Mechanistic studies of the oxidation of substituted phenethyl alcohols by N-metallo-N-haloarylsulphonamides: Kinetic isotope studies

Article abstract of DOI:10.1002/poc.415

The oxidation of para-substituted phenethyl alcohols (PEA, 2) by N-metallo-N-haloarylsulphonamides (1) in the presence of dilute HCl to the corresponding phenacetaldehydes (4) is first order with respect to oxidant (1) and [H⁺] and a fract ional order each in [PEA] and [Cl^-]. Addition of the reaction product (3), ionic strength variations and variation of dielectric constant of the medium had no effect on the rate. The oxidation of PhCH₂CD₂OH (2) exhibited a substantial primary kinetic isotope effect (k^H/k^D=5.83). The rates correlate satisfactorily with the Hammett free energy relationship. The activation parameters ΔH^≠, ΔS^≠, ΔG^≠ and logA were calculated for the i reaction. The proposed mechanism is consistent with the observed results. Copyright

Full text of DOI:10.1002/poc.415

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2001; 14: 684–690
DOI:10.1002/poc.415
Mechanistic studies of the oxidation of substituted
phenethyl alcohols by N-metallo-N-haloarylsulphonamides:
kinetic isotope studies
Kanchugarakoppal S. Rangappa*
Department of Studies in Chemistry, University of Mysore, Manasagangotri, Mysore 570 006, India
Received 15 May 2000; revised 15 December 2000; accepted 9 May 2001
ABSTRACT: The oxidation of para-substituted phenethyl alcohols (PEA, 2) by N-metallo-N-haloarylsulphonamides
(
1) in the presence of dilute HCl to the corresponding phenacetaldehydes (4) is first order with respect to oxidant (1)
‡
À
and [H ] and a fractional order each in [PEA] and [Cl ]. Addition of the reaction product (3), ionic strength variations
and variation of dielectric constant of the medium had no effect on the rate. The oxidation of PhCH CD OH (2)
exhibited a substantial primary kinetic isotope effect (k /k = 5.83). The rates correlate satisfactorily with the
Hammett free energy relationship. The activation parameters DH , DS , DG and logA were calculated for the
reaction. The proposed mechanism is consistent with the observed results. Copyright  2001 John Wiley & Sons, Ltd.
2
2
H
D
≠
≠
≠
KEYWORDS: substituted phenethyl alcohols; oxidation; N-metallo-N-haloarylsulphonamides; mechanism; kinetic
isotope effect
INTRODUCTION
state (X = Cl or Br, bonded to nitrogen is positive).
Generally they undergo a two-electron change to form
the respective halide ions and the corresponding
sulphonamides (3). Depending on the pH of the medium,
these oxidants (1) furnish different types of reactive
species. Since these oxidants react with a wide variety of
functional groups, they are used as reagents in analytical
The N-metallo-N-haloarylsulphonamides (1) are a class
of compounds capable of producing halonium cations,
hypohalites and N-anions which behave both as bases and
as nucleophiles, depending on the reaction conditions.
The subject has been extensively reviewed by Campbell
1
2
3–6
and Johnson and Bremner. These oxidants contain a
chemistry. The important aspects of the chemistry of
strongly polarized N-linked halogen which is in the ‡1
these oxidants are that (i) they perform very specific and
selective oxidations and (ii) they can also be used as mild
oxidants. For example, mild oxidation of alcohols to
carbonyl compounds is a very important operation in
7,8
organic synthesis. During a detailed study
of the
primary and secondary kinetic isotope effects for E2
reactions, it was anticipated that the oxidation of
substituted phenethyl alcohols (2) with the halosulpho-
namides 1 could shed light on the mechanism of
oxidation of 2. Therefore, with the available data, it is
important to report the detailed kinetic isotope effects for
the oxidation of 2 by 1 in an aqueous homogeneous
medium. It is found that addition of HCl catalyses the
‡
reaction, indicating simultaneous catalysis by H and
À
Cl ions. Attempts have been made to correlate the
structure–reactivity relationships for these reactions
through Hammett plots.
*
Correspondence to: K. S. Rangappa, Department of Studies in
Chemistry, University of Mysore, Manasagangotri, Mysore 570 006,
India.
E-mail: yangappaks@yahoo.com
RESULTS AND DISCUSSION
Contract/grant sponsor: Department of Science and Technology, New
Delhi; Contract/grant number: SP/S1/GO5/97.
Oxidation of 2 by 1 revealed that 1 mol of 1 was
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 684–690

