Effect of substitution of oxygen by sulfur in the npnleaving group of a carbonate: Kinetics of the phenolysis and benzenethiolysis of 5-methyl aryl thiocarbonates

Article abstract of DOI:10.1002/poc.1192

The phenolysis and benzenethiolysis of S-methyl 4-nitrophenyl thiocarbonate (1) and S-methyl 2,4-dinitrophenyl thiocarbonate (2) in water are studied kinetically. The Brnsted plots (log k_N versus nucleophile basicity) are linear for all reactions. The Bronsted slopes for 1 and 2 are, 0.51 and 0.66 (phenolysis) and 0.55 and 0.70 (benzenethiolysis), respectively. These values suggest a concerted mechanism for these reactions, as found in the corresponding carbonates. Namely, substitution of OMe by SMe in the nonleaving group does not change the mechanism. Copyright 2007 John Wiley & Sons, Ltd.

Full text of DOI:10.1002/poc.1192

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2007; 20: 533–538
Published online 9 May 2007 in Wiley InterScience
(www.interscience.wiley.com) DOI: 10.1002/poc.1192
Effect of substitution of oxygen by sulfur in the
nonleaving group of a carbonate: kinetics of the
phenolysis and benzenethiolysis of S-methyl aryl
thiocarbonates
*
´
Enrique A. Castro, Margarita Aliaga and Jose G. Santos
´
´
Facultad de Quımica, Pontificia Universidad Catolica de Chile, Casilla 306, Santiago 6094411, Chile
Received 5 March 2007; revised 16 March 2007; accepted 16 March 2007
ABSTRACT: The phenolysis and benzenethiolysis of S-methyl 4-nitrophenyl thiocarbonate (1) and S-methyl
2,4-dinitrophenyl thiocarbonate (2) in water are studied kinetically. The Brønsted plots (log kN versus nucleophile
basicity) are linear for all reactions. The Brønsted slopes for 1 and 2 are, 0.51 and 0.66 (phenolysis) and 0.55 and 0.70
(benzenethiolysis), respectively. These values suggest a concerted mechanism for these reactions, as found in the
corresponding carbonates. Namely, substitution of OMe by SMe in the nonleaving group does not change the
mechanism. Copyright # 2007 John Wiley & Sons, Ltd.
Supplementary electronic material for this paper is available in Wiley InterScience at http://www.mrw.interscience.
wiley.com/suppmat/0894-3230/suppmat/
KEYWORDS: phenolysis; benzenethiolysis; kinetics; thiocarbonates
INTRODUCTION
evaluate the effect of the nucleophile (phenoxide versus
benzenethiolate) on the kinetics and mechanism of these
reactions.
Although there have been some studies on the kinetics
and mechanisms of the phenolysis of thioesters1,2 and
thiocarbonates,1,3 the benzenethiolysis of thiocarbonates
has attracted less attention.4 The latter report investigates
the kinetics of the benzenethiolyses of alkyl aryl
carbonates and O-alkyl S-aryl thiocarbonates, and
discusses the effect of changing the oxygen atom in
the leaving group by sulfur. Nevertheless, to our
knowledge, there are no reports in the literature on the
benzenethiolysis and phenolysis of thiocarbonates where
the change of the oxygen atom of a carbonate by sulfur is
in the nonleaving group.
In this work, we examine the kinetics and mechanisms
of the phenolysis and benzenethiolysis of S-methyl
4-nitrophenyl and S-methyl 2,4-dinitrophenyl thiocarbo-
nates (1 and 2, respectively). By comparison of these
reactions with the phenolysis and benzenethiolysis of
the corresponding O-methyl carbonates (3 and 4),4,5 the
effect of substitution of O-methyl by S-methyl as the
nonleaving group will be assessed. Other goal is to
RESULTS AND DISCUSSION
For all the reactions, pseudo-first-order coefficients (kobs
)
were obtained (under nucleophile excess). The exper-
imental conditions of the reactions and the values of kobs
are shown in Tables 1–4.
