Kinetics of the pH-Independent Hydrolysis of Bis(2,4-dinitrophenyl) Carbonate in Acetonitrile-Water Mixtures: Effects of the Structure of the Solvent

Article abstract of DOI:10.1021/jo970070x

The pH-independent hydrolysis of bis(2,4-dinitrophenyl) carbonate, DNPC, in aqueous acetonitrile was studied spectrophotometrically from 20 to 45 °C. The binary solvent composition covers [H₂O] from 0.02 to 51.39 M, corresponding to the water mole fraction, χ_w, from 0.100 to 0.971. The dependence of log (k_obs), the observed rate constant, on χ_w is sigmoidal and is similar to the dependence of the solvent polarity scale E_T(30) on χ_w for the same solvent mixture. As a function of decreasing χ_w, the Gibbs free energy of activation gradually increases, but ΔH? and ΔS? show a complex, quasi-mirror image dependence on χ_w. Plots of log (k_obs) versus log [water] do not allow calculation of a single kinetic order with respect to water over the entire range of [water]. The structure of the transition state was probed by a proton inventory study carried out at χ_w = 0.453, 0.783, and 0.871, respectively. Plots of observed rate constants versus the atom fraction of deuterium in the solvent curve downward, and the results were fitted to a transition-state model that contains DNPC and two water molecules. Thus, the sigmoidal dependence of log (k_obs) on log [water] is not due to an increase in the number of water molecules in the transition state as a function of increasing [water]. The similarity of plots of log (k_obs) versus χ_w and E_T(30) versus χ_w suggest similar solute- solvent interaction mechanisms, namely H-bonding and dipolar interactions. Kinetic results are discussed in terms of effects of the structure of acetonitrile-water mixtures on the solvation of reactant and transition states.

Full text of DOI:10.1021/jo970070x

5928
J . Org. Chem. 1997, 62, 5928-5933
Kin etics of th e p H-In d ep en d en t Hyd r olysis of
Bis(2,4-d in itr op h en yl) Ca r bon a te in Aceton itr ile-Wa ter Mixtu r es:
Effects of th e Str u ctu r e of th e Solven t
Omar A. El Seoud,* Monica I. El Seoud, and J oa˜o P. S. Farah
Instituto de Quı´mica, Universidade de Sa˜o Paulo, C.P. 26077, 05599-970 Sa˜o Paulo, S.P., Brazil
Received J anuary 14, 1997X
The pH-independent hydrolysis of bis(2,4-dinitrophenyl) carbonate, DNPC, in aqueous acetonitrile
was studied spectrophotometrically from 20 to 45 °C. The binary solvent composition covers [H2O]
from 0.02 to 51.39 M, corresponding to the water mole fraction, øw, from 0.100 to 0.971. The
dependence of log (kobs), the observed rate constant, on øw is sigmoidal and is similar to the
dependence of the solvent polarity scale ET(30) on øw for the same solvent mixture. As a function
of decreasing øw, the Gibbs free energy of activation gradually increases, but ∆Hq and ∆Sq show a
complex, quasi-mirror image dependence on øw. Plots of log (kobs) versus log [water] do not allow
calculation of a single kinetic order with respect to water over the entire range of [water]. The
structure of the transition state was probed by a proton inventory study carried out at øw ) 0.453,
0.783, and 0.871, respectively. Plots of observed rate constants versus the atom fraction of deuterium
in the solvent curve downward, and the results were fitted to a transition-state model that contains
DNPC and two water molecules. Thus, the sigmoidal dependence of log (kobs) on log [water] is not
due to an increase in the number of water molecules in the transition state as a function of increasing
[water]. The similarity of plots of log (kobs) versus øw and ET(30) versus øw suggest similar solute-
solvent interaction mechanisms, namely H-bonding and dipolar interactions. Kinetic results are
discussed in terms of effects of the structure of acetonitrile-water mixtures on the solvation of
reactant and transition states.
In tr od u ction
solvent “polarity” is a deceptively simple term because
calculated ET values contain contributions from dipolar-
ity/polarizability and H-bond donation (acidity) and ac-
ceptance (basicity) of both the solvent and the solvato-
chromic probe. Consequently, correlations between, e.g.,
rate constants or equilibrium constants and the solvent
polarity are complex both in pure and mixed solvents.
In the latter case, preferential solvation of the relevant
species (probes, reactants, and transition states) by one
component of the solvent mixture introduces additional
complexity, and the importance of this factor has been
only recently analyzed in some detail.2,7-10
Binary mixtures of water and organic solvents are
extensively used in organic synthesis and in physical
organic chemistry studies of, e.g., acyl-transfer reactions.
