds is a very important syn-
thetic tool in organic synthesis [7,8]. A significant
amount of work has been done on the mechanistic
studies of oxidation of alcohols by transition metal
ions such as chromium(VI), vanadium(V), co-
balt(III), manganese(VII), and cerium(IV) in acid
medium [9] and copper(II) [10], ruthenium tetroxide,
and ferrate(VI) ions in alkaline medium [11,12]. How-
ever, there is no report in the literature on the oxidation
kinetics of secondary aromatic alcohols by N-me-
tallo-N-haloarylsulfonamides. Preliminary experi-
ments involving several N-haloarylsulfonamides have
shown that CAT is an excellent oxidant which allows
controlled conversion of alcohols to corresponding
carbonyl products in the presence of the catalyst ru-
thenium(III) chloride in acid medium. Furthermore,
this redox system is adoptable for large scale opera-
tions. In an effort to understand the mechanism of al-
cohol-CAT redox reaction, we have studied the kinet-
ics of oxidation of six substituted benzhydrols or
diphenyl carbinols by CAT. Optimum conditions of
the reaction have been devised for the formation of
benzophenone which, in addition to being an impor-
tant constituent of perfumes, finds applications in the
manufacture of antihistamines, hypnotics, and insec-
ticides. Activation parameters have been calculated
and a Hammett-free energy relationship has been
tested. An isokinetic relationship has also been de-
duced.
Catalyst
A solution of RuCl3 (Arora Mathey, Bombay), pre-
pared in 0.50 mol dmϪ hydrochloric acid, was used
as the catalyst. Allowance was made for the amount
of acid present in the catalyst solution while preparing
the solutions for kinetic runs.
All other reagents used were of analytical grade.
Solvent isotope studies were made with D2O (99.2%)
supplied by the Bhabha Atomic Research Centre,
Bombay, India. Triple distilled water was used for pre-
paring all aqueous solutions. The ionic strength of the
reaction mixtures was kept at a constant value
(0.40 mol dmϪ ) using a concentrated solution of so-
dium perchlorate. Regression analysis of experimental
data was carried out on an EC-72 Statistical Calculator
(Electronic Corporation, India).
3
3
Kinetic Measurements
The reaction was carried out in stoppered Pyrex boil-
ing tubes whose outer surfaces were coated black to
eliminate photochemical effects. Kinetic runs were
performed under pseudo-first-order conditions with a
known excess of [YBH] over [CAT] at 35ЊC. For each
run, requisite amounts of solutions of YBH, HCl,
NaClO4, and RuCl3 were taken in the tube. Required
amounts of MeOH and water (to maintain a 30%
MeOH medium and a constant total volume) were
added. The tube was thermostated in a water bath at a
set temperature (35ЊC). A measured amount of CAT
solution, also preequilibrated at the same temperature,
was added to the above mixture to initiate the reaction.
The reaction mixture was periodically shaken for uni-
form concentration. The progress of the reaction was
monitored by withdrawing known aliquots from the
reaction mixture at regular time intervals and deter-
mining the unreacted CAT concentration iodometri-
cally. The reaction was followed for at least two
EXPERIMENTAL
Oxidant
Chloramine-T or CAT (N. R. Chem., Bombay) was
purified by the method of Morris et al. [13]. An aque-
ous solution of CAT was prepared, standardized io-
dometrically, and stored in amber colored stoppered
bottles until further use. The concentration of the stock
solution was periodically checked.
half-lives. The pseudo-first-order rate constants, kЈ or
kobs , calculated were reproducible within Ϯ3%.
1
Reaction Stoichiometry
Preliminary studies were performed with mixtures of
varying mole ratios of CAT to YBH (CAT ϾϾ YBH),
5
in the presence of 0.50 molar HCl and 4.82 ϫ 10Ϫ
Substrates (YBH)
molar RuCl3 in 30% MeOH, thermostated for 24 h or
longer at 35ЊC.
Benzhydrol, 4-chlorobenzhydrol, 4-bromobenzhydrol,
4-nitrobenzhydrol, 4-methylbenzhydrol, and 4-meth-
oxybenzhydrol (Aldrich Chemical Co., USA) were
of acceptable grades of purity and used without
further purification to prepare solutions in 30% MeOH
(v/v).
