Mechanistic studies of oxidation of diphenylmethanols by sodium N-chlorobenzenesulphonamide catalysed by ruthenium(III)

Article abstract of DOI:10.1002/(SICI)1099-1395(199703)10:3<159::AID-POC867>3.0.CO;2-V

The kinetics of oxidation of six para-substituted diphenylmethanols (Y-DPM, where Y=H, Cl, Br, NO₂, CH₃ and OCH₃) by sodium N-chlorobenzenesulphonamide [chloramine-B (CAB)] in the presence of HCI and catalysed by RuCl₃ in 30% (v/v) methanol medium was studied at 35 °C. The experimental rate law is rate=k′[CAB]₀[DPM]₀^x[RuCl ₃]^y[H⁺]^z, where x, y and z are fractions. Addition of reaction product, benzenesulphonamide (BSA), retards the reaction. An increase in the dielectic constant of the medium decreases the rate. Rate studies in D₂O medium showed that the solvent isotope effect k′ (H₂O)/k′ (D₂O)=0·53. Proton inventory studies were carried out using H₂O-D₂O mixtures. The rates correlate satisfactorily with the Hammett σ relationship and the plot is biphasic. The reaction constant ρ is -2·8 for electron-releasing groups and -0·31 for electron-withdrawing groups at 35 °C. The activation parameters ΔH^?, ΔS^?, ΔG^? and logA were calculated. ΔH^? and ΔS^? are linearly related and an isokinetic relationship is observed with β=343 K, indicating enthalpy as a controlling factor,

Full text of DOI:10.1002/(SICI)1099-1395(199703)10:3<159::AID-POC867>3.0.CO;2-V

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, VOL. 10, 159–166 (1997)
MECHANISTIC STUDIES OF OXIDATION OF DIPHENYLMETHANOLS
BY SODIUM N-CHLOROBENZENESULPHONAMIDE CATALYSED BY
RUTHENIUM(III)
K. S. RANGAPPA,1* H. RAMACHANDRA2 AND D. S. MAHADEVAPPA1
1 Department of Studies in Chemistry, Mysore University, Manasagangothri, Mysore 570 006, India
2 Department of Chemistry, PES College of Engineering, Mandya, India
The kinetics of oxidation of six para-substituted diphenylmethanols (Y-DPM, where Y=H, Cl, Br, NO2 , CH3 and OCH3 )
by sodium N-chlorobenzenesulphonamide [chloramine-B (CAB)] in the presence of HCl and catalysed by RuCl3 in 30%
(v/v) methanol medium was studied at 35 °C. The experimental rate law is rate=kЈ[CAB]0[DPM]x0[RuCl3 ]y[H+ ]z, where
x, y and z are fractions. Addition of reaction product, benzenesulphonamide (BSA), retards the reaction. An increase in
the dielectic constant of the medium decreases the rate. Rate studies in D2O medium showed that the solvent isotope
effect kЈ(H2O)/kЈ(D2O)=0·53. Proton inventory studies were carried out using H2O–D2O mixtures. The rates correlate
satisfactorily with the Hammett ␴ relationship and the plot is biphasic. The reaction constant ␳ is ؊2·8 for electron-
releasing groups and ؊0·31 for electron-withdrawing groups at 35 °C. The activation parameters ⌬H‡, ⌬S‡, ⌬G‡ and
logA were calculated. ⌬H‡ and ⌬S‡ are linearly related and an isokinetic relationship is observed with ␤=343 K,
indicating enthalpy as a controlling factor. © 1997 by John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 10, 159–166 (1997) No. of Figures: 5 No. of Tables: 6 No. of References: 20
Keywords: diphenylmethanols; chloramine-B; oxidation; kinetics, HCl medium; ruthenium(III)
Received 21 May 1996; revised 28 September 1996; accepted 14 October 1996
INTRODUCTION
some light on the mechanism of CAB oxidation of
secondary alcohols, we have studied the reactions of six
substituted diphenylmethanols with this oxidant. Optimum
conditions were established for the formation of benzophe-
none, which is an important constituent of perfumes and is
also used in the manufacture of antihistamines, hypnotics
and insecticides. The Hammett free energy relationship was
tested and an isokinetic relationship was deduced together
with the computed thermodynamic parameters.