The rate law obtained for all the reactions studied is given
by Eqns (1) and (2), where P, S and ArX– represent the
product (either 4-nitrophenoxide or 2,4-dinitrophenoxide
anions), the substrate and a substituted phenoxide (X ¼ O)
or benzenethiolate (X ¼ S) nucleophile, respectively; k0 is
´
*Correspondence to: J. G. Santos, Facultad de Quımica, Pontificia
Universidad Catolica de Chile, Casilla 306, Santiago 6094411, Chile.
´
E-mail: jgsantos@uc.cl
Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2007; 20: 533–538

534
E. A. CASTRO, M. ALIAGA AND J. G. SANTOS
Table 1. Experimental conditions and kobs values for the phenolysis of S-methyl 4-nitrophenyl thiocarbonate (1)a
b
Phenoxide substituent
4-OCH3
pH
FN
103[ArOH]totc/mol ꢂ dmꢁ3
105kobs sꢁ1
no. of runs
10.0
10.3
10.6
9.6
9.9
10.2
8.3
8.6
8.9
7.5
7.8
8.1
7.5
0.33
0.50
0.67
0.33
0.50
0.67
0.33
0.50
0.67
0.33
0.50
0.67
0.7152
0.9755
0.9937
1.0
8.87–35.5
3.56–35.6
3.71–37.1
1.91–19.1
1.96–19.6
1.93–19.3
1.54–15.4
1.32–13.2
3.31–13.2
1.73–17.3
2.02–20.2
1.51–15.1
5.97–50.7
6.36–63.6
6.36–63.6
1.52–15.2
1.41–14.1
1.63–16.3
171–386
77.1–594
88.2–846
6.59–44.0
8.94–70.2
7.02–119
1.62–15.5
3.65–26.8
9.47–32.4
1.51–3.69
1.37–5.55
1.55–5.90
2.07–7.71
1.58–12.2
3.02–12.5
0.126–0.464
0.453–0.678
0.739–1.12
6
6
6
7
7
7
7
6
6
7
6
7
7
7
7
7
6
6
H
3-CN
4-CN
2,6-F2
2,3,4,5,6-F5
8.7d
9.3d
8.5d
9.0d
9.5d
1.0
1.0
a In aqueous solution, at 25.0 8C, ionic strength 0.2 mol ꢂ dmꢁ3 (KCl).
b Fraction of free phenoxide.
c Concentration of total phenol (free phenoxide plus conjugate acid).
d Borate buffer 0.02 mol ꢂ dmꢁ3
.
the rate coefficient for hydrolysis and kN is the rate
coefficient either for phenolysis or benzenethiolysis of the
substrate.
The value of k0 was much lower than that of kN [ArX–]
in Eqn (2), except for the slow reactions of 2,3,4,5,6-
pentafluorophenol and 2,3,4,5,6-pentafluorobenzenethiol
with both substrates and those of thiolcarbonate 1 with
2,6-difluorophenol, where the kN [ArX–] term in Eqn (2)
was also small. The values of kN for all reactions were
obtained as the slopes of the linear plots of kobs versus
[ArX–], and were found to be pH independent. These kN
d½Pꢀ
¼ kobs½Sꢀ
(1)
(2)
dt
kobs ¼ k0 þ kN½ArXꢁꢀ
Table 2. Experimental conditions and kobs values for the phenolysis of S-methyl 2,4-dinitrophenyl thiocarbonate (2)a
b
Phenoxide substituent
4-OCH3
pH
FN
103[ArOH]totc/mol ꢂ dmꢁ3
103kobs/sꢁ1
no. of runs
10.0
10.3
10.6
9.6
9.9
10.2
8.3
8.6
8.9
7.5
7.8
8.1
6.8
7.1
0.33
0.50
0.67
0.33
0.50
0.67
0.33
0.50
0.67
0.33
0.50
0.67
0.33
0.50
0.715
1.0
3.55–35.5
3.56–35.6
3.71–37.1
1.91–16.3
1.96–19.6
4.83–19.3
1.54–13.1
1.32–13.2
1.32–13.2
1.73–17.3
3.78–15.1
1.51–15.1
5.55–47.2
5.61–47.7
5.97–50.7
1.52–15.2
1.41–14.1
1.63–16.3
4.04–22.5
3.14–29.9
4.24–38.5
0.339–1.66
0.415–2.97
1.59–4.07
0.078–0.831
0.211–1.54
0.254–2.03
0.0504–0.29
0.0995–0.397
0.0376–0.538
0.073–0.690
0.118–0.932
0.064–1.18
0.022–0.063
0.0173–0.064
0.044–0.102
6
7
7
6
7
6
6
6
6
6
6
6
6
6
7
7
6
7
H
3-CN
4-CN
2,6-F2
2,3,4,5,6-F5
7.5
8.5d
9.0d
9.5d
1.0
1.0
a In aqueous solution, at 25.08C, ionic strength 0.2 mol ꢂ dmꢁ3 (KCl).