OXIDATION OF PHENETHYL ALCOHOLS BY N-METALLO-N-HALOARYLSULPHONAMIDES
685
ꢀ1†
‡
consumed by 1 mol of 2 as shown in Eqn. (1) and the
product is the corresponding phenacetaldehyde (4).
With the substrate (PEA, 2) in excess, at constant
was varied on adding HCl and plots of log k' vs log [H ]
were linear (Fig. 3, r = 0.9986) with slopes of unity. At
‡
constant [H ] = 0.4 M, the rate increased with addition of
NaCl and plots of log k' vs log [Cl ] were linear
(r = 0.998) with fractional slopes (Fig. 4, Table 1).
Addition of the reaction product 3 (0.0005–0.002 M) had
a negligible effect on the rate, indicating that it is not
involved in a pre-equilibrium to the rate-limiting step.
Variation of the ionic strength of the medium was
À
[
(
[
HCl] and [PEA] , plots of log[ox] vs time were linear
r = 0.999), indicating a first-order dependence of rate on
0
ox] . Further, the values of k' were unaltered with
0
variation of [ox] (0.0002–0.002 M) confirming the first-
0
order dependence of rate on [oxidant] . The rate initially
0
increases with increase in [PEA] (0.005–0.05 M). Plots
0
of log k' vs log [PEA] were linear (Fig. 1, r = 0.998) with
fractional slopes, thus showing a fractional order
carried out by adding NaClO (0.2–1.0 M) and the rate
was unaffected.
0
4
dependence on [substrate]0.
The dielectric constant of the medium was varied by
adding methanol (0–40%, v/v) to the reaction mixture but
the rates were not significantly altered. The kinetic orders
observed in the present set of experiments are summar-
ized in Table 1. The reaction was studied at different
temperatures and from the Arrhenius plots of log k' vs
The rate increases with increase in [HCl] (0.08–0.4 M)
and plots of log k' vs log[HCl] were linear (Fig. 2,
r = 0.9986) and the slopes (order) are given in Table 1. At
À
constant [Cl ] = 0.4 M, maintained by adding NaCl, the
‡
rate increased with increase in [H ] (0.1–0.4 M), which
Figure 1. Plot of logk' vs log[PEA] at 308 K
Copyright  2001 John Wiley & Sons, Ltd.
Figure 2. Plot of logk' vs log [HCl] at 308 K
J. Phys. Org. Chem. 2001; 14: 684–690

6
86
K. S. RANGAPPA
Table 1. Kinetic data for the oxidation of substitu-
ted phenethyl alcohols ꢀ2) by N-metallo-N-haloarylsul-
À3
phonamides ꢀ1), with [1] = 0.0005 mol dm
0
T = 35°C
,
[2] =
À3
À3
À3
.001 mol dm , [HCl] = 0.1 mol dm , m = 0.5 mol dm
,
N-Metallo-N-haloarylsulphonamide
Observed order w.r.t
CAT
CAB
BAT
BAB
[
[
[
[
[
[
1]
1.00
0.75
1.54
1.00
0.56
1.00
0.72
1.60
1.00
0.65
1.00
0.80
1.50
1.00
0.58
1.00
0.77
1.60
1.00
0.70
2]
HCI]
‡
H ]
À
CI ]
3]
No effect
Ionic strength (m)
No effect
No effect
Dielectric constant
(
D) variation
À
Figure 4. Plot of log[Cl ] vs logk'
1/T, values of the activation parameters for the compo-
site reaction were calculated and these are shown in
parentheses in Table 2.
Tests for free radicals
Addition of acrylamide to the reaction mixture did not
initiate polymerization, showing the absence of free
radical species.
Discussion
N-Metallo-N-haloarylsulphonamides (1) act as oxidizing
agents in both acidic and alkaline media. Regarding the
oxidizing species, it has been thoroughly discussed from
‡
Figure 3. Plot of log[H ] vs logk' at 308 K
3–5
1,2
our work and that of others and shown in the case of
À4
À3
Table 2. Activation parameters for the rate-limiting step of Scheme 1, with [ox] = 5.0 Â 10 mol dm , [PEA] =
0
0
À3
À3
À3
À3
1
0.0 Â 10 mol dm , [HCl] = 0.1 mol dm , m = 0.5 mol dm
À1
≠
À1
≠
À1
À1
≠
À1
Oxidant
Ea (kJ mol
)
DH (kJ mol
)
DS (J K mol
)
DG (kJ mol
)
Log A
CAT
CAB
BAT
BAB
81.3 a
54.6)
66.9
78.8
(52.1)
64.4
(57.2)
65.7
(54.8)
71.8
(58.2)
À45.4
(À144)
À87.8
93.0
(96.9)
91.6
(95.9)
93.7
(93.8)
94.5
(91.9)
10.3
(8.80)
11.7
(9.80)
11.6
(9.80)
12.6
(11.2)
(
(
(
(
59.8)
(À125)
À90.3
68.3
57.4)
74.4
(À125)
À73.2
60.8)
(À116)
a
Values in parentheses refer to the parameters calculated for the overall reaction (1).
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 684–690