N-Metallo N-arylhalosulphonamides (organic haloamines)
contain halogen in the +1 state. They behave as mild
oxidants and are suitable for the limited oxidation of
specific groups. The important chlorine compound of this
class, chloramine-T (CAT), is a byproduct of saccharin
manufacture, and is a well known analytical reagent. The
benzene analogue sodium N-chlorobenzenesulphonamide
[chloramine-B (C6H5SO2NClNa · 1·5H2O) (CAB)] can be
easily prepared and is effective in oxidizing aromatic and
aliphatic primary alcohols1, 2 to the aldehyde stage. A
considerable amount of work has been performed on the
mechanistic studies involving oxidations of alcohols by
transition metal ions3 such as chromium(VI), vanadium(V),
cobalt(III), manganese(VII) and cerium(IV) in an acidic
medium and with copper(II)4 and ruthenium tetroxide in an
alkaline medium and ferrate(VI)5, 6 ion. However, informa-
tion is lacking concerning the oxidation of secondary
aromatic alcohols by organic haloamines. A detailed
examination of CAB showed that it is an excellent reagent
for performing oxidations of secondary alcohols to ketones
in an acidic medium in the presence of RuCl3 catalyst and
this system is adaptable for large-scale operations. To shed
EXPERIMENTAL
Chloramine-B (CAB) was prepared as reported previ-
ously.1, 2 An aqueous solution of the compound was
prepared, standardized iodometrically and stored in brown
bottles to prevent its photochemical deterioration.
Diphenylmethanols (Aldrich) were of accepted grades of
purity and were used without further purification; solutions
were prepared in 30% (v/v) methanol. A solution of RuCl3
(Arora Matthey) in 0·5 mol dmϪ3 HCl was used as a
catalyst in an acidic medium. Allowance was made for the
amount of HCl present in the catalyst solution while
preparing solutions for kinetic runs. All other chemicals
were of analytical grades. Triply distilled water was used for
preparing aqueous solutions. Solvent isotope studies were
carried out with D2O (99·2%) supplied by the Bhabha
* Correspondence to: K. S. Rangappa.
© 1997 by John Wiley & Sons, Ltd.
CCC 0894–3230/97/030159–08 $17.50

160
RUTHENIUM-CATALYSED OXIDATION OF DIPHENYLMETHANOLS
Atomic Research Centre (Bombay, India). The ionic
strength of the reaction mixture was kept at high value with
a concentrated solution of NaClO4 . Regression analysis of
the experimental data was carried out on an EC-72
statistical calculator supplied by the Electronic Corporation
(India).
dichloromethane as the solvent and iodine as the spray
reagent (Rf =0·82). The benzophenone was further identified
as its 2,4-dinitrophenylhydrazone (2,4-DNP) derivative,
which was recrystallized from ethanol (recovery 72·2%)
and was found to be identical with the DNP derivative of an
authentic sample; m.p.=237–238 °C, known m.p.=238 °C.
Kinetic measurements. The reaction was carried out in
glass-stoppered Pyrex boiling tubes whose outer surface
was coated black to eliminate photochemical effects.
Solutions containing appropriate amounts of DPM, HCl,
RuCl3 and water (to keep the total volume constant for all
runs) were placed in the tube, which was thermostated at
35 °C. A measured amount of CAB solution, also thermo-
stated at 35 °C, was rapidly added to the mixture. The
progress of the reaction was monitored by withdrawing
aliquots from the reaction mixtures at different time
intervals and determining the unreacted CAB iodomet-
rically. The course of the reaction was studied up to two
half-lives. The calculated pseudo-first-order rate constants,
kЈ, were reproducible to within ±3%.
Benzenesulphonamide (BSA). BSA was recrystallized
from dichloromethane and light petroleum and detected by
thin-layer chromatography using light petroleum–chloro-
form–butan-1-ol (2:2:1, v/v/v) as the solvent and iodine
for detection (Rf =0·88).
RESULTS
Effect of reactants
With the substrate in excess, at constant [HCl], [DPM]0 and
[RuCl3 ], plots of log[CAB] vs time are linear (r=0·9989),
indicating a first-order dependence of rate on [CAB]0 .