Product Analysis
The reaction products were analyzed by column chro-
matography with silica gel (60–200 mesh) using gra-

OXIDATION OF BENZHYDROLS
775
Table I Effect of Varying Reactant Concentrations on the Ratea
[CAT]0 ϫ 103 [YBH]0 ϫ 102 kЈ ϫ 104 [CAT]0 ϫ 103
[YBH]0 ϫ 102
kЈ ϫ 104
(s )
Ϫ
3
Ϫ
3
b
Ϫ
1
Ϫ
3
Ϫ
3
b
Ϫ1
(mol dm
)
(mol dm
)
(s
)
(mol dm
)
(mol dm
)
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
5.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
3.74
3.36
3.26
3.09
3.39
3.56
3.62
3.46
3.71
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
0.50
0.75
1.00
1.50
2.00
3.00
4.00
5.00
6.00
1.69
2.29
3.36
4.17
6.39
9.56
11.26
12.92
12.86
a [HCl] ϭ 0.100 mol dmϪ3, [RuCl]3 ϭ 4.82 ϫ 10Ϫ5 mol dmϪ3, ␮ ϭ 0.40 mol dmϪ3, MeOH ϭ 30% v/v, and Temp ϭ 308 K.
b YBH ϭ benzhydrol where Y ϭ H.
dient elution (dichloromethane). After initial sepa-
ration, the products were further purified by
recrystallization and characterized by comparing their
data with that of authentic samples available commer-
cially.
Benzophenone: The sample recrystallized from eth-
anol has an uncorrected m.p. of 49–50ЊC
(lit. m.p. ϭ 49–51ЊC, Merck Index 11, 1108). The Rf
value of 0.82 was obtained from TLC using dichlo-
romethane as solvent and iodine as developing agent.
Benzophenone was further identified by preparing its
2,4-dinitrophenylhydrazone (2,4-DNP) derivative
which, after recrystallization from ethanol (recovery
72.2%), was found to be identical with the 2,4-DNP
of the authentic sample with m.p. ϭ 237–238ЊC
(lit. m.p. ϭ 238) [14].
p-Toluenesulfonamide (PTS): The sample recrys-
talized from dichloromethane and petroleum ether was
found to have an uncorrected m.p. of 137–139ЊC
(lit. m.p. ϭ 137–140ЊC, Beil 11, 104). An R f value
of 0.26 was obtained from TLC using dichloro-
methane as solvent and iodine as developing agent
[6,14].
Kinetics
Effect of Reactants on the Rate: Under pseudo-first-
order conditions of [YBH] O ϾϾ [CAT]O at constant
[YBH]O , [HCl], [RuCl3 ], and temperature, plots of
log [CAT] vs. time were linear indicating a first-order
dependence of the reaction rate on [CAT]. The values
of pseudo-first-order rate constant, kЈ, are given in Ta-
1
ble I. Furthermore, the rate constant did not change
with the change in [CAT]O confirming the first-order
dependence on [CAT]O .
The rate increased initially with increase in
[YBH]O and a plot of log k1Ј vs. log [YBH]O was linear
(r ϭ 0.9989, Table I) with a fractional slope of 0.87
showing a fractional-order dependence of the rate on
[YBH]O . The rate, however, levels off at higher
[YBH]O .
Effects of HCl and Hϩ Concentrations on the Rate:
When the [HCl] was increased, keeping the other ex-
perimental conditions the same, the rate increased. A
plot of log kЈ vs. log [HCl] was linear with a fractional
1
slope of 0.87 (r ϭ 0.998, Table II). At constant [ClϪ]
of 0.40 mol dmϪ , maintained by adding NaCl, the
3
rate increased with increasing [Hϩ] which was caused
by the addition of HCl. A plot of log k1Ј vs. log [Hϩ]
was linear with a fractional slope of 0.88 (r ϭ
0.9998, Table II) indicating that the effect of HCl was
due to [Hϩ] only.