The reasons are partially practical and are often con-
nected with solubility constraints. On the other hand,
there is an increasing interest in understanding the
effects of pure solvents and their mixtures on chemical
processes, i.e., on rates and equilibria of chemical
reactions.1-6 In the analysis of solvent effects, extensive
use is made of their microscopic polarities determined
with solvatochromic probes. The so called ET scale is
calculated from the relationship
We report here on the pH-independent hydrolysis of
bis(2,4-dinitrophenyl) carbonate, DNPC, in acetonitrile-
water mixtures over [H2O] 0.02 to 51.39 M, corresponding
to the water mole fraction range øw 0.100 to 0.971. We
used DNPC because mechanisms of “water reactions” are
relatively simple1 and because data are available for the
spontaneous hydrolysis of the structurally similar bis(4-
nitrophenyl) carbonate, NPC.11-13 Additionally, water is
ET (kcal/mol) ) 28591.5/λmax (nm)
(1)
where λmax is the wavelength of maximum UV-vis
absorption of the probe.2 It is now realized, however, that
* To whom correspondence should be addressed. Fax/Phone: +55-
11-818-3874. E-mail: elseoud@iq.usp.br.
X Abstract published in Advance ACS Abstracts, J uly 1, 1997.
(1) Engberts, J . B. F. N. Pure Appl. Chem. 1982, 54, 1797 and
references cited therein.
(7) (a) Suppan, P. J . Chem. Soc., Faraday Trans. 1 1987, 83, 495.
(b) Suppan, P. J . Chem. Soc., Faraday Trans. 1 1988, 85, 173. (c)
Suppan, P. J . Photochem. Photobiol. 1990, 59, 293 and references cited
therein. (d) Lerf, C.; Suppan, P. J . Chem. Soc., Faraday Trans. 1992,
88, 963.
(8) (a) Novaki, L. P.; El Seoud, O. A. Ber. Bunsenges. Phys. Chem.
1996, 100, 648. (b) Novaki, L. P.; El Seoud, O. A. Ber. Bunsenges. Phys.
Chem, in press. (c) Novaki, L. P.; El Seoud, O. A. Ber. Bunsenges. Phys.
Chem. In press.
(9) (a) Toselli, N. B.; Silber, J . J .; Anunziata, J . D. Spectrochim. Acta
1988, 44A, 829. (b) Cattana, R.; Silber, J . J .; Anunziata, J . Can. J .
Chem. 1992, 70, 2677. (c) Boggetti, H.; Anunziata, J . D.; Cattana, R.;
Silber, J . J . Spectrochim. Acta 1994, 50, 719. (d) Nitsche, K. S; Suppan,
P. Chimia 1982, 36, 346.
(10) (a) Marcus, Y.; Migron, Y. J . Phys. Chem. 1991, 95, 400. (b)
Marcus, Y. Chem. Soc. Rev. 1993, 22, 409 and references cited therein.
(11) (a) Fife, T. H.; McMahon, D. M. J . Am. Chem. Soc. 1969, 91,
7481. (b) Fife, T. H.; McMahon, D. M. J . Org. Chem. 1970, 35, 3699.
(12) Menger, F. M.; Venkatasubban, K. S. J . Org. Chem. 1976, 41,
1868.
(2) (a) Reichardt, C. Solvents and Solvent Effects in Organic
Chemistry; VCH: New York, 1988 and references cited therein. (b)
Reichardt, C. Chem. Soc. Rev. 1992, 21, 147. (c) Reichardt, C., Chem.
Rev. 1994, 94, 2319.
(3) (a) Isaacs, N. S. Physical Organic Chemistry; Longman: London,
1987; p 171. (b) Carey, F. A.; Sundberg, R. J . Advanced Organic
Chemistry; Plenum: New York, 1990; Part B, pp 20, 128.
(4) (a) Dawber, J . G.; Ward, J .; William, R. A. J . Chem. Soc., Faraday
Trans. 1 1988, 84, 713. (b) Dawber, J . G.; Ward, J .; William, R. A. J .
Chem. Soc., Faraday Trans. 1990, 86, 287.
(5) (a) Ferris, D. C.; Drago, R. S. J . Am. Chem. Soc. 1994, 116, 7509.
(b) Wijnen, J . W; Zavarise, S.; Engberts, J . B. F. N. J . Org. Chem. 1996,
61, 2001.
(6) (a) Shaskus, J .; Haake, P. J . Org. Chem. 1983, 48, 2036. (b)
Gopalakrishnan, G.; Hogg, J . L. J . Org. Chem. 1983, 48, 2038. (c) Haak,
J . R.; Engberts, J . B. F. N. J . J . Org. Chem. 1984, 49, 2387. (d)
Neuvonen, H. J . Chem. Soc., Perkin Trans. 2 1987, 1723. (e) El Seoud,
O. A.; Novaki, L. P. An. Assoc. Bras. Quı´m. 1996, 45, 10 and references
cited therein.