b Fraction of free phenoxide.
c Concentration of total phenol (free phenoxide plus conjugate acid).
d Borate buffer 0.02 mol ꢂ dmꢁ3
.
Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2007; 20: 533–538
DOI: 10.1002/poc

KINETICS OF S-METHYL ARYL THIOCARBONATES
535
Table 3. Experimental conditions and kobs values for the benzenethiolysis of S-methyl 4-nitrophenyl thiocarbonate (1)a
103[ArSH]totc/mol ꢂ dmꢁ3
105kobs/sꢁ1
no. of runs
b
Benzenethiolate substituent
pH
FN
H
6.5
7.0
5.7
6.0
0.557
0.799
0.33
0.50
0.67
0.969
0.99
1
1
1
1
1
2.45–9.80
0.980–9.80
0.184–1.44
0.179–1.28
0.179–1.28
0.429–3.45
0.429–2.15
0.534–5.34
0.958–9.58
0.534–4.54
0.525–1.87
0.375–1.50
0.375–1.87
9.98–138
16.5–168
0.264–5.27
0.359–9.97
2.43–10.5
2.76–30.0
6.20–20.4
3.40–32.8
7.78–48.5
5.54–22.5
6
7
6
6
6
6
6
6
6
5
5
5
6
4-Cl
6.3
3-Cl
7.0d
7.5d
6.5d
6.6d
6.9d
7.0d
7.3d
7.6d
2,4-F2
2,3,4,5,6-F5
0.0317–0.0854
0.0332–0.0786
0.0403–0.0888
1
a In aqueous solution, at 25.08C, ionic strength 0.2 mol ꢂ dmꢁ3 (KCl).
b Fraction of free benzenethiolate.
c Concentration of total benzenethiol (free benzenethiolate plus conjugate acid).
d Buffer phosphate 0.01 mol ꢂ dmꢁ3
.
Table 4. Experimental conditions and kobs values for the benzenethiolysis of S-methyl 2,4-dinitrophenyl thiocarbonate (2)a
b
Benzenethiolate substituent
H
pH
FN
103[ArSH]totc/mol ꢂ dmꢁ3
103kobs/sꢁ1
No. of runs
6.0
6.5
7.0
5.7
6.0
6.3
5.8
0.285
0.557
0.799
0.33
0.50
0.67
0.67
0.969
0.99
1
0.98–8.33
2.45–8.33
0.98–8.33
0.184–1.44
0.179–1.28
0.179–1.02
0.862–8.62
0.429–2.15
0.429–4.74
0.534–5.34
1.33–5.34
0.525–2.10
0.375–1.87
0.375–2.10
4.80–63.5
6.36–99.9
8.59–172
0.290–3.54
0.317–3.84
0.354–4.08
5.13–38.9
9.70–20.4
9.92–42.6
5.74–36.8
6.91–35.3
6
5
6
6
6
6
5
5
6
6
6
5
6
7
4-Cl
3-Cl
7.0d
7.5d
6.5d
6.9d
7.0d
7.3d
7.6d
2,4-F2
1
1
1
1
2,3,4,5,6-F5
0.0304–0.108
0.0226–0.0994
0.0249–0.104
a In aqueous solution, at 25.08C, ionic strength 0.2 mol ꢂ dmꢁ3 (KCl).