Values of pseudo-first-order rate constants (kЈ) are given in
Table 1. Further, the values of kЈ are unaltered with variation
in [CAB]0 , confirming the first order dependence on
[oxidant]0 .
Stoichiometry. Investigations under the conditions
[CAB]ӷ[DPM] revealed that 1 mol of CAB was consumed
by 1 mol of DPM according to the equation
The rate increases initially with increase in [DPM]0 . A
plot of logkЈ vs log[DPM]0 is linear (r=0·998, Table 1) with
a fractional slope of 0·81. The rate levels off at higher
[DPM]0 .
35 °C
____
Y-C6H4CHOHPh+PhSO2NClNa
→
Y-C6H4COPh+PhSO2NH2 +Na+ +Cl (1)
where Y=H, Cl, Br, CH3 , OCH3 and NO2 .
Effect of [HCl]
Product analysis. The reaction products were subjected to
column chromatography on silica gel (60–200 mesh) using
gradient elution (dichloromethane). After initial separation,
the products were further purified by recrystallization.
Materials were identified by comparison with commercially
available samples.
The rate increases with increase in [HCl] and the plot of
logkЈ vs log[HCl] is linear (r=0·9979, Table 2) with a
fractional slope of 0·80.
Effect of [H+ ]
Benzophenone. This was recrystallized from ethanol,
m.p.=49–50 °C; known m.p.=49–51 °C (Merck Index 11,
1108). The compound was identified by TLC using
At constant [ClϪ ]=0·4 mol dmϪ3, maintained by adding
NaCl, the rate increased with increase in [H+ ], which was
varied by adding HCl. A plot of logkЈ vs log[H+ ] is linear
Table 1. Effect of varying reactant concentration on the ratea
[CAB]0 ϫ104
[DPM]0 ϫ103 kЈϫ104
(mol dmϪ3 (sϪ1
[CAB]ϫ104
(mol dmϪ3
[DPM]0 ϫ103
kЈϫ104
(sϪ1
(mol dmϪ3
)
)
)
)
(mol dmϪ3
)
)
05·0
10·0
15·0
20·0
25·0
30·0
35·0
40·0
50·0
10·0
4·09
10·0
10·0
10·0
10·0
10·0
10·0
10·0
10·0
10·0
05·0
07·5
10·0
15·0
20·0
30·0
40·0
50·0
60·0
1·99
2·95
4·18
6·30
8·91
10·0
10·0
10·0
10·0
10·0
10·0
10·0
10·0
4·18
4·20
4·29
4·05
4·26
4·09
4·30
4·32
12·6
18·2
19·5
20·0
a [HCl]=0·1 mol dmϪ3, [RuCl3 ]=4·82ϫ10Ϫ5 mol dmϪ3, ␮=0·4 mol dmϪ3, MeOH=30% (v/v),
T=308 K.
© 1997 by John Wiley & Sons, Ltd.
JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, VOL. 10, 159–166 (1997)

K. S. RANGAPPA, H. RAMACHANDRA AND D. S. MAHADEVAPPA
Table 2. Effect of varying HCl, H+ and RuCl3 concentrations on the ratea
161
[HCl]ϫ102
kЈϫ104
(sϪ1
[H+ ]b ϫ102
kЈϫ104
(sϪ1
[RuCl3 ]ϫ105
kЈϫ104
(sϪ1
(mol dmϪ3
)
)
(mol dmϪ3
)
)
(mol dmϪ3
)
)
05·0
08·0
10·0
15·0
20·0
30·0
40·0
2·51
3·30
4·18
5·62
6·92
9·55
10·0
15·0
20·0
25·0
30·0
40·0
50·0
4·18
5·49
7·08
8·51
10·0
12·6
—
1·96
2·52
3·64
4·82
7·24
9·30
11·21
2·34
2·82
3·46
4·18
5·37
6·30
7·08
11·5
a [CAB]0 =10·0ϫ10Ϫ4 mol dmϪ3
MeOH=30% (v/v), T=308 K.