RESULTS
Effect of Ruthenium(III) Concentration: The in-
crease in [RuCl3 ], when the other conditions are kept
Reaction Stoichiometry
constant, increased the reaction rate. A plot of log kЈ
Results of preliminary experiments, performed with
varying compositions of CAT and YBH, showed that
one mole of CAT reacted with one mole of the sub-
strate.
1
vs. log [RuCl3 ] was linear (r ϭ 0.9988), with a frac-
tional slope (0.56), indicating a positive fractional-or-
der dependence on [RuCl3 ] (Table II).
Effects of Halide Ions and Ionic Strength on the
The oxidation of YBH by the haloamine, CAT, in
HCl medium in the presence of ruthenium(III) chlo-
ride catalyst follows a pseudo-first-order kinetics.
Rate: Addition of the halide ions, ClϪ or BrϪ, in the
4
3
form of their sodium salts (5.0 ϫ 10Ϫ Ϫ2.0 ϫ 10Ϫ

776
RAMACHANDRA ET AL.
Table II Effect of Varying HCl, Hϩ, and RuCl3 Concentrations on the Ratea
c
ϩ
[HCl] ϫ 10
kЈ ϫ 104
[H ]a ϫ 10
kЈ ϫ 104
[RuCl]3 ϫ 105
kЈ ϫ 104
(s )
Ϫ
3
Ϫ
1
Ϫ
3
Ϫ
1
Ϫ
3
Ϫ1
(mol dm
)
(s
)
(mol dm
)
(s
)
(mol dm
)
0.50
0.80
1.00
1.50
2.00
3.00
4.00
1.74
2.75
3.36
4.78
6.30
8.71
10.96
1.00
1.50
2.00
2.50
3.00
4.00
5.00
3.36
4.89
5.88
7.58
8.91
1.96
2.52
3.64
4.82
7.24
9.26
11.22
2.02
2.39
2.85
3.36
3.94
4.46
5.59
11.22
13.48
3
a [CAT]0 ϭ 1.00 ϫ 10Ϫ3 mol dmϪ3, [YBH]0 ϭ 1.00 ϫ 10Ϫ2 mol dmϪ where Y ϭ H, ␮ ϭ 0.40 mol dmϪ3, MeOH ϭ 30% v/v, and
Temp ϭ 308 K.
b
3
At constant [ClϪ] ϭ 0.40 mol dmϪ
.
c [HCl] ϭ 0.100 mol dmϪ
.
3
mol dmϪ3), to the reaction mixture had no effect on
the rate. Also, the ionic strength of the reaction me-
dium varied by adding NaClO4 (0.20–1.00 mol
dmϪ3) did not influence the rate.
isotope effect, kЈH2O/kЈD2O, of 0.60. The proton in-
1 1
ventory studies were made in H2O—D2O mixtures
with varying deuterium atom fraction ‘n’ in the solvent
n
(Table IV). A plot of the rate constant kobs vs. ‘n’ is
Effect of p-Toluenesulfonamide (PTS) on the Rate:
shown in Figure 1.
4
Addition of the reaction product, PTS (2.0 ϫ 10Ϫ
Ϫ
Effect of Temperature on the Rate: The reaction
was studied with varying temperature, 303 K to
318 K, keeping the other experimental conditions con-
stant. The determined rate constants are presented in
Table V. Activation parameters calculated from the
3
3
1.5 ϫ 10Ϫ mol dmϪ ), to the reaction mixture de-
creased the rate. A plot of log kЈ1 vs. log [PTS]O was
linear with a fractional slope of Ϫ0.35 (r ϭ 0.989,
Table III) indicating that PTS is involved in a fast
preequilibrium to the rds.
Arrhenius and Eyring plots of log kЈ vs. 1/T and log
1
Effect of Varying Dielectric Constant on the Rate:
The dielectric constant (D) of the solvent medium was
varied by adding MeOH (30–60%). The rate de-
creased with increase in MeOH content and gave a
(k1Ј/T ) vs. 1/T, respectively, are given in Table VI.