(13) Gopalakrishnan, G.; Hogg, J . L. J . Org. Chem. 1984, 49, 3161.
S0022-3263(97)00070-4 CCC: $14.00 © 1997 American Chemical Society

Hydrolysis of Bis(2,4-dinitrophenyl) Carbonate
J . Org. Chem., Vol. 62, No. 17, 1997 5929
Sch em e 1
Ta ble 1. Dep en d en ce of th e Obser ved Ra te Con sta n ts
(kobs) on [Wa ter ] or th e Mole F r a ction of Wa ter (øw ) a t
Differ en t Tem p er a tu r es for th e p H-In d ep en d en t
Hyd r olysis of DNP C in Aceton itr ile-Wa ter Mixtu r es
a solvent and a reactant; therefore, kinetic results can
be used to probe the properties of the binary solvent
mixture and solute-solvent interactions. We show that
the complex dependence of reaction rate constants and
activation parameters on [H2O] is not due to a change in
the number of water molecules in the transition state. It
reflects, however, effects of acetonitrile-water interac-
tions on solvation of reactant and transition states.
[water],
M/water
mole fraction,
øw
104kobs
s-1
20 °C
,
104kobs
s-1
25 °C
,
104kobs
s-1
30 °C
,
104kobs
s-1
35 °C
,
104kobs
s-1
45 °C
,
,
,
,
,
,
[2.02]/0.1
0.22
1.44
3.41
4.92
6.39
0.29
1.98
4.89
7.38
9.75
12.97
15.39
18.36
22.89
42.18
56.27
93.53
123.04
0.32
2.70
6.98
0.41
3.60
9.62
0.71
6.20
[4.91]/0.221
[8.74]/0.351
[12.29]/0.453
[16.31]/0.552
[21.86]/0.649
[25.25]/0.701
[28.36]/0.746
[31.31]/0.783
[39.52]/0.871
[42.81]/0.902
[48.43]/0.948
[51.39]/0.971
18.23
28.54
40.08
51.02
63.11
79.71
97.25
168.38
225.85
320.37
396.17
Resu lts An d Discu ssion
10.49
14.74
19.13
22.29
28.55
34.38
63.81
84.58
135.26
162.11
15.13
20.50
27.30
32.51
38.81
45.59
78.26
105.57
160.80
207.94
The reaction investigated is given by Scheme 1. It is
a water reaction over the entire øw range because kobs is
independent of [HCl] from 0.005 to 0.05 M for øw ) 0.221,
0.783, and 0.948, respectively. Hydrolysis of DNPC
produces 2 equiv of 2,4-dinitrophenol, and the reaction
shows a sharp isosbestic point at λ ) 272 nm (25 °C, 30%
acetonitrile in water). Additionally, excellent pseudo-
first-order kinetics were always observed; identical rate
constants were obtained in the presence or absence of
1.5 × 10-5 M 2,4-dinitrophenol; an initial rapid release
of 2,4-dinitrophenol was not detected. Thus, by analogy
to the pH-independent hydrolysis of NPC, the intermedi-
ate of the first step (the monoester of carbonic acid, eq
2) does not accumulate; i.e., the first step is rate
limiting.11-13
8.24
10.01
11.65
14.70
27.89
37.62
62.03
83.34
reduced dimensionless form [ET(30)red, eq 5], and øw. This
ET (30)red ) [ET(30)binary mixture - ET(30)acetonitrile]/
[ET(30)water - ET(30)acetonitrile] (5)
polarity scale is based on 2,6-diphenyl-4-(2,4,6-triphenyl-
1-pyridinio)-1-phenolate, RB, the most studied solvato-
chromic probe both in pure and in mixed solvents.2 The
points are from the literature,2 and the solid sigmoidal
line was calculated by a fifth power polynomial depen-
dence of ET(30)red on øw. Although this gives an excellent
fit (correlation coefficient, cc, ) 0.9998, ∑Q ) 0.0056), a
fourth power polynomial dependence also gives a satis-
factory fit (cc ) 0.9963, ∑Q ) 0.0221) The plots in Figure
1A,B show a remarkable resemblance, a fact that will
be discussed below.
The slope of a linear plot of log (kobs) versus log [water]
is the reaction kinetic order with respect to water;
however, such plots (not shown) are sigmoidal for DNPC.