OXIDATION OF PHENETHYL ALCOHOLS BY N-METALLO-N-HALOARYLSULPHONAMIDES
687
‡
À
chloramine-T (CAT; 1, R = CH , X = Cl) that it behaves
like a strong electrolyte in aqueous solutions and
dissociates according to
Since both H and Cl ions catalyze the reaction,
Scheme 1 can be proposed for the oxidation of 2 by 1,
involving simultaneous catalysis by both these ions.
3
ꢀRNCl† ‡ Na‡
À
ꢀ2†
RNClNa
K1
‡
À
À
RNHX ‡ H ‡ Cl
RNH X Á Á Á Cl
fast
fast
2
0
ꢀi†
The anion picks up a proton in acidic solutions to give
the free acid monochloramine-T, RNHCl (N-chloro-p-
toluenesulphonamide):
ꢀX †
K2
Y-PEA ‡ X0
X
00
ꢀ
ii†
À
ꢀ
RNCl† ‡ H‡
RNHCl
ꢀ3†
k3
0
0
X ‡ H O
products slow rate-determining step
2
À5
ꢀiii†
K ˆ 2:82  10
a
Scheme 1
Although the free acid has not been isolated, there is
experimental evidence for its formation in acidic
solutions. It undergoes disproportionation giving rise to
9
Scheme 1 assumes the formation of a tight ion pair
X'), which is an intermediate and at the same time
indicates simultaneous catalysis by H and Cl ions.
This ion pair reacts with PEA through an equilibrium step
to form a complex (X@) which decomposes in a rate-
limiting step to the products. The Michaelis–Menten
kinetics obeyed by the substrate indicate a pre-equi-
(
p-toluenesulphonamide (RNH ) and dichloramine-T
2
‡
À
(
RNCl2):
Kd
2RNHCl
RNH ‡ RNCl
ꢀ4†
2
2
À2
ꢁ
K ˆ 6:1  10 at 25 C
d
librium step (ii) in Scheme 1. Assuming [OX] =
t
[
RNHX] ‡ [X'] ‡ [X@], rate law (9) can be derived
The dichloramine-T and the free acid hydrolyse to give
hypochlorous acid (HOCl):
for the oxidation of 2 by 1.
d[ox]
dt
0
0
Kh
rate ˆ À
ˆ k3‰X Š
ꢀ8†
ꢀ9†
RNCl ‡ H O
RNHCl ‡ HOCl
ꢀ5†
ꢀ6†
2
2
‡
À
d[ox]
k K K ‰oxŠ ‰H Š‰Cl Š‰PEAŠ
À7
ꢁ
3
1
2
t
0
K ˆ 8:0  10 at 25 C
À
ˆ
h
À
‡
dt
1 ‡ K ‰H Š‰Cl Šf1 ‡ K ‰PEAŠ g
1
2
0
0
Kh
RNHCl ‡ H O
RNH ‡ HOCl
2
2
Equation (9) can be transformed into
0
À8
ꢀ
ꢁ
K ˆ 4:88  10
h
1
1
1
1
k3
ˆ
‡ 1 ‡
ꢀ10†
0
‡
À
k
k K ‰PEAŠ K ‰H Š‰Cl Š
3
2
1
Finally, HOCl ionizes according to
From the double reciprocal plots of k' vs [PEA] and k'
0
Ka
2
‡
À
‡
vs [HCl] , since [H ] = [Cl ] = [HCl], values of k , K
HOCl
H ‡ OClÀ
ꢀ7†
3
1
and K can be evaluated (Table 3). Using k values
2
3
À8
ꢁ
K ˆ 3:3  10 at 25 C
(obtained by varying [PEA]
0
at each temperature),
a
activation parameters can be determined for the rate-
limiting step (Table 2).
In the light of the above, we can show that the
oxidation potential of the oxidant–3 couple is pH
dependent and decreases with increase in the pH of the
medium. Depending on the pH of the medium, 1
Kinetic isotope effect
3–5
furnishes
different types of reactive species in
solutions such as RNHX, RNX , HOX and possibly
To ascertain the importance of the cleavage of the C —H
bond in the rate-determining step, the oxidation of
2
a
‡
À
À
2
H O X (R = CH C H SO or C H SO ). If RNX
2
2
3
6
4
2
6
5
were to be the oxidant, the reaction predicts a second-
YC H CH CD OH (2, Y = H) was studied. The observed
6
4
2
2
order dependence on [oxidant] , which is not observed
substantial primary kinetic isotope effect (Table 4) in the
0
experimentally. Similarly, the absence of a retardation
oxidation of 2 by 1 confirms the cleavage of the C —H
a
effect on the rate by the added sulphonamide rules out the
bond in the rate-determining step. The intermediate (X@)
loses X (Scheme 2) creating O species, which in turn
‡
À
‡
involvement of HOX or H O X. The results obtained in
2
the present investigation (around pH 0–3) show that
RNHX is the active oxidant, in the present oxidations.
makes C —H more acidic. Therefore, the formation of an
electron-deficient reaction centre in the activated com-
a
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 684–690