b Fraction of free benzenethiolate.
c Concentration of total benzenethiol (free benzenethiolate plus conjugate acid).
d Buffer phosphate 0.01 mol ꢂ dmꢁ3
.
values are shown in Tables 5 and 6, together with the pKa
values of the series of nucleophiles employed.
Figure 1 shows the Brønsted-type plots obtained with
the values of kN and pKa in Tables 5 and 6 (measured
under the same experimental conditions).
The Brønsted-type plots for the reactions of the
two thiocarbonates, 1 and 2, are linear (Figure 1), with
slopes (b) 0.51 ꢃ 0.06 and 0.66 ꢃ 0.06, respectively, for
phenolysis, and 0.55 ꢃ 0.05 and 0.70 ꢃ 0.1, respectively,
for benzenethiolysis.
The values of the Brønsted slopes found for the title
reactions are in accordance with those obtained in the
concerted phenolysis of 2,4-dinitrophenyl acetate (b ¼
0.57)6 and 4-chloro-2-nitrophenyl acetate (b ¼ 0.64).7
They are also in agreement with those found in the
concerted phenolysis of 3-nitrophenyl, 4-nitrophenyl, and
3,4-dinitrophenyl formates (b ¼ 0.64, 0.51, and 0.43,
respectively)8 and the corresponding acetates (b ¼ 0.66,
0.59, and 0.53, respectively)8, and phthalic and maleic
anhydrides (b ¼ 0.45 and 0.56, respectively).9 The
Brønsted slopes exhibited by the reactions under investi-
gation are also similar to those found in the concerted
benzenethiolysis of ethyl 2,4-dinitrophenyl dithiocarbo-
nate (b ¼ 0.66),10 ethyl 2,4,6-trinitrophenyl dithioca-
rbonate (b ¼ 0.66),10 and methyl 2,4-dinitrophenyl
thionocarbonate (b ¼ 0.58).10 Therefore, the b values
obtained in the phenolysis and benzenethiolysis of the
two thiolcarbonates studied in this work suggest that these
Copyright # 2007 John Wiley & Sons, Ltd.
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DOI: 10.1002/poc

536
E. A. CASTRO, M. ALIAGA AND J. G. SANTOS
Table 5. Values of pKa for the phenols and kN values for the reactions of phenoxides with 4-nitrophenyl (1) and
2,4-dinitrophenyl (2) S-methyl thiocarbonatesa
102kN/sꢁ1 ꢂ molꢁ1 ꢂ dm3
Phenoxide substituent
pKa
1
2
4-OCH3
H
3-CN
4-CN
2,6-F2
2,3,4,5,6-F5
10.3
9.9
8.6
7.8
7.1
5.3
30.7 ꢃ 0.8
8.8 ꢃ 0.4
150 ꢃ 5
31 ꢃ 2
3.6 ꢃ 0.1
23 ꢃ 1
0.41 ꢃ 0.04
0.19 ꢃ 0.01
0.020 ꢃ 0.003
5.2 ꢃ 0.1
3.4 ꢃ 0.3
0.34 ꢃ 0.01
a Both the pKa and kN values were determined in aqueous solution, at 25.08C, ionic strength 0.2 mol ꢂ dmꢁ3 (KCl).