,
[DPM]0 =10·0ϫ10Ϫ3 mol dmϪ3
,
␮=0·4 mol dmϪ3
,
b Variation at constant [ClϪ ]=0·4 mol dmϪ3
.
(r=0·996, Table 2) with a fractional slope of 0·79. The
results were the same even when [ClϪ ] was not maintained
constant.
Effect of halide ions
Addition of ClϪ or BrϪ ions in the form of NaCl or NaBr
(5ϫ10Ϫ4–20ϫ10Ϫ4 mol dmϪ3 ) at fixed [H+ ]=0·1 mol
dmϪ3 had no effect on the reaction rate.
Effect of RuCl3
The rate increases with increase in [RuCl3 ] (r=0·976, Table
2) and the plot of logkЈ vs log[RuCl3 ] is linear with a
fractional slope of 0·62.
Effect of benzenesulphonamide (BSA)
The rate decreased with the addition of BSA. A plot of logkЈ
vs log[BSA] is linear with a negative fractional slope of
0·47 (r=0·989, Table 3), indicating that BSA is involved in
a pre-equilibrium before the rate-limiting step.
Figure 1. Proton inventory plot of kЈ vs the deuterium atom
n
fraction (n) in H2O–D2O mixtures
Effect of varying the dielectric constant of the medium
Effect of ionic strength
The dielectric constant (D) of the medium was varied by
adding methanol (30–60%, v/v) to the reaction mixture.
The rate decreased with increase in methanol content. A plot
Variation of the ionic strength of the medium by adding
NaClO4 (0·2–1·0 mol dmϪ3 ) had a negligible effect on the
rate.
Table 3. Effect of varying BSA concentration and dielectric
constant on the ratea
Table 4. Proton inventory studies for DPM in H2O–D2O mixtures
at 308 Ka
[BSA]ϫ104
kЈϫ104
(sϪ1
MeOH
(% v/v)
Dielectric
constant (D)
kЈϫ104
(sϪ1
Atom fraction of
deuterium (n)
(mol dmϪ3
)
)
)
knobs ϫ104 (sϪ1
)
2·0
4·0
6·0
10·0
15·0
3·75
2·63
2·13
1·73
1·48
30·0
40·0
50·0
55·0
60·0
56·73
51·08
45·30
42·66
40·40
4·18
3·46
2·81
2·45
2·00
0·00
0·15
0·30
0·45
0·56
4·18
4·72
5·50
6·42
7·80
a [CAB]0 =10·0ϫ10Ϫ4 mol dmϪ3
,
[DPM]0 =10·0ϫ10Ϫ3 mol dmϪ3
,
,
a [CAB]0 =10ϫ10Ϫ4 mol dmϪ3
, ,
MeOH=30% (v/v), T=308 K.
,
[DPM]0 =10ϫ10Ϫ3 mol dmϪ3
,
,
[HCl]Ϫ0·1 mol dmϪ3, [RuCl3 ]=4·82ϫ10Ϫ5 mol dmϪ3, ␮=0·4 mol dmϪ3
T=308 K.
[HCl]=0·1 mol dmϪ3 [RuCl3 ]=4·82ϫ10Ϫ5 mol dmϪ3 ␮=0·4 mol dmϪ3
© 1997 by John Wiley & Sons, Ltd.
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162
RUTHENIUM-CATALYSED OXIDATION OF DIPHENYLMETHANOLS
Table 5. Temperature dependence of the oxidation of substituted
diphenylmethanols by CABa
Test for free radicals
Addition of the reaction mixtures to acrylamide did not
initiate polymerization, showing the absence of free radical
species.
