Lack of linear free energy relationships in the p-toluenesulfonic acid mediated chromium(VI) oxidation of organic sulfides

Article abstract of DOI:10.1007/s007060070019

The oxidation of methylphenyl sulfide by imidazolium dichromate in acetonitrile mediated by p-toluenesulfonic acid is first order in IDC and TsOH and zero order in MeSPh in the concentration range studied. The IDC oxidation of 14 para-, meta-, and ortho-s

Full text of DOI:10.1007/s007060070019

Monatshefte fuÈr Chemie 131, 1123±1128 (2000)
Lack of Linear Free Energy Relationships in the
p-Toluenesulfonic Acid Mediated Chromium(VI)
Oxidation of Organic Sul®des
Ã
Chockalingam Karunakaran and Vaithiyalingam Chidambaranathan
Department of Chemistry, Annamalai University, Annamalainagar-608002, India
Summary. The oxidation of methylphenyl sul®de by imidazolium dichromate in acetonitrile
mediated by p-toluenesulfonic acid is ®rst order in IDC and TsOH and zero order in MeSPh in the
concentration range studied. The IDC oxidation of 14 para-, meta-, and ortho-substituted
phenylmethylsul®des at 20±45ꢀC conforms to the isokinetic relationship but not to any of the linear
free energy relationships; the isokinetic temperature lies within the experimental temperature
range.
Keywords. Linear free energy relationships; Activation parameters; Isokinetic temperature.
Introduction
Linear free energy relationships are empirical relationships between thermodynamic
quantities and are known as extrathermodynamic equations. Substituent constants
represent inductive and resonance effects which in¯uence the potential energies of
the molecules and hence the activated complexes [1]. Kinetic studies on the
oxidation of organic sul®des are numerous, and the reactions conform to the linear
free energy relationships [2±6].
Chromium(VI) complexes of heterocyclic bases are mild oxidants soluble in
organic solvents and are used under anhydrous conditions [7]. Kinetic and
mechanistic studies on oxidations by chromium(VI) complexes are numerous. The
oxidations are ®rst order in chromium(VI) reagents and are catalyzed by mineral
acids. The reactions are of either ®rst order or fractional order with respect to the
substrates. The mechanism of oxidation is of two types: one involves the rapid
formation of a substrate ± protonated oxidant complex followed by its slow
decomposition, and the other one is characterized by slow complex formation and
fast decomposition. Recently we have shown that the oxidation by chromium(VI)
complexes follows a common mechanism [8, 9]. Chromium(VI) has the largest
reduction potential in acetonitrile (acetonitrile: 1632 mV, acetic acid: 1533 mV,
water: 875 mV, dimethyl sulfoxide: 541 mV [7]), and therefore the oxidation was
studied in this solvent. Imidazolium dichromate (IDC) is an effective oxidant
Ã
Corresponding author

1124
C. Karunakaran and V. Chidambaranathan
[10], and the IDC oxidation of organic sul®des in acetonitrile is catalyzed by p-
toluenesulfonic acid (TsOH).
Results and Discussion
Reaction order
The oxidation of MeSPh by IDC is ®rst order in the oxidant. A plot of
log(absorbance) vs. time is linear up to about one-half life of the oxidation. The
reaction is zero order with respect to the sul®de; the pseudo-®rst order rate constant
remains constant at different concentrations of MeSPh. Table 1 presents represen-
tative rate data. The oxidation proceeds only in the presence of p-toluenesulfonic
acid (TsOH), and mineral acids such as perchloric acid and carboxylic acids such as
chloroacetic acid do not mediate the oxidation. The oxidation is ®rst order with
respect to TsOH. The oxidation rate increases with increasing TsOH concentration,
and a plot of k0 vs. [TsOH] is linear and passes through the origin (correlation
3
5
1
coef®cient: r ˆ 0:986; standard error: s ˆ 4:8  10 s ; slope: 5:3  10 2 dm Á
1
1
mol Á s ).
Mechanism
The activation enthalpy and activation entropy of the title reaction vary with the
substituents at the benzene ring (vide infra). This demonstrates that the substrate is
encountered in the reaction prior to the rate-limiting step; it is involved such that the
rate is independent of the substrate concentration. The oxidation proceeds under
anhydrous condition, and hence hydrolysis of the dichromate ion can be ruled out.
In non-aqueous media, chromium(VI) reagents form complexes with the sul®des,
and the reactions obey Michaelis-Menten kinetics with respect to the sul®de [8, 11,
12]. If the formation constant K of the IDC-sul®de complex is large, the oxidation
exhibits zero order dependence with respect to sul®de concentration (Scheme 1).
Rate-limiting decomposition of the IDC-sul®de complex mediated by p-toluene-
sulfonic acid accounts for the observed kinetic orders.
Linear free energy relationships
The measurement of the rates of oxidation of 14 para-, meta-, and ortho-substituted
phenylmethyl sul®des at different temperatures (20±45ꢀC) yields the activation
Table 1. IDC oxidation of methylphenyl sul®de in acetonitrile ([IDC]0 ˆ 4.9Â10 4 mol Á dm 3, 35ꢀC)
102 Á ‰MeSPhŠ
102‰TsOHŠ
104k0
0
0
3
3
1
mol Á dm
mol Á dm
s
3.8
7.7
15
2.9
2.9
2.9
2.3
2.6
3.2
3.5
15
15
14
12
14
16
18
3.8
3.8
3.8
3.8