An order of 2.1 has previously been calculated for the
spontaneous hydrolysis of NPC in aqueous acetonitrile
for øw from 0.23 to 0.43.13 For øw from 0.100 to 0.453,
our data yield a similar order (1.9 ( 0.2), but for øw 0.871
to 0.971 they yield temperature-dependent orders that
decrease from 4.1 at 20 °C to 3.2 at 45 °C. Thus, a single
kinetic order with respect to water cannot be obtained
from our data over the entire range of [H2O]!
There is some discrepancy between rate constants of
the hydrolysis of NPC, which were reported by two
different groups using almost identical experimental
conditions;12,13 inhomogeneity problems may have led to
13
the reported differences in kobs
.
Because DNPC is more
hydrophobic than NPC, we paid attention to solution
homogeneity, especially in the water-rich region. We
found that kobs is independent of [ester] in the range (1.0-
4.0) × 10-5 M. In the log (absorbance) versus time plots
we judged the quality of fit both from value of the sum
of squares of the residuals, ∑Q, and from change of
residuals with time, always at random. The adverse
consequences of solution inhomogeneity on the accuracy
of rate data have been discussed elsewhere.14
Table 1 shows the dependence of kobs on [water] or øw
from 20 to 45 °C. A plot of log (kobs) versus øw is shown
in part (A) of Figure 1; the points are experimental, and
the solid sigmoidal lines were calculated by a fourth
power polynomial dependence of log (kobs) on øw (eq 4),
log (kobs) ) a + b(øw) + c(øw)2 + d(øw)3 + e(øw)4 (4)
The dependence of the activation parameters on øw is
listed in Table 2 and is shown graphically in Figure 2.
As a function of increasing øw, ∆Gq gradually decreases
due to complex variations, i.e., an initial increase then
decrease of both ∆Hq and ∆Sq.
How can the dependence of log (kobs) and of the
activation parameters on øw be explained? The above-
mentioned kinetic orders may be taken to indicate that
where a-e are regression coefficients. Figure 1B shows
the relationship between ET(30) values, plotted in its
(14) (a) Bunnett, J . F. In Techniques of Chemistry; Bernasconi, C.
F., Ed.; Wiley-Interscience: New York, 1986; p 299. (b) El Seoud, M.
I.; El Seoud, O. A. Educ. Chem. 1994, 31, 105. (c) El Seoud, O. A.; da
Silva, R. L.; El Seoud, M. I. An. Assoc. Bras. Qu´ım. 1994, 43, 109.

5930 J . Org. Chem., Vol. 62, No. 17, 1997
El Seoud et al.
F igu r e 1. (A) Dependence of log (kobs), the observed rate constant on øw, the mole fraction of water in aqueous acetonitrile, at
different temperatures. The points are experimental and the solid curve was calculated from eq 4. (B) Dependence of the solvent
polarity scale ET(30) on øw. The points were taken from ref 20b, and the solid curve was calculated from a fifth power polynomial
dependence of ET(30) on øw.
Ta ble 2. Activa tion P a r a m eter s for th e p H-In d ep en d en t
Hyd r olysis of DNP C in Aceton itr ile-Wa ter Mixtu r esa
water mole
fraction, øw
∆Hq,
kcal/mol
∆Sq,
cal/K mol
∆Gq,
kcal/mol
0.1
7.8
10.2
11.8
12.4
13.0
12.9
13.0
13.5
13.2
12.4
12.4
11.2
10.7
-53.1
-41.2
-34.0
-31.3
-28.8
-28.7
-27.8
-25.7
-26.4
-27.7
-27.1
-30.2
-31.5
23.6
22.5
21.9
21.7
21.6
21.5
21.3
21.2
21.1
20.7
20.5
20.2
20.1
0.221
0.351
0.453
0.552
0.649
0.701
0.746
0.783
0.871
0.902
0.948
0.971
a
The errors are ( 0.1 kcal/ mol (∆Hq and ∆Gq) and 0.5 cal/K
mol (∆Sq).
the number of water molecules in the transition state,
TS, depends on [H2O]. Examples of such dependence are
known, e.g., the kinetic order with respect to water for
the spontaneous hydration of chloral varies from ca. 4 to
ca. 2 when [water] is increased from 0.25 to 46.3 M.15
The TS for the hydration of 1,3-dichloroacetone contains
the ketone and either one, two, or three molecules of
water, depending on whether the reaction is carried out
in the presence of detergent aggregates in n-hexane (i.e.,
reverse micelles and water-in-oil microemulsion) or in
aqueous dioxane, respectively.16,17 Finally, the number
of water molecules in the transition state of the imida-
F igu r e 2. Dependence of the reaction activation parameters
on øw.
zole-catalyzed hydrolysis of 4-nitrotrifluoroacetanilide in
aqueous acetonitrile was reported to increase from one
to four as a function of increasing [water].18
In order to probe the number of water molecules in the
TS, we subjected the reaction to a proton inventory study
that we carried out at øw ) 0.453, 0.783, and 0.871,
respectively. The use of proton inventory to probe the
structure of TS is documented,13,16,19 and only a brief
(15) Sørensen, P. S. Acta Chem. Scand. 1976, A30, 673.