Test for Free Radicals: Addition of the reaction
mixture to the acrylamide monomer in the dark did
not initiate polymerization indicating the absence of
free radicals. Proper control experiments were also
simultaneously performed.
linear plot of log kЈ vs. 1/D with a negative slope
1
(r ϭ 0.999, Table III) supporting a rate-limiting step
involving partial ionization [15]. As the control ex-
periments with MeOH showed a slight decomposition
of MeOH, under the experimental conditions, the rate
constants were corrected to represent only the oxida-
tion of YBH.
DISCUSSION
Solvent Isotope Studies: As the rate was dependent
on [Hϩ], solvent isotope studies were made using D2O
as the solvent medium. The oxidation of YBH by CAT
Reaction Stoichiometry
The 1 : 1 stoichiometry of the reaction of Chlor-
amine-T (TsNClMa or CAT) with the substrate
(YBH) is as shown in eq. (1).
4
was faster in D2O medium (kЈH2O ϭ 3.36 ϫ 10Ϫ
1
1
sϪ1 and kЈD2O ϭ 5.60 ϫ 10Ϫ4 sϪ ) showing a solvent
1
Table III Effect of Varying p-Toluenesulfonamide (PTS) Concentration and Dielectric Constant on the Ratea
[PTS] ϫ 104
kЈ ϫ 104
% MeOH
(v/v)
Dielectric
Constant (D)
kЈ ϫ 104
(s )
Ϫ
3
Ϫ
1
Ϫ1
(mol dm
)
(s
)
2.0
4.0
6.0
10.0
15.0
3.07
2.54
2.18
1.86
1.54
30.0
40.0
50.0
55.0
60.0
56.73
51.08
45.30
42.66
40.40
3.36
2.56
1.92
1.51
1.27
3
3
3
a [CAT]0 ϭ 1.00 ϫ 10Ϫ3 mol dmϪ
,
[YBH]0 ϭ 1.00 ϫ 10Ϫ2 mol dmϪ where Y ϭ H, [HCl] ϭ 0.100 mol dmϪ
,
[RuCl3] ϭ 4.82 ϫ
10Ϫ mol dmϪ3, ␮ ϭ 0.40 mol dmϪ3, and Temp ϭ 308 K.
5

OXIDATION OF BENZHYDROLS
777
Table IV Proton Inventory Studies of Benzhydrols
(YBH) in H2O—D2O Mixtures at 308 Ka
Table V Temperature Dependence of the Oxidation
of Substituted Benzhydrols by CAT
Ϫ
1
kЈ ϫ 104/(s
)
Ϫ1
Atom Fraction of Deuterium (n)
kobsn ϫ 104/(s
)
Substrate (YBH)
Y ϭ
303 K
308 K
313 K
318 K
0.00
0.15
0.30
0.45
0.56
3.36
3.65
4.20
4.75
5.60
4-NO2
4-Cl
4-Br
4-H
4-CH3
4-OCH3
1.26
1.55
1.74
2.51
4.26
7.08
1.52
2.10
2.39
3.36
7.76
13.80
1.78
2.39
3.06
5.00
12.30
22.38
2.14
3.02
3.71
6.94
19.95
39.81
a [CAT]0 ϭ 1.00 ϫ 10Ϫ3 mol dmϪ
,
[YBH]0 ϭ 1.00 ϫ 10Ϫ
3
2
mol dmϪ where Y ϭ H, [HCl] ϭ 0.100 mol dmϪ
, [RuCl3] ϭ
3
3
4.82 ϫ 10Ϫ mol dmϪ3, ␮ ϭ 0.40 mol dmϪ3, MeOH ϭ 30% v/v,
5
[CAT]0 ϭ 1.00 ϫ 10Ϫ3 mol dmϪ
,
[YBH]0 ϭ 1.00 ϫ 10Ϫ
[RuCl3] ϭ 4.82 ϫ 10Ϫ
3
2
and Temp ϭ 308 K.
3
3
5
mol dmϪ
,
[HCl] ϭ 0.100 mol dmϪ
,
mol dmϪ3, ␮ ϭ 0.40 mol dmϪ3, and MeOH ϭ 30% v/v.