Kinetics of the Oxidation of Organic Sul®des with Cr(VI)
1125
IDC ‡ MeSPh „ IDC-MeSPh
IDC-MeSPh ‡ TsOH ! MeSOPh ‡ HCrO4 ‡ CrO2 ‡ TsO ‡ 2 C3H5N2
K
k
‡
CrꢁIV† ‡ CrꢁVI† ! 2CrꢁV†
CrꢁV† ‡ MeSPh ! products
fast ‰7--9Š
fast ‰8Š
Scheme 1. d‰IDCŠT=dt ˆ k‰IDC-MeSPhŠ‰TsOHŠ ˆ Kk‰IDCŠ ‰MeSPhŠ‰TsOHŠ=ꢁ1 ‡ K‰MeSPhŠ†
T
where ‰IDCŠ ˆ ‰IDC-MeSPhŠ ‡ ‰IDCŠ; if K‰MeSPhŠ ꢂ 1, the rate law of the suggested
T
mechanism is
d‰IDCŠT=dt ˆ k‰IDCŠ ‰TsOHŠ with a pseudo-®rst order rate constant of
T
k0 ˆ k‰TsOHŠ
parameters (Table 2). The reaction is neither isoenthalpic nor isoentropic but
conforms to the compensation law @Ea=@lnA ˆ Rꢀ; @ꢁÁH#†=@ꢁÁS#† ˆ ꢀ, also
known as isokinetic relationships, where Ea is the energy of activation, A is the
frequency factor, ÁH# is the enthalpy of activation, ÁS# is the entropy of activat-
ion, R is the gas constant, and ꢀ is the isokinetic temperature, i.e. the temperature at
which all substrates react at equal rate. In other words, at the isokinetic temperature
the substituent has no in¯uence on the free energy of activation. In an isoentropic
reaction, the isokinetic temperature is in®nity, and only the enthalpy of activation
determines the reactivity. The isokinetic temperature is zero for an isoenthalpic
series, and the reactivity depends on the activation entropy. In the title reaction the
activation energy varies linearly with log(frequency factor) (r ˆ 0:999, slope ˆ
1
1
2.58 kJ Á mol , interceptˆ 13.9 kJ Á mol ), and the enthalpy of activation is linearly
1
related to the entropy of activation (Fig. 1; r ˆ 0:999, s ˆ 1:22 kJ Á mol , slope ˆ
310 K). The maximum possible errors in activation enthalpy (ꢁ) and activation
entropy are 3 kJÁ mol and 10 J Á K Á mol . The error criterion is satis®ed in the
1
1
1
Table 2. Activation parameters for the IDC oxidation of para-, meta-, and ortho- substituted
phenylmethyl sul®des in acetonitrile
Ea
ÁH#
ÁS#
ÁG#
Substituent
lnA
1
1
1
1
1
kJ Á mol
kJ Á mol
J Á K Á mol
kJ Á mol
ꢁ308 K†
H
42
80
74
85
69
49
35
45
42
69
106
91
92
65
11
26
23
28
21
13
8
39
78
72
82
66
47
32
43
39
66
104
89
89
62
157
37
87
89
91
89
91
92
89
89
91
89
90
91
90
90
p-CH3
p-OCH3
p-Cl
61
22
p-Br
80
p-NO2
m-CH3
m-Cl
145
184
148
169
73
13
10
22
36
30
30
19
m-NO2
o-CH3
o-OCH3
o-Cl
45
7
o-Br
2
o-NO2
92

1126
C. Karunakaran and V. Chidambaranathan
ˆ
ˆ
Fig. 1. Isokinetic plot of ÁH vs. ÁS
1
1
present study, i.e. ÁÁH# (72 kJ Á mol ) ꢂ 2ꢁ (6 kJ Á mol ), and hence the cor-
relation between ÁH# and ÁS# is signi®cant. The existence of isokinetic relation-
ship suggests that all substituted phenylmethyl sul®des studied are oxidized via a
common mechanism.
The free energies of activation of the IDC oxidation of para- and meta-
substituted phenylmethyl sul®des, calculated from the activation enthalpies and
activation entropies, were analyzed using the linear free energy relationships. The
linear free energy relationship is based on the ionization of benzoic acids; hence, a
correlation of free energy of activation, enthalpy of activation, and entropy of
activation of the IDC oxidation with the free energy, enthalpy, and entropy of
ionization of benzoic acids [1] should be meaningful. However, the correlation is
poor in all cases. The substituent in¯uence on the free energy of activation concerns
the enthalpic and entropic components [1]. The biparametric correlation of the free
energy of activation with the enthalpic (ꢂH) and entropic (ꢂS) substituent constants
is nonetheless unsatisfactory and the activation enthalpy and entropy also fail to
correlate with ꢂH and ꢂS. In addition, the free energy of activation does not correlate
with the usual Hammett substituent (ꢂ) constant (Fig. 2). The p-nitro group is likely
to establish conjugation with the reaction center. Also, expansion of the valence
shell of sulfur beyond the octet enables conjugation with the p-methoxy group [13].
‡
Therefore, the ꢂ and ꢂ constants were used in the single parameter correlation,
but also without success. The failure of all single parameter equations to correlate
the free energy of activation turned our interest towards dual substituent parameter
(DSP) equations. However, all known DSP equations including that of Swain [14]
failed to correlate the free energy of activation with the substituent constants ꢂI, ꢂ0R,