(16) El Seoud, O. A.; Vieira, R. C.; Farah, J . P. S. J . Org. Chem.
1981, 46, 1231.
(17) Bell, R. P.; Critchlow, J . E. Proc. R. Soc. London, Ser. A 1971,
325.
(18) Henderson, J . W.; Haake, P. J . Org. Chem. 1977, 42, 3989.

Hydrolysis of Bis(2,4-dinitrophenyl) Carbonate
J . Org. Chem., Vol. 62, No. 17, 1997 5931
Ta ble 3. Dep en d en ce of th e Ra te Con sta n ts (kn ) for th e
Hyd r olysis of DNP C on th e Atom F r a ction of Deu ter iu m
(n ) in Mixtu r es of Aceton itr ile-H2O-D2O a t Differ en t
Wa ter Mole F r a ction s, øw , a t 25 °C
atom fraction of
104kn, s-1
,
104kn, s-1
,
104kn, s-1
,
deuterium, n
øw ) 0.453
øw ) 0.783
øw ) 0.871
0.000
0.167
0.327
0.477
0.662
0.834
0.998
7.380
6.652
5.978
5.366
4.655
4.061
3.252
22.891
20.423
18.165
16.413
14.098
12.308
10.400
42.181
37.824
34.143
30.689
26.806
23.631
20.690
account will be given here. One uses the observed solvent
kinetic isotope effect to estimate the number of hydrogens
(hence water molecules in our case) that participate in
the TS. This number, and other chemical information,
e.g., activation parameters, enable one to suggest a
structure for the TS. The observed rate constant, kn, in
a H2O-D2O mixture is related to the observed rate
constant in pure H2O, ko, by the equation
F igu r e 3. Results of the proton inventory study of the
reaction, showing the dependence of observed rate constant
(kn) on the atom fraction of deuterium (n) in acetonitrile-H2O-
D2O mixtures. The points are experimental, and the solid
curves were calculated by eq 8.
q
TS
Π
i
GS
kn/ko )
(1 - n + næi )/ (1 - n + næj) (6)
Π
j
where n is the atom fraction of deuterium in the solvent.
All TS hydrogens (i) that contribute to the isotope effect
possess a term in the right-hand numerator of eq 6.
These terms are multiplied, as are the terms in the right-
hand denominator, corresponding to the exchangeable
ground state, GS, hydrogens (j). Each hydrogen site is
associated with a fractionation factor defined as, e.g., for
an exchangeable site (s);
æS ) [D/H]s/[D/H]solvent
(7)
F igu r e 4. Structure of TS for the spontaneous hydrolysis of
NPC.13
i.e., æs expresses the preference of deuterium for the site
(s) relative to that of bulk solvent. Analysis of the present
water reaction is simplified by the fact that DNPC carries
no exchangeable hydrogens, and the other reactant,
water, has a æ ) 1 by definition. Consequently, the
denominator of eq 6 is unity, and we need consider only
hydrogens of the TS.
kinetic isotope effect.12 Gopalakrishnan and Hogg stud-
ied the hydrolysis of NPC under almost identical experi-
mental conditions and found that the kn versus n plot
curves downward. They fitted their data to eq 8
q
kn ) ko(1 - n + næa )(1 - n + næcq)2
(8)
Table 3 and Figure 3 show the dependence of kn on n
for three different øw.
q
where Ha contributes a primary isotope effect (æa
)
0.565), Hb does not contribute an isotope effect, and Hc
contributes a secondary isotope effect (æcq ) 0.889).13 That
is, both groups agree that the TS contains NPC and two
water molecules, but there is discrepancy regarding the
degree of transfer of Ha. It is interesting to note that
although Menger’s data were fitted to a straight line, they
The next step is to fit our data to a chemically plausible
model for the transition state.20 Before doing this, it is
instructive to discuss proton inventory results of the
spontaneous hydrolysis of NPC.12,13 Menger and Ven-
katasubban adjusted their data to a straight line. This
would lead to a TS that contains one water molecule, a
suggestion that was discarded in favor of a TS containing
NPC and two water molecules. In order to account for
the linearity of their kn versus n plot, they argued that
the TS is reached at an early stage of the reaction, i.e.,
when there is little transfer of Ha from the “nucleophilic”
water to the “general base” water, as shown in Figure 4
(for sake of simplicity, we draw Ha symmetrically be-
tween the two water molecules). According to their
assumption, the observed isotope effect is essentially due
to Ha, whereas Hb is expected to have a fractionation
factor of unity and should not contribute to the observed
can be made to fit eq 8 with æa ) 0.51 and æcq ) 0.92
simply by taking into account the reported error in ko!