Table 6. Values of pKa for the benzenethiols and kN values for the reactions of benzenethiolates with 4-nitrophenyl (1) and
2,4-dinitrophenyl (2) S-methyl thiocarbonatesa
102kN/sꢁ1 ꢂ molꢁ1 ꢂ dm3
Benzenethiolate substituent
pKa
1
2
H
4-Cl
3-Cl
2,4-F2
2,3,4,5,6-F5
6.4
6.0
5.5
4.9
2.7
21.9 ꢃ 2.3
13.9 ꢃ 0.9
2370 ꢃ 160
645 ꢃ 37
8.6 ꢃ 0.6
507 ꢃ 70
5.2 ꢃ 0.3
669 ꢃ 35
0.0334 ꢃ 0.0027
4.37 ꢃ 0.19
a Both the pKa and kN values were determined in aqueous solution, at 25.08C, ionic strength 0.2 mol ꢂ dmꢁ3 (KCl).
reactions are ruled by a concerted mechanism. This is
shown in the Scheme (X ¼ O or S, Y ¼ 4-nitro or
2,4-dinitro).
of the Brønsted curvature (pK0a) would be 7.1 (the pKa of
4-nitrophenol). The absence of the Brønsted break is a
clear indication that this mechanism is not stepwise but
concerted.11 On the other hand, if the reaction of 2 were
stepwise the predicted pK0a value would be 4.1 (the pKa
of 2,4-dinitrophenol), which is outside the pKa range
studied. Nevertheless, although the Brønsted break can-
not be observed, the concerted nature of this reaction can
be deduced from the fact that the putative tetrahedral
intermediate formed in this reaction should be even more
unstable (in view of the better leaving group involved)
than that formed in the phenolysis of 1.
Clearly, the magnitude of b is not sufficient to validate
the concerted mechanism.11 If the mechanism for the
phenolysis of 1 were stepwise the pKa value for the center
For the benzenethiolysis of 2 the magnitude of the
Brønsted slope (b¼ 0.70) is near the lower limit of the
Brønsted slopes found in stepwise aminolyses of similar
substrates when the breakdown to products of the
zwitterionic tetrahedral intermediate (Tꢃ) is the rate-
determining step. For instance, Brønsted slope values of
0.8 or slightly lower have been reported for the stepwise
aminolysis of 2,4-dinitrophenyl acetate and 1-acetoxy-
4-methoxypyridinium ion.12
Nevertheless, the b value for the benzenethiolysis of 2
is also near the upper limit found for concerted reactions.
In fact, linear Brønsted plots with slope values similar or
greater than 0.7 have been obtained for the concerted
phenolysis of 4-nitrophenyl and 4-formylphenyl acetates
(b ¼ 0.75 and 0.79, respectively).7
Figure 1. Brønsted-type plots obtained in the phenolysis
(*) and benzenethiolysis (*) of (a) S-methyl 2,4-dinitro-
phenyl carbonate (2) and (b) S-methyl 4-nitrophenyl carbon-
ate (1), in aꢁq3ueous solution, 25.0 ꢃ 0.1 8C, ionic strength
0.2 mol ꢂ dm (KCl)
The above analysis indicates that the benzenethiolysis
of 2 is in the borderline between concerted and stepwise
mechanisms. Nevertheless, we are more inclined toward
Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2007; 20: 533–538
DOI: 10.1002/poc

KINETICS OF S-METHYL ARYL THIOCARBONATES
537
the concerted process as shown in the Scheme (Y ¼
2,4-(NO2)2, X ¼ S) for the following reasons: (i) If the
mechanism of this reaction were stepwise, the pKa0 value
would be ca. 4 (the pKa of 2,4-dinitrophenol). This is
due to the fact that there is no much difference between
the leaving abilities of 2,4-dinitrophenoxide and
2,4-dinitrobenzenethiolate from the anionic tetrahedral
intermediate.2a,4 The lack of a Brønsted break for the
benzenethiolysis of 2, covering a pKa range 2.7–6.4,
indicates that this mechanism is not stepwise. (ii) The
instability of the putative anionic tetrahedral intermediate
that would be formed if the reaction were stepwise would
be very high due to the great compression brought about
by the two sulfur atoms attached to the anionic tetrahedral
intermediate.