Substrate
Y-DPM:
kЈϫ104 (sϪ1
)
Y=
303 K
308 K
313 K
318 K
4-NO2
4-Cl
4-Br
4-H
4-CH3
4-OCH3
1·46
2·08
2·24
3·16
7·58
12·5
1·94
2·69
2·95
4·18
12·0
22·4
2·38
3·38
4·36
5·96
18·3
40·7
2·82
4·26
5·52
8·32
28·3
60·8
DISCUSSION
Sodium N-chlorobenzenesulphonamide (chloramine-B) acts
as an oxidizing agent in both acidic and alkaline media. In
general, CAB undergoes a two-electron change in its
reactions. The oxidation potential of CAB–PhSO2NH2 is pH
dependent and decreases with increase in the pH of the
medium. Depending on the pH of the medium, CAB
furnishes different types of reactive species in solutions
[equations (2)–(7)], such as PhSO2NHCl, PhSO2NCl2 ,
HOCl and possibly H2O+ Cl in acidic solutions.7, 8
a [CAB]0 =10ϫ10Ϫ4 mol dmϪ3
,
[DPM]0 =10ϫ10Ϫ3 mol dmϪ3
,
,
[HCl]=0·1 mol dmϪ3
MeOH=30% (v/v).
,
[RuCl3 ]=4·82ϫ10Ϫ5 mol dmϪ3 ␮=0·4 mol dmϪ3
,
of logkЈ vs 1/D is linear (r=0·9978, Table 3) with a negative
slope, supporting a rate-limiting step with partial ionization.
A blank experiment with methanol showed that there is a
slight decomposition of solvent under the experimental
conditions. This was allowed for in the calculation of the net
reaction rate constant for the oxidation of diphenylmetha-
nols.
Ϫ
+
*
PhSO2NClNa)PhSO2NCl +Na
(2)
(3)
Ϫ
+
*
PhSO2NCl +H )PhSO2NHCl
Ka
Kd
*
2PhSO NHCl PhSO NCl +PhSO NH
(4)
)
2
2
2
2
2
Solvent isotope studies
*
Studies of the rate of oxidation of DMP by CAB in D2O
revealed that whereas kЈ(H2O) is 4·18ϫ10Ϫ4 sϪ1, kЈ(D2O) is
7·80ϫ10Ϫ4 sϪ1. The solvent isotope effect kЈ(H2O)/kЈ(D2O)
is thus found to be 0·53. Proton inventory studies were
performed by carrying out the reaction in H2O–D2O
mixtures with varying atom fractions n of deuterium (Figure
1, Table 4).
PhSO2NHCl+H2O)PhSO2NH2 +HOCl
(5)
(6)
(7)
+
+
*
PhSO2NHCl+H )PhSO2N H2Cl
+
+
*
PhSO2N H2Cl+H2O)PhSO2NH2 +H2O Cl
If the dichloramine, PhSO2NCl2 , were to be the reactive
species, then the rate law would predict a second-order
dependence of rate on [CAB]0 [equation (4)], which is
contrary to the experimental observations. The rate
increases with increase in [H+ ] but is retarded by the added
reaction product benzenesulphonamide. Hence equations
(6) and (7) play a dominant role in the oxidation of DPM by
CAB.
Effect of temperature on the rate
The reaction was studied at different temperatures
(303–318 K) and, from the Arrhenius plots of logkЈ vs 1/T,
values of the activation parameters for the composite
reaction were calculated (Tables 5 and 6).
Table 6. Thermodynamic parameters for the oxidation of substituted di-
phenylmethanols by CABa
Substrate
Y-DPM:
Y=
Ea
⌬H‡
Ϫ⌬S‡
⌬G‡
(kJ molϪ1
)
(kJ molϪ1
)
(J KϪ1 molϪ1
)
(kJ molϪ1
)
Log A
4-NO2
4-Cl
4-Br
4-H
4-CH3
4-OCH3
33·2
40·2
46·7
57·4
70·5
77·0
31·0
37·6
44·2
54·8
67·8
73·9
213·4
192·0
170·0
132·1
80·8
98·4
97·4
97·1
96·1
93·5
91·6
8·1
9·2
10·4
12·4
15·0
16·3
55·7
a [CAB]0 =10ϫ10Ϫ4 mol dmϪ3
,
[DPM]0 =10ϫ10Ϫ3 mol dmϪ3
,
[HCl]=0·1 mol dmϪ3
,
[RuCl3 ]=4·82ϫ10Ϫ5 mol dmϪ3, ␮=0·4 mol dmϪ3, MeOH=30% (v/v).