We attribute these possibilities of fit to the small degree
of curvature of the kn versus n plot.
q
Because of the structural similarity between NPC and
DNPC and the similarity of their solvent kinetic isotope
effects, 2.2 for NPC,13 2.27, 2.20, and 2.04 for the
hydrolysis of DNPC at øw ) 0.453, 0.783, and 0.871,
respectively, we fitted our data, Figure 3, to eq 8 The
quality of the data fit is shown by ∑Q, between 1.2 ×
10-6 and 5 × 10-5, and the resulting fractionation factors
are æa ) 0.583, 0.588, and 0.708 and æcq ) 0.901, 0.818,
q
and 0.832 for øw ) 0.453, 0.783, and 0.871, respectively.
These fractionation factors are similar to those obtained
for NPC; they show some dependence on øw, probably
because of slight differences in the degree of transfer of
(19) Schowen, K. B. In Transition States for Biochemical Processes;
Gandour, R. D., Schowen, R. L., Eds.; Plenum Press: New York, 1978;
p 225.
(20) Alvarez, F. J .; Schowen, R. L. Isot. Org. Chem. 1987, 7 (Second.
Solvent Isot. Eff.), 1 and references cited therein.

5932 J . Org. Chem., Vol. 62, No. 17, 1997
El Seoud et al.
Ha. The relevant point is that three hydrogens are
contributing to the kinetic solvent isotope effect, i.e., the
TS contains DNPC and two water molecules, over the
entire range of øw.
10-6 M),2 is solubilized in the ranges 0.4 g øw g 0.8 by
acetonitrile-water complexes that are either rich in
acetonitrile (for øw e 0.4) or rich in water (for øw g 0.8).
Alternatively, it is solubilized by the acetonitrile clusters
in the region of microheterogeneity (0.4 e øw e 0.8). On
the other hand, 1-methyl-3-oxypyridinium betaine (solu-
bility in pure water >2.0 M)8a forms its own solvation
sphere, the composition of which is always rich in water.8c
The remarkable similarity of Figure 1A,B indicates that
both processes, i.e., solvation of RB and hydrolysis of
DNPC are sensitive to the same interaction mechanisms,
mainly H-bonding and dipolar interactions. (iii) Both RB
and DNPC are hydrophobic substrates, and there are
certainly solvent effects both on GS and TS. One way to
explain our data is that solvation of the TS does not
change much as a function of øw, but the observed solvent
effects could be due to the smaller number of GS
molecules with adequate solvation to reach the TS. This
limiting case does not fit our results because it implies
an almost constant ∆Hq;6a this is not observed, Table 2.
Because the Gibbs free energies of transfer of several
nonelectrolytes from water to mixtures of water and
aprotic solvents are relatively insensitive to the binary
solvent composition,27,28 it is plausible that kinetic solvent
effects on the GS are not dominant in the present
reaction, so that we concentrate on solvation of the TS.6b-e
(iv) In the acetonitrile molecule the partial negative
dipole charge is localized on the nitrogen while the
positive charge of the dipole is diffused onto the methyl
group. Hence, it can solvate positive centers much
stronger than negative centers. On the other hand, the
water molecule is capable of solvating both types of
centers effectively. In the TS shown in Figure 4, the
partial negative charge is still localized on the carbonyl
oxygen while the positive charge is diffused; i.e., the TS
is more efficiently solvated by water, and solvent kinetic
effects should be sensitive to the state of water in the
binary solvent mixture.