6
88
K. S. RANGAPPA
Table 3. Values of formation constants, K and K , and decomposition constant, k , for the oxidation of PEA by arylhaloamines
1
2
3
À4
À3
À3
À3
À3
À3
,
ꢀ
Scheme 1), with [ox] = 5.0 Â 10 mol dm , [PEA] = 10.0 Â 10 mol dm , [HCl] = 0.1 mol dm , m = 0.5 mol dm
0
0
T = 35°C
3
À1
k  10 (s
)
3
À2
6
À1
3
Oxidant
K (mol dm )
K (mol dm )
303 K
308 K
313 K
318 K
1
2
CAT
CAB
BAT
BAB
10.9
3.50
4.90
4.40
266
890
500
568
0.66
1.34
3.57
1.66
1.25
1.67
5.00
2.94
2.00
2.86
6.25
5.00
3.33
5.00
10.0
6.25
À4
À3
,
Table 4. Kinetic isotope effect in the oxidation of phenethyl alcohols by arylhaloamines, with [ox] = 5.0 Â 10 mol dm
0
À3
À3
À3
À3
[
PEA] = 10.0 Â 10 mol dm , [HCl] = 0.1 mol dm , m = 0.5 mol dm , T = 35°C
0
2
H
À1 À1
3
D
À1 À1
H D
1
0 k l mol
s
10 k 1.mol .s
k /k
[
(
PEA]
mol dm
À3
)
CAT CAB BAT BAB
CAT CAB BAT BAB
CAT
CAB
BAT
BAB
0
0
0
.005
.010
.050
3.40 4.40 9.96 7.24
2.90 3.68 8.52 5.98
9.90 2.30 6.49 3.90
5.86 7.65 17.0 12.5
4.96 6.20 14.7 10.1
1.72 4.00 11.0 6.67
5.80
5.84
5.80
5.75
5.93
5.75
5.85
5.78
5.90
5.79
5.89
5.84
a
Average
5.81 Æ 0.02 5.81 Æ 0.10 5.84 Æ 0.06 5.84 Æ 0.05
a
H D
Mean k /k = 5.83 Æ 0.02.
Scheme 2
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 684–690