© 1997 by John Wiley & Sons, Ltd. JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, VOL. 10, 159–166 (1997)

K. S. RANGAPPA, H. RAMACHANDRA AND D. S. MAHADEVAPPA
163
K1
Electronic spectral studies by Cady and Connick9 and
Connick and Fine10 reveal that species such as
(i)
PhSO2NHCl+H3O+
(fast)
*PhSO2NH2 +H2O+ Cl
)
[RuCl5(H2O)]2Ϫ
,
[RuCl4(H2O)2 ]Ϫ ,
[RuCl3(H2O)3 ],
K2
+
(fast)
(fast)
(rds)
*
(ii)
(iii)
(iv)
DPM+H O Cl
X
)
2
[RuCl2(H2O)4 ]+ and [RuCl(H2O)5 ]2+ do not exist in an
aqueous solution of RuCl3 . A study on the oxidation states
of ruthenium has shown that Ru(III) exists in the following
equilibrium11–13 in acidic medium:
H2O
K3
*XЈ
)
X+[RuCl6 ]3Ϫ
k4
Products
XЈ
→
[Ru(III)Cl6 ]3Ϫ +H2O)[Ru(III)Cl5(H2O)]2Ϫ +Cl
*
H2O
Singh and co-workers14, 15 employed the above equilibrium
in the Ru(III)-catalysed bromamine-T oxidation of some
primary alcohols in acidic medium and in the Ru(III)
chloride-catalysed oxidation of ethylene glycols by N-
bromoacetamide (NBA) in HClO4 medium. However in the
present case, addition of ClϪ ion in the form of NaCl at
fixed [H+ ] had no effect on the rate, indicating that
[Ru(III)Cl6 ]3Ϫ itself is the most likely catalysing species.
Ultraviolet spectral measurements showed that di-
phenylmethanol solution has two sharp absorption bands at
255·2 and 222·4 nm while bands around 204 and 226·4 nm
were noticed for Ru(III) and CAB solutions, respectively, in
the presence of 0·1 mol dmϪ3 HCl. With mixtures of (i)
diphenylmethanol and Ru(III) and (ii) CAB and Ru(III)
solutions, there were no changes in ␭max , but when CAB and
Scheme 1
Scheme 1 assumes the formation of H2O+ Cl due to the
protonation of PhSO2HNCl by H+ ion and subsequent
hydrolysis, which complexes with DPM (X). The latter
reacts with RuCl3 through an equilibrium step to form a
complex (XЈ) which decomposes in a rate-limiting step to
yield products. The fractional order on [RuCl3 ] indicates a
pre-equilibrium step (iii) in Scheme 1. Assuming
[CAB]t =[PhSO2NHCl]+[H2O+ Cl]+[X]+[XЈ], the follow-
ing rate law can be derived for the oxidation of DPM by
CAB:
d[CAB] k4K1K2K3[CAB]t[H3O+ ][DPM]0[Ru]
Ϫ
=
dt
[PhSO2NH2 ]+K1[H3O+ ]
DPM solutions are mixed in presence of 0·1 mol dmϪ3
=
+K1K2[H3O+ ][DPM]0(1+K3[Ru])
(8)
HCl a single, sharp absorption band was noticed at
216·2 nm, indicating that complex formation takes place
only between CAB and DPM. In view of these facts,
Scheme 1 is proposed for the oxidation of diphenylmethanol
by CAB.
The detailed mode of oxidation of diphenylmethanols by
CAB and the probable structures of the intermediates is
shown in Scheme 2
It is interesting that the rate has increased in D2O
Scheme 2
© 1997 by John Wiley & Sons, Ltd.
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RUTHENIUM-CATALYSED OXIDATION OF DIPHENYLMETHANOLS
medium. Since D3O+ ion is a stronger acid than H3O+ ion
by a factor of 2–3, a solvent isotope effect of this magnitude
is to be expected. However, the observed inverse solvent
isotope effect kЈ(D2O(/kЈ(H2O) of 1·86 probably shows that
since the protonation is followed by hydrolysis in the rate-
limiting step which involves O–H bond scisson, the normal
kinetic isotope effect kЈ /kЈ >1 could counterbalance the
solvent isotope effect. Proton inventory studies in H2O–D2O
mixtures could throw light on the nature of the transition
state. The dependence of the rate constant (kn ) on n, the
atom fraction of deuterium in a solvent mixture of D2O and
H2O, is given16, 17 by a form of Gross–Butler equation:
the present case is a curve and comparison with the standard
curves18 indicates the involvement of a single proton or
H–D exchange during the reaction sequence. Hence the
participation of H+ ion in the formation of transition state is
inferred.