Before addressing solvent effects on the hydrolysis of
DNPC, we discuss some relevant aspects of the structure
of acetonitrile-water mixtures:2,7-10,21-24 when the or-
ganic solvent is added to water it replaces the uncoordi-
nated water molecules. The limit of øW beyond which
acetonitrile cannot be accommodated within the cavities
of ordinary water is øw ≈ 0.85.10,21 Below this øw limit
solvent microheterogeneity sets in, and there exists two
“microdomains”, one highly structured consisting pre-
dominantly of coordinated water molecules and a rela-
tively disordered one containing mostly acetonitrile
H-bonded to water molecules.21,22 Calculations of the
Kirkwood-Buff integral functions showed that the prob-
ability of finding a water molecule close to an acetonitrile
molecule is minimum at øw ≈ 0.65.10,23a,b Addition of more
acetonitrile leads to a change in the relative concentra-
tions of the two microdomains, but their compositions do
not vary appreciably. At øw e 0.3 the water clusters have
become so few and far apart that new interactions set
in. Water-acetonitrile interactions become important;
this results in the formation of complexes, e.g., (CH3CN)m
- H2O where m ) 1-4.10,23c,24a,b At still lower øw values,
the structure of acetonitrile dominates, and species such
as (CH3CN)m and CH3CN are abundant.23c It should be
born in mind, however, that the onset of formation of the
different regions is not sharp and is dependent on the
method used to study the system.10
In addressing the results shown in Figures 1 and 2,
the following points are relevant: (i) A detailed analysis
of solvent kinetic effects on hydrolysis of DNPC over the
whole øw range is beyond the scope of the present work
and may not even be possible because of the above-
mentioned solvent microheterogeneity. This explains the
fact that studies in which overall solvent kinetic effects
have been separated into contributing interactions of
solvent components with GS and TS have been confined
to the water-rich region.6c,25,26 Additionally, experimental
determination of thermodynamic functions of transfer of
DNPC from a reference solvent to aqueous acetonitrile
mixtures that could shed light on solvent interactions
with GS and TS27 is not feasible because of the facile
hydrolysis of this ester. (ii) Our results indicate that
solvatochromism of RB in acetonitrile-water mixtures
is very different from that of the small, hydrophilic probe
1-methyl-3-oxypyridinium betaine.8c This indicates that
RB, being very hydrophobic (solubility in water ) 7.2 ×
The initial addition of acetonitrile to water (øw from
0.971 to 0.871) results in a decrease of kobs and increase
in ∆Hq and ∆Sq since uncoordinated water molecules
would be considerably more polar (i.e., of higher kinetic
reactivity) and form stronger H-bonds than their coor-
dinated counterparts.1,29 In the intermediate øw region
(øw from 0.783 to 0.552), the structure of each micro-
domain should remain relatively constant so long as they
exist.21 Consequently, the solvent-dependent process, i.e.,
hydrolysis of DNPC or solvation of RB, may vary little
as shown in Figure 1A,B. Addition of more acetonitrile
(øw e 0.453, Table 2) results in formation of mobile
acetonitrile-water clusters. In this acetonitrile-rich
region the rate constant decrease simply reflects the
decrease in [water]. The observed decrease of ∆Hq may
be due to the fact that water-acetonitrile interactions
are weaker that water-water interactions.21-24 The
accompanied entropy decrease occurs because the reac-
tant water has more degrees of freedom. It is interesting
(21) (a) Easteal, A. J . Aust. J . Chem. 1979, 32, 1379. (b) Balakrish-
nan, S.; Easteal, A. J . Aust. J . Chem. 1981, 34, 943. (c) Easteal, A. J .;
Woolf, L. A. J . Chem. Thermodyn. 1982, 14, 755.
(22) (a) Gorbunov, B. Z.; Naberukhin, Yu. I. J . Mol. Struct. 1972,
14, 113. Gorbunov, B. Z.; Naberukhin, Yu. I. J . Struct. Chem. 1975,
16, 755.
(23) (a) Matteoli, E.; Lepori, L. J . Phys. Chem. 1984, 80, 2856. (b)
Blandamer, M. J .; Blundell, N. J .; Burgess, J .; Cowles, H. J .; Horn, I.
M. J . Chem. Soc., Faraday Trans. 1990, 86, 277. (c) Rowlen, K. L.;
Harris, J . M. Anal. Chem. 1991, 63, 964.
(24) (a) Wakisaka, A.; Shimizu, Y.; Nishi, N.; Tokumaru, K.;
Sakuragi, H. J . Chem. Soc., Faraday Trans. 1992, 88, 1129. (b)
Wakisaka, A.; Takahashi, S.; Nishi, N. J . Chem. Soc., Faraday Trans.
1995, 91, 4063.
that the gradual increase in ∆Gq (∆∆Gq ) ∆Gqøw0.1
-
∆Gqøw0.453) is due to a loss of entropy (T∆∆Sq ) -6.5 kcal/
mol, Table 2) not compensated by a gain in enthalpy
(∆∆Hq ) -4.6 kcal/mol, Table 2).
(25) Blandamer; M. J .; Burgess, J . Chem. Soc. Rev. 1975, 4, 55 and
references cited therein.
Con clu sion s
(26) (a) Blokzijl, W.; Engberts, J . B. N.; J ager, J .; Blandamer, M. J .
J . Phys. Chem. 1987, 91, 6022. (b) Blokzijl, W.; Blandamer, M. J .;
Engberts, J . B. N. J . Org. Chem. 1991, 56, 1832.