Y9C6H4 9CH(OH)9C6H5 ϩ TsNClNa !:
Y9C6H4 9CO9C6H5 ϩ TsNH2
ϩ Naϩ ϩ ClϪ (1)
Kd
2TsNHCl ERF TsNCl2 ϩ TsNH2;
(PTS)
K d ϭ 6.1 ϫ 10Ϫ2 at 25ЊC (4)
Kinetics and Mechanism
TsNHCl ϩ H2O EF TsNH2 ϩ HOCl
TsNHCl ϩ Hϩ EF TsNH2Clϩ
(5)
(6)
Chloramine-T acts as a mild oxidant in both acidic and
alkaline media. In general, CAT undergoes a two-elec-
tron change in its reactions. The oxidation potential of
CAT-TsNH2 couple is pH dependent and decreases
with increase in the pH of the medium (Eo is 1.14 V
at pH 0.65 and 0.50 V at pH 12). Depending on the
pH, CAT furnishes different types of reactive species
(eqs. (2)–(7) ) such as TsNHCl, TsNCl2 , HOCl, and
possibly H2OClϩ in acid solutions [5,16,17].
TsNH2Clϩ ϩ H2O EF TsNH2 ϩ H2OClϩ (7)
Since the involvement of TsNCl2 as the reactive
species predicts a second-order dependence of the rate
on [CAT]O , contrary to the experimental observation
of first-order, TsNCl2 is ruled out in the mechanism.
As the rate increases with increase in [Hϩ] and with
decrease in [PTS], eqs. (6) and (7) should play a dom-
inant role in the oxidation of YBH by CAT.
TsNClNa EF TsNClϪ ϩ Naϩ
(2)
(CAT)
Electronic spectral studies of Cady and Connick
[18], and Connick and Fine [19] have shown that
Ka
TsNClϪ ϩ Hϩ E
RF TsNHCl;
Ka ϭ 2.82 ϫ 10Ϫ5 M at 25ЊC (3)
2
the complex species such as [RuCl5(H2O)]Ϫ ,
[RuCl4(H2O)2]Ϫ, [RuCl3(H2O)3 ], [RuCl2(H2O)4 ]ϩ,
and [RuCl(H2O)5]ϩ2 do not exist in aqueous solutions
of RuCl3 . A study on the oxidation states of ruthenium
has shown that Ru(III) exists in the following equilib-
rium in acidic solutions [20–22]:
[RuCl6 ]Ϫ3 ϩ H2O EF [RuCl5(H2O)]Ϫ2 ϩ ClϪ (8)
Singh et al. [23,24] used the above equilibrium in
Ru(III)-catalyzed oxidation of primary alcohols by
bromamine-T in acid medium and ethylene glycols by
N-bromoacetamide in HClO4 medium. However, in
the present study, the zero effect of chloride ion on the
3
rate indicates that the complex ion, [RuCl6 ]Ϫ , is the
catalyzing species.
n
Figure 1 Proton inventory plot of kobs vs. the deuterium
atom fraction (n) in H2O9D2O solvent mixtures:
Ultraviolet spectral measurements showed that
benzhydrol solution has two sharp absorption bands at
255.2 and 222.4 nm while Ru(III) and CAT solutions
have bands at 204.0 and 227.2 nm, respectively, in the
presence of 0.10 molar HCl. Mixtures of (a) benzhy-
Ϫ3
Ϫ3
[CAT]O ϭ 1.00 ϫ 10 mol dm
;
[YBH]0 ϭ 1.00 ϫ
10 mol dm where Y ϭ H; [HCl] ϭ 0.100 mol dm
[RuCl3] ϭ 4.82 ϫ 10 mol dm 3; ␮ ϭ 0.40 mol dm
Ϫ2
Ϫ3
Ϫ3
;
;
Ϫ5
Ϫ
Ϫ3
MeOH ϭ 30% v/v; and Temp. ϭ 308 K.

778
RAMACHANDRA ET AL.