The value of the Brønsted slope for the benzenethio-
lysis of 1 (b ¼ 0.55) also suggests a concerted mechanism
(Scheme, Y ¼ 4-NO2, X ¼ S). This can be confirmed by
the following reasoning: If these mechanisms were
stepwise a Brønsted break at pK0a ca. 7 would be observed
(near the pKa of 4-nitrophenol, see discussion above).
Although this value is outside the pKa range of the
nucleophiles employed, it can be concluded that if the
break occurred at this pKa, the rate-determining step for
the experimental pKa range (2.7–6.4) would be the
breakdown of the putative Tꢁ to products. The value of
the Brønsted slope found (b ¼ 0.55) is too small
compared with those obtained for stepwise mechanisms
when breakdown of the anionic tetrahedral intermediate
is rate-limiting (b ¼ 0.8–1.1).2a,12
(3 and 4, respectively) are driven by a concerted
mechanism,5 as are the phenolyses of 1 and 2 (this
work). This indicates that substitution of SMe by OMe as
the ‘nonleaving’ group of the substrates does not change
the reaction mechanism.
Comparison of the nucleophilic rate constants (kN)
between the phenolysis of thiocarbonates 1 and 2 (this
work) and the phenolysis of the corresponding carbonates
(3 and 4),5 shows that the latter are more reactive (5 to
10-fold) toward phenoxides than thiocarbonates.
The higher reactivity of carbonates than thiocarbonates
toward phenoxide anions seems at first sight surprising in
view of the stronger electron-withdrawing effect of
SMe in a thiocarbonate compared to OMe in the
corresponding carbonate,14 which should result in the
carbonyl carbon of the thiocarbonate being more
positively charged and therefore more prone to nucleo-
philic attack by phenoxide, relative to carbonate. The
lower kN values for the phenolysis of thiocarbonates can
be attributed to steric hindrance toward phenoxide attack
by the bulkier sulfur atom in a thiocarbonate compared to
the oxygen atom in the corresponding carbonate. This is
also true when the S and O atoms are in the leaving group:
the nucleophilic rate constants (kN) for the phenolysis of
methyl aryl carbonates are larger than those of the
corresponding S-aryl thiocarbonates.3,5
Comparison of the nucleophilic rate constants for the
benzenethiolysis of 2 (this study) with those for the
corresponding carbonate (4)4 shows that the reactivities
of these compounds toward benzenethiolates are similar.
In view of the relative soft nature of benzenethiolates, it
should be expected a greater reactivity of these
nucleophiles toward the carbonyl group of thiocarbonate
2 compared with that of carbonate 4, since the carbonyl
group of a thiocarbonate should be softer than that of a
carbonate.13 Nevertheless, it is likely that this greater
affinity is counterbalanced by a greater steric hindrance,
toward the attack of benzenethiolate anion, of the sulfur
atom in 2, relative to oxygen in the corresponding
carbonate.
Phenolysis versus Thiolysis
Figure 1 shows a comparison of the Brønsted plots of the
nucleophilic rate constants for the benzenethiolysis and
phenolysis of thiocarbonates 1 and 2. It can be observed
that in both cases benzenethiolysis is faster than
phenolysis toward a carbonyl carbon within the pKa
range studied. A similar result was found in the same
reactions of 2,4-dinitrophenyl and 2,4,6-trinitrophenyl
methyl carbonates,4,5S-(2,4-dinitrophenyl) and S-(2,4,6-
trinitrophenyl) ethyl thiocarbonates,3,4 4-nitrophenyl
acetate2a and 4-nitrophenyl formate.2b This was explained
by the softer character of the sulfur nucleophile,
compared with the oxygen nucleophile, which favors
the binding of the former to the relatively soft carbonyl
carbon.13 This can be confirmed by the fact that
benzenenethiolates show an additional rate enhancement,
relative to isobasic phenoxide ions, toward 4-nitrophenyl
thiolacetate, which has a softer electrophilic group than
that of the corresponding carbonate.2a
CONCLUSIONS
The reactions of S-methyl 4-nitrophenyl thiocarbonate
(1), and S-methyl 2,4-dinitrophenyl thiocarbonate (2)
with a series of phenols and benzenethiols are studied
kinetically in water. The Brønsted slopes, together with
other evidence suggest a concerted mechanism for these
reactions.