Kinetics of the Oxidation of Organic Sul®des with Cr(VI)
1127
ˆ
Fig. 2. Lack of linear relationship between ÁG and ꢂ
‡
ꢂRꢁBA†, ꢂR , and ꢂR [15] (F- and R-values: Ref. [14]). The possible reason for the
lack of any linear free energy relationship is that the isokinetic temperature lies
within the experimental temperature (293±318 K); the isokinetic temperature
calculated from the isokinetic plots is 310 Æ 5 K.
Experimental
The para-, meta-, and ortho-substituted phenylmethyl sul®des were prepared by standard methods
[16] and distilled. Imidazolium dichromate ((C3H5N2)2Cr2O7) was prepared by a reported procedure
[10], dissolved in acetonitrile (HPLC grade), and standardized iodometrically. A solution of p-
toluenesulfonic acid (TsOH) in acetonitrile was prepared and standardized. The kinetics of the
oxidation were investigated under pseudo-®rst order conditions ([TsOH] and [MeSPh] ꢂ [IDC]) at
constant temperature. The reaction was initiated by the addition of sul®des, and the progress of the
oxidation was followed spectrophotometrically (UVIDEC-7800, Jasco) at 350 nm. The pseudo-®rst
order rate constants (k0) were calculated from the slopes of log(absorbance) vs. time plots (least
squares ®ts). The oxidation stops at the sulfoxide; under the chosen experimental conditions, further
oxidation of sulfoxide is slow. The reduction product of IDC is chromium(III), identi®ed by the UV/
Vis spectrum of the reaction solution after completion of the oxidation. Hence, the oxidation follows
Eq. (1).
ꢁC3H5N2†2Cr2O7 ‡ 8 p-CH3C6H4SO3H ‡ 3 CH3SC6H5
‡
! 2 Cr3‡ ‡ 2 C3H5N2 ‡ 8 p-CH3C6H4SO3 ‡ 3 CH3SOC6H5 ‡ 4H2O
ꢁ1†
References
[1] Klumpp GW (1982) Reactivity in Organic Chemistry. Wiley Interscience, New York
[2] Chellamani A, Alhaji NMI, Rajagopal S, Sevvel R, Srinivasan C (1995) Tetrahedron 51: 12677
[3] Chellamani A, Alhaji NMI, Rajagopal S (1997) J Chem Soc Perkin Trans 2, 299
[4] Bohra A, Sharma PK, Banerji KK (1997) J Org Chem 62: 3562
[5] Bharathy JB, Ganesan TK, Sheriff AIM, Rajagopal S (1997) Tetrahedron 53: 1131
[6] Ganesan TK, Rajagopal S, Bharathy JB, Sheriff AIM (1998) J Org Chem 63: 21
[7] Cainelli G, Cardillo G (1984) Chromium Oxidation in Organic Chemistry. Springer, Berlin

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C. Karunakaran and V. Chidambaranathan: Kinetics of the Oxidation of Organic Sulfides
[8] Karunakaran C, Venkataramanan R, Kamalam R (1999) Monatsh Chem 130: 1461
[9] Karunakaran C, Suresh S (2000) J Chem Res (S) 114
[10] Lopez C, Gonzalez A, Cossio FP, Palomo C (1985) Synth Commun 15: 1197
[11] Panigrahi GP, Mahapatro DD (1981) Int J Chem Kinet 13: 85
[12] Rajasekaran K, Baskaran T, Gnanasekaran G (1984) J Chem Soc Perkin Trans 2, 1183
[13] Shorter J (1991) In: Zalewski RI, Krygowski TM, Shorter J (eds) Similarity Models in Organic
Chemistry, Biochemistry and Related Fields. Elsevier, Amsterdam
[14] Swain GC, Unger SH, Rosenquist NR, Swain MS (1983) J Am Chem Soc 105: 492
[15] Dayal SK, Ehrenson S, Taft RW (1972) J Am Chem Soc 94: 9113
[16] Vogel AI (1959) Practical Organic Chemistry. Longman, London
Received April 18, 2000. Accepted (revised) July 12, 2000