(i) Calculation of the kinetic order with respect to one
component of a binary solvent mixture should be based
(27) (a) Abraham, M. H. Prog. Phys. Org. Chem. 1974, 11, 2. (b)
Yates, K.; McClelland, R. A. Ibid. 1974, 11, 324. (c) Buncel, E.; Wilson,
H. Acc. Chem. Res. 1979, 12, 42.
(28) Cox, B. G. J . Chem. Soc., Perkin Trans. 2 1973, 607.
(29) Symons, M. C. R. Acc. Chem. Res. 1981, 14, 179.

Hydrolysis of Bis(2,4-dinitrophenyl) Carbonate
J . Org. Chem., Vol. 62, No. 17, 1997 5933
148 °C,31b and 130-131 °C.31c IR (KBr, Perkin-Elmer FT-
1750): 3116, 1803, 1617, 1540, 1346 cm-1. Anal. Calcd for
C13H6N4O11: C, 39.59; H, 1.53; N, 14.22. Found: C, 39.61; H,
1.53; N, 14.24.
on rate constants obtained over the entire range of
solvent composition.6b Care should be exercised when
kinetic orders with respect to water are interpreted in
terms of the number of water molecules in the TS. The
reason is that slopes of plots of log k versus log [water]
may be affected by the nonideal behavior of the solvent
mixture and by preferential solvation of GS and/or TS
by one component of the mixture.7-10 The proton inven-
tory technique can be fruitfully used to solve this
problem. (ii) The TS of the present reaction contains
DNPC and two water molecules, independent of [water]
in the binary solvent mixture. (iii) Solvation of RB and
hydrolysis of DNPC seem to be sensitive to the same
solute-solvent interactions, namely, H-bonding and di-
polar interactions; these are modulated by the water-
acetonitrile interactions.
Kin etic Mea su r em en ts. Glass double-distilled water was
used throughout. Binary solvent mixtures were prepared by
weight; they contained 0.01 M HCl or 0.01 M DCl in order to
suppress any base-catalyzed hydrolysis. The deuterium con-
tent of D2O and DCl in D2O was determined by NMR (Bruker
AC-200) by the internal reference method.19 The DNPC stock
solution was 0.005 M in dry acetonitrile.
Kinetic runs were carried out with a Beckman DU-70 UV-
vis spectrophotometer, interfaced to a microcomputer and
fitted with a thermostated cell holder whose temperature was
kept constant to ( 0.05 °C (checked with a Lauda R 42/2 digital
thermometer). All runs were carried out in triplicate, under
pseudo-first-order conditions, and were initiated by injecting
20 µL of the DNPC stock solution into 7 mL of the thermally
equilibrated binary solvent mixture, contained in a 5 cm path
length quartz cell. The reaction mixture was homogenized by
a hand-held, battery-operated Hellma-type 338.004 microstir-
rer. The reaction was followed by monitoring the liberation
of 2,4-dinitrophenol at λ ) 302 nm as a function of time. The
log (absorbance) versus time plots were rigorously linear over
more than 5 half-lives. Values of kobs were calculated from
the slopes of these plots. The relative standard deviation in
kobs, i.e., ((standard deviation/kobs)100), was e0.2%, that be-
tween kobs of a triplicate was e0.5%.
Exp er im en ta l Section
Ma ter ia ls. All chemicals were obtained from Aldrich or
Merck and were purified as described elsewhere.30 DNPC was
prepared by stirring a suspension of sodium 2,4-dinitrophe-
noxide (20 mmol) with phosgene (15 mmol) in toluene for 6 h
at room temperature. The solvent was evaporated and the
residue recrystallized from benzene. It was further purified
by sublimation under reduced pressure. The impurity, i.e.,
2,4-dinitrophenol, sublimes, leaving the pure carbonate ester
as a slightly yellowish solid, mp 137-138 °C. Several melting
points are reported for DNPC in the literature, e.g., 125.5 °C,31a
Ack n ow led gm en t. We thank the FAPESP and
FINEP research foundations for financial support.
O.A.S. thanks the CNPq for a research productivity
fellowship. We thank Paula P. Brotero, Ana Paola V.
Triffoni, and Reinaldo C. Bazito for their help.
(30) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory
Chemicals, 3rd ed.; Pergamon Press: New York, 1988.
(31) (a) Kempf, Th. J . Prakt. Chem. 1870, 1, 407. (b) Glatthard, R.;
Matter, M. Helv. Chim. Acta 1963, 46, 795. (c) Gresser, M. J .; J encks,
W. P. J . Am. Chem. Soc. 1977, 99, 6971.
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