2128 J . Org. Chem., Vol. 63, No. 7, 1998
Cevasco and Thea
of aryl 4-hydroxybenzenesulfonates (8.7) and that both
are larger than those found for the carboxylate esters
(6.0-6.5). These observations add further support to our
previous indication13 that aryl hydroxyarenesulfonates
seem to be more prone to hydrolyze through dissociative
pathways than the corresponding carboxylate esters.
Moreover, as far as 2,4-dinitrophenyl esters are in-
volved, such indication is sustained by the values of the
ratio (R, Table 5) between the apparent second-order rate
constant (kaKa/Kw) for attack of hydroxide ion on neutral
p-hydroxy esters, this term being related to the dissocia-
tive pathway, and the bimolecular rate constant for the
attack of hydroxide ion onto the esters lacking the
hydroxy group. As shown in Table 5, the kinetic advan-
tage of the dissociative mechanism over the associative
one for the