Table VI Activation Parameters for the Oxidation of Substituted Benzhydrols by CATa
Þ
Þ
Þ
Substrate (YBH)
Ea
(kJ mol
⌬H
⌬S
⌬G
Ϫ
1
Ϫ
1
Ϫ
Ϫ
1
Ϫ1
Y ϭ
)
(kJ mol
)
(JK 1 mol
)
(kJ mol
)
log A
4-NO2
4-Cl
4-Br
4-H
4-CH3
4-OCH3
25.8
31.6
36.3
49.7
88.9
93.9
20.4
24.1
30.8
47.2
86.6
89.9
Ϫ252.0
Ϫ236.9
Ϫ213.8
Ϫ158.0
Ϫ 23.9
Ϫ 10.4
98.5
97.8
97.3
96.2
93.9
92.5
5.8
6.9
7.8
10.3
17.2
18.1
3
3
3
3
a [CAT]0 ϭ 1.00 ϫ 10Ϫ3 mol dmϪ
,
[YBH]0 ϭ 1.00 ϫ 10Ϫ2 mol dmϪ
,
[HCl] ϭ 0.100 mol dmϪ
,
[RuCl3] ϭ 4.82 ϫ 10Ϫ5 mol dmϪ
,
␮ ϭ 0.40 mol dmϪ3, and MeOH ϭ 30% v/v.
drol and Ru(III) and (b) CAT and Ru(III) in the pres-
ence of 0.10 molar HCl showed no changes in the
k4
XЈ ϩ H2O 9
9
: products slow/rds
(iv)
␭
max , whereas CAT-YBH mixtures in the presence of
Scheme I
0.100 molar HCl exhibited sharp band at
a
216.8 nm. This suggests that a complexation occurs
only between CAT and YBH. In view of the preceding
facts, a mechanism, Scheme I, is proposed for the ox-
idation of YBH by CAT.
In Scheme I, the protonation and hydrolysis of
TsNHCl form the reactive oxidant species, (H2OCl)ϩ,
which interacts with the substrate to form complex I
(X). Complex I reacts with the active catalyst species,
K1
R
TsNHCl ϩ H3Oϩ E
F TsNH2
3
[RuCl6 ]Ϫ , in an equilibrium step to form complex II
(oxidant)
(XЈ) which hydrolyzes to form products. The frac-
ϩ (H2OCl)ϩ fast (i)
3
tional order in [RuCl6 ]Ϫ supports a fast preequili-
brium, step (iii), in the mechanism.
K2
RF
YBH ϩ (H2OCl)ϩ E
X
fast (ii)
A detailed mechanistic scheme, involving struc-
tures of the participating species including YBH,
CAT, and complexes X and XЈ, for the CAT oxidation
of benzhydrols catalyzed by RuCl3 in HCl medium is
presented in Scheme II.
(substrate)
(complex I)
K3
X ϩ [RuCl6 ]Ϫ3 E
RF
XЈ
fast
(iii)
(catalyst)
(complex II)
ϩ
ϩ
K1
H3C
SO2 NHCl ϩ H3O
H3C
SO2 NH2 ϩ H2OCl
ϩ
OH
H
O
C
Cl
ϩ
K2
Y
Y
C
H
ϩ H2O Cl
Y
ϪH2O
H
(X)
ϩ
ϩ
Ϫ3
H
O
Cl
H
O
Cl
RuCl6
K3
Ϫ3
C
H
ϩ RuCl6
Y
C
H
(X1)
k4
rds
H2O
O
C
ϩ
Ϫ
RuCl6 Ϫ3 ϩ Y
ϩ H3Oϩ ϩ H ϩ Cl
Scheme II

OXIDATION OF BENZHYDROLS
779
rate ϭ k4 [XЈ] [H2O]
CAT is given by eq. (10), the rate law (eq. (11) can
be derived.
ϭ k4Ј [XЈ], based on the rds in Scheme I (9)
[CAT]t ϭ [TsNHCl] ϩ [H2OClϩ]
ϩ[X] ϩ [XЈ] (10)
Since the total effective concentration of the oxidant
Ϫd[CAT]
K1K2K3kЈ [CAT]t [YBH] [H3Oϩ] [Ru(III)]
4
[TsNH2] ϩ K1 [H3Oϩ] ϩ K1K2[H3Oϩ] [YBH] {1 ϩ K3 [Ru(III)]}
rate ϭ
ϭ
(11)
dt
where kЈ ϭ k4 [H2O].