By the comparison of the kinetics and mechanisms of
these reactions with those of similar reactions, the
following conclusions arise: (i) Thiocarbonates 1 and 2
react with phenoxide and benzenethiolate ions through
concerted mechanisms. (ii) Substitution of OMe in
carbonates by SMe as the nonleaving group does not
affect the mechanism. (iii) Benzenethiolates are more
Carbonates vs. Thiocarbonates
It has been found that the phenolyses of methyl
4-nitrophenyl and methyl 2,4-dinitrophenyl carbonates
Copyright # 2007 John Wiley & Sons, Ltd.
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DOI: 10.1002/poc

538
E. A. CASTRO, M. ALIAGA AND J. G. SANTOS
reactive than isobasic phenoxide ions toward both 1 and 2.
(iv) S-Methyl thiocarbonates are less reactive than
the corresponding carbonates toward phenoxide ions.
(v) Thiocarbonate 2 is as reactive as the corresponding
carbonate (4) toward benzenethiolate ions.
and ionic strength 0.2 mol ꢂ dmꢁ3 maintained with KCl).
The pKa value found was 4.9 ꢃ 0.2.
Product studies
EXPERIMENTAL
Materials
4-Nitrophenoxide and 2,4-dinitrophenoxide anions were
identified as one of the products of the phenolysis and
benzenethiolysis of 1 and 2, respectively. This was
achieved by comparison of the UV-vis spectra after
completion of these reactions with those of authentic
samples of 4-nitrophenol and 2,4-dinitrophenol under the
same reaction conditions.
The series of phenols employed were purified by
distillation or recrystallization. The series of benze-
nethiols were used as purchased. Thiolcarbonates 1 and 2
were synthesized as described.15
Acknowledgements
Kinetics
The authors thank MECESUP of Chile (Projects
PUC-0004 and RED QUIMICA UCH-01) and FONDE-
CYT of Chile (project 1060593). The second author
thanks CONICYT of Chile (for a doctoral fellowship
and project AT-2405019).
The reactions were followed spectrophotometrically
(300–500 nm) by monitoring the corresponding
4-nitrophenoxide or 2,4-dinitrophenoxide anions by
means of a Hewlett-Packard 8453 instrument. The
reactions were studied in aqueous solutions, at
25.0 ꢃ 0.18C, and an ionic strength of 0.2 mol ꢂ dmꢁ3
(maintained with KCl). Phosphate and borate buffers
were used in some reactions (see Tables 1–4). The
reactions were started by injection of a substrate stock
solution in acetonitrile (10 ml) into the nucleophile
aqueous solution (2.5 ml) in the spectrophotometric cell.
The initial substrate concentration was 5 ꢄ 10ꢁ5 mol ꢂ
dmꢁ3. A rapid mixer apparatus was used in some of the
fastest reactions. At least a 10-fold excess of total phenol
or total benzenethiol (the anion plus its conjugate acid)
over the substrate was employed in all reactions.
Pseudo-first-order rate coefficients (kobs) were found
in all cases. These were obtained by means of the kinetic
software of the spectrophotometer, after at least 4
half-lives, except for the slower reactions (1 with 2,6-difl-
uorophenol, 2,3,4,5,6-pentafluorophenol, 4-chlorobence-
nethiol, 2,4-difluorobenzenethiol and 2,3,4,5,6-penta-
fluorobenzenethiol and those of 2 with 2,3,4,5,6-
pentafluorobenzenethiol) where the initial rate method
was used.16
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Determination of pKa
The pKa value for 2.4-difluorbenzenethiol was deter-
mined spectrophotometrically at 250 nm by the method
reported.17 The experimental conditions used were the
same as those for the kinetic measurements (25.0 ꢃ 0.18C
Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2007; 20: 533–538
DOI: 10.1002/poc