The moderate values of the enthalpy of activation, a large
negative entropy of activation and the fairly high ⌬G‡
values support the mechanism. The near constancy of the
⌬G‡ values indicates a solvated state and operation of a
similar mechanism for the oxidation of all diphenylmetha-
nols.
H
D
Structure–reactivity correlations
TS
k0obs ∏ (1Ϫn+n⌽i )
=
(9)
The Hammett plot shows two distinct (Figure 2) lines for
each of which there is a good correlation between the
substituent constants and the logarithm of the rate constants,
particularly when the McDaniel–Brown constant (␴p ),19 is
used. Of these, one has a much larger value of ␳=Ϫ2·8 for
electron-releasing groups and the other relatively low value
of ␳=Ϫ0·31 for electron-withdrawing groups at 35 °C. The
break in the Hammett plot could suggest a concerted
mechanism, the degree of concertedness depending on
whether the hydride transfer from the C—H bond to the
oxidant is synchronous with the removal of a proton from
RS (1Ϫn+n⌽j )
knobs
where ⌽i and ⌽j are the isotopic fractionation factors for
isotopically exchangeable hydrogen sites in the transition
states (TS) and reactant site (RS), respectively. Equation (9)
allows the calculation of the fractionation factor of TS, if
the reactant fractionation factors are known. However, the
curvature of the proton inventory plot could reflect the
number of exchangeable protons in the reaction.18 A plot of
knobs vs the deuterium atom fraction n (Figure 1, Table 4) in
Figure 2. Plot of logk2 vs ␴P for diphenylmethanols at 308 K
JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, VOL. 10, 159–166 (1997)
© 1997 by John Wiley & Sons, Ltd.

K. S. RANGAPPA, H. RAMACHANDRA AND D. S. MAHADEVAPPA
165
Figure 3. (a) Isokinetic plot of ⌬H‡ vs ⌬S‡ for the oxidation of diphenylmethanols by
CAB. (b) Exner plot of logkЈ (318 K) vs logkЈ (303 K)
the OH group by a water molecule. In earlier work on the
T1(1Ϫq)
oxidation of primary alcohols by organic haloamines,1 it
was noted that electron-donating groups increase the rate.
This indicates that the rupture of the C—H bond occurs
ahead of O—H bond cleavage, creating a carbonium ion
centre which is stabilized by the electron-donating groups.
In the present case, the decrease in rate with electron-
withdrawing groups is in agreement with this observation.
Also, a reaction involving a carbonium ion in the transition
state will be aided by electron-donating substituents and the
value of ␳ will be negative. On the other hand, a reaction
involving a decrease in the carbonium ion charge will be
facilitated by electron-withdrawing substituents. Therefore,
the carbonium ion is more stable when the electron donors
are attached to benzene ring system, which disperse the
positive charge on the carbonium ion (␳=Ϫ2·8), than the
electron-withdrawing groups on the benzene ring system
(␳=Ϫ0·31). Because of the above facts, no linearity was
found in the Hammett plot (Figure 2).
␤=
(T1 /T2 )Ϫq
where q is the slope of the Exner plot and T1 >T2 . The value
of ␤ is 353 K [Figure 3(b)]. It is seen that the value of ␤ is
higher than the experimental temperature (308 K), indi-
cating enthalpy control of the reactions.
ACKNOWLEDGEMENTS
One of the authors (D.S.M.) thanks the University Grant
Commiossion, New Delhi, for the award of an Emeritus
Fellowship during these investigations. H.R. is grateful to
the Principal, PES college of Engineering, Mandya, for
permittting him to work in the Department of Studies in
Chemistry, Mysore University.
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calculated from the equation
© 1997 by John Wiley & Sons, Ltd.
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© 1997 by John Wiley & Sons, Ltd.
JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, VOL. 10, 159–166 (1997)