4
Structure-Reactivity Correlations
It is interesting to note that the rate in D2O medium
is faster than that in H2O. Since the D3Oϩ ion is a
stronger acid than H3Oϩ by a factor of 2–3, a solvent
isotope effect of this magnitude is to be expected. But
the observed inverse solvent isotope effect,
kЈD O/kHЈ O, is 1.66. This probably shows that since the
A plot of log k2 (second-order rate constant) vs. ␴
P
parameters for the ring substituents was obtained (Fig.
2). This Hammett plot shows two distinct lines for
each of which there is a good correlation between the
substituent constants and log of rate constant, partic-
ularly when the McDaniel–Brown constant, ␴P, is
used [28]. Of these, one has a much lower value for
the reaction constant (␳ ϭ Ϫ2.3) for electron-releas-
ing groups while the other line has a relatively high
value (␳ ϭ Ϫ0.32) for electron-withdrawing groups
at 35ЊC. The break in the Hammett plot could suggest
a concerted mechanism, the degree of concertedness
depending on whether the hydride transfer from
C9H bond to the oxidant is synchronous with the
removal of a proton from the O9H group of YBH
by a water molecule. Puttaswamy and Mahadevappa
[29], have studied the kinetics of oxidation of primary
alcohols by organic haloamines and noted that the
electron-donating groups tend to increase the rate. This
2
2
protonation is followed by hydrolysis in the rds which
involves O9H bond rupture, the normal kinetic iso-
tope effect kЈ /kЈ Ͼ 1 could counterbalance the sol-
H
D
vent isotope effect. The proton inventory studies made
in H2O9D2O mixtures could throw light on the na-
ture of the transition state. The dependence of the rate
constant, kobsn, on the deuterium-atom fraction ‘n’ in
the solvent mixture is given by the following form of
Gross–Butler equation [25,26]:
n
kobso/kobs ϭ ͟TS (1 Ϫ n
ϩ n ⌽i)/͟RS (1 Ϫ n ϩ n ⌽j) (12)
where ⌽i and ⌽j are isotopic fractionation factors for
isotopically exchangeable hydrogen sites in the tran-
sition state (TS) and in the ground/reactant state (RS),
respectively. The Gross–Butler equation (eq. 12) per-
mits the evaluation of ⌽i when the value of ⌽j is
known. However, the curvature of the proton inven-
tory plot could reflect the number of exchangeable
protons in the reaction [25]. A plot of kobsn vs. n (Fig.
1, Table IV) is a curve, in the present case, which in
comparison with the standard curves indicates the in-
volvement of a single proton or H9D exchange in
the reaction sequence [27]. This proton exchange in-
fers the participation of hydrogen ion in the formation
of transition state.
The moderate values of enthalpy of activation
Þ
Þ
(⌬H ), entropy of activation (⌬S ), and free energy
Þ
of activation (⌬G ), obtained for the oxidation of
benzhydrols by CAT support the reaction mechanism
(Schemes I and II) (Table VI). For each YBH reaction,
Þ
the negative ⌬S value suggests the formation of an
associative rigid transition state. In addition, the near
Figure 2 A Hammett plot of log k2 vs. ␴ parameters for
p
Þ
constancy of ⌬G values are indicative of a similar
benzhydrols: [CAT]O ϭ 1.00 ϫ 10 mol dm 3; [YBH]O
Ϫ3
Ϫ
Þ
ϭ 1.00 ϫ 10 mol dm 3; [HCl] ϭ 0.100 mol dm
;
;
Ϫ2
Ϫ
Ϫ3
solvated transition state. The values of both ⌬S and
Þ
Ϫ
5
Ϫ
Ϫ3
[RuCl3] ϭ 4.82 ϫ 10 mol dm 3; ␮ ϭ 0.40 mol dm
⌬G support the existence of a similar mechanism for
the oxidation of all benzhydrols.