OXIDATION OF PHENETHYL ALCOHOLS BY N-METALLO-N-HALOARYLSULPHONAMIDES
689
Table 5. Rate constants for the reaction of phenethyl
alcohols
with
,
arylhaloamines,
with
[ox] = 5.0 Â
0
À4
À3
À3
À3
1
0
0
mol dm
[PEA] = 10.0 Â 10 mol dm
,
[HCl] =
0
À3
À3
.1 mol dm , m = 0.5 mol dm , T = 35°C
4
À1
k' Â 10 (s
)
Y-PEA: Y =
CAT
CAB
BAB
BAT
OCH3
CH3
H
24.0
10.4
2.90
1.67
1.55
1.25
29.5
13.2
3.68
2.60
2.30
1.74
44.7
20.0
5.98
4.10
3.80
2.69
60.3
28.2
8.52
5.88
5.37
1.97
Br
Cl
NO2
substrates follow a Michaelis–Menten plot and yielded
definite intercepts, which supports the proposed mech-
anism. The presence of a substantial primary kinetic
isotope effect indicates that the rupture of the C—H bond
occurs after the O—H bond cleavage, creating a
carbocation centre or a positive character in the transition
state, which is stabilized by the electron-donating groups,
and the decrease in rate with electron-withdrawing
groups is in agreement with this observation. The rate
of oxidation decreased in the order BAT > BAB > CAB
YF i- gP uE Ar e w5 i .t hH a1 mmet plot ꢀ4 ‡ logk' vs sp) for the reaction of
>
CAT (Table 5).
Detailed studies were carried out by changing R = H or
plex indicates that the cleavage of the C —H bond is
CH , X = Cl or Br and Y = p-NO , p-Cl, p-Br, p-H, p-
a
3 2
ahead of the formation of the C
=
O bond. Detailed
CH or p-OCH in reaction (1) to ascertain the proposed
3 3
kinetic isotope studies were performed with different
oxidants and at different concentrations and the results
are shown in Table 4.
mechanism and confirm the conclusions drawn. Similar
results were obtained in supporting the proposed mech-
anism.
The activation parameters were calculated with the
The detailed mode of oxidation of 2 by 1 and the
probable structures of the intermediates are shown in
Scheme 2. Overall, the mechanistic scheme appears to be
of E2 type.
help of k values for the rate-determining step, obtained
3
by Michaelis–Menten kinetics. The values of k and the
3
formation constant K K for all the oxidants were
1
2
calculated and are given in Table 3.
10
A Hammett plot of log k' vs sp was drawn (Fig. 5) to
correlate the structure–reactivity for these reactions. The
rate increased with electron-donating groups and de-
creased with respect to electron-withdrawing groups.
EXPERIMENTAL
7
,8
Materials. Phenethyl-1,1-d alcohol was prepared by
2
11
12
Swain and Langsdorf and other workers have studied
various reactions of this type and observed similar
behaviour. The results showed lower rates with respect
to electron-withdrawing groups and higher rates with
electron-donating groups (Table 5). Electron-donating
substituents stabilize a transition state having a carboca-
tion centre by resonance. This facilitates the bond-
breaking process in the transition state. On the other
hand, an electron-withdrawing substituent increases the
capacity of the carbocation ion centre, suggesting the
development of a loose or partial positive charge at the
transition state, which in turn helps the bond-making
process, thus leading to a decrease in rate.
the reduction of phenethyl acetate with lithium alumi-
13
nium deuteride in dry diethyl ether by the method of
1
3
Amundsen and Nelson. The isotopic purity, determined
1
by H NMR spectroscopy, was 95 Æ 3%. The oxidants
(CAT, CAB, BAT and BAB) were prepared by the
1
4–16
reported procedure.
An aqueous solution of the
compound was prepared, standardized iodimetrically and
stored in brown bottles to prevent photochemical
deterioration. All other chemicals were of analytical-
reagent grade. Triply distilled water was used for
preparing aqueous solutions. The ionic strength of the
reaction mixture was kept at high with a concentrated
solution of NaClO4.
≠
≠
A plot of DH vs DS gave a very good correlation,
which supports the existence of a similar mechanism for
the oxidation of substituted phenethyl alcohols. These
Product analysis. The reaction products were subjected to
column chromatography on silica gel (60–200 mesh)
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 684–690

6
90
K. S. RANGAPPA
using gradient elution (from dichloromethane to chloro-
form). After initial separation, the products were further
purified by recrystallization. Materials were identified by
comparison with commercially available samples.
Acknowledgements
I thank the Department of Science and Technology
(DST), New Delhi, for financial support (SP/S1/GO5/97).
The technical assistance given by Mr N. D. Dinesh is
gratefully acknowledged.
Phenacetaldehyde. Recrystallized from diechloro-
methane–light petroleum, m.p. 32–33°C, known m.p.
3
3–34°C (Merck Index, 112236). Phenacetaldehyde was
further purified as its 2,4-dinitrophenylhydrazone
2,4DNP) derivative, which was recrystallized from
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1
2
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4
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1
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1
1
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were placed in the tube and thermostated at 35°C. A
measured amount of oxidant solution, also thermostated
at the same temperature, was rapidly added to the
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time intervals and determining the unreacted oxidant
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to two half-lives. The calculated pseudo-first-order rate
constants k' were reproducible to within Æ3%.
2
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1
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