MeOH ϭ 30% v/v; and Temp. ϭ 308 K.

780
RAMACHANDRA ET AL.
7. For a review see: A. H. Haines, Methods for the Oxi-
dation of Organic Compounds, Academic Press, New
York, 1988.
8. C. Palomo, J. M. Ontolia, J. M. Odriozola, J. M. Aizpu-
rua, and I. Ganboa, J. Chem. Soc. Chem. Commun., 248
(1990).
9. K. B. Wiberg, Oxidation in Organic Chemistry, Part A,
Academic Press, New York, 1965, pp. 142, 159, 198,
200, 47, and 247.
indicates the formation of a carbonium ion or a posi-
tive character in the transition state which is stabilized
by electron-donating groups. The complex XЈ can de-
velop a positive character due to the cleavage of the
O9Cl bond which begins ahead of other bond rup-
tures. In the present case, the increase in the rate due
to the presence of electron-donating groups is in agree-
ment with the above observation.
10. W. S. Trahanovsky, Oxidation in Organic Chemistry,
Part B, Academic Press, New York, 1973, pp. 35 and
197.
11. R. J. Andette, J. W. Quail, and P. J. Smith, J. Chem.
Soc. Chem. Commun., 38 (1972).
Isokinetic Relationship
Þ
The values of ⌬H are low for the oxidation of benz-
Þ
hydrols by CAT (Table VI). The values of ⌬H and
Þ
12. R. M. E. Richards, J. Pharmacol., 21, 68 (1969); ibid,
24, 145 (1972).
⌬S are linearly related (r ϭ 0.9996) giving an iso-
kinetic temperature ␤. The genuine nature of the iso-
kinetic relationship was verified by the Exner criterion
[30] by plotting log kЈ (318 K) vs. log kЈ (303 K). The
value of ␤ was calculated from the equation, ␤ ϭ T1
(1 Ϫ q)/(T1/T2) Ϫ q, where q is the slope of the Exner
plot and T1 Ͼ T2 . The calculated value of ␤ is 346 K.
It is seen that the ␤ value is higher than the experi-
mental temperature (308 K) indicating a common
enthalpy-controlled pathway for the reactions (Ta-
ble VI).
13. J. C. Morris, J. A. Salazar, and M. A. Wineman, J. Am.
Chem. Soc., 70, 2036 (1948).
14. Vogel’s Textbook of Practical Organic Chemistry, 5th
ed, 1989, p. 1338.
15. E. S. Amis, Solvent Effects on Reaction Rates and
Mechanisms, Academic Press, New York, 1966, p. 351.
16. D. S. Mahadevappa, S. Ananda, N. M. Made Gowda,
and K. S. Rangappa, J. Chem. Soc., Perkin Trans II, 39
(1985).
17. F. E. Hardy and J. P. Johnston, J. Chem. Soc., Perkin
Trans. 2, 642 (1973).
18. H. H. Cady and R. E. Connick, J. Amer. Chem. Soc.,
80, 2646 (1958).
One of the authors (DSM) thanks the University Grant’s
Commission, New Delhi, for the award of an Emeritus Fel-
lowship during this study. Another author (HR) is grateful
to the Principal and the Management, the PES College of
Engineering, Mandya, Karnataka, for permitting him to pur-
sue his graduate research in the Department of Studies
in Chemistry, University of Mysore, Manasagangothri,
Mysore, India.
19. R. E. Connick and D. A. Fine, J. Amer. Chem. Soc., 82,
4187 (1960).
20. J. R. Backhours, F. D. Doyer, and N. Shales, Proc. Roy.
Soc., 83, 146 (1950).
21. T. Davfokratova, Analytical Chemistry of Ruthenium,
Academy of Sciences, USSR, 1963, pp. 54, 71, and 97.
22. W. P. Griffith, The Chemistry of Rare Platinum Metals,
Inter Science, New York, 1967, p. 141.
23. B. Singh, N. B. Singh, and B. B. L. Saxena, J. Ind.
Chem. Soc., 61, 319 (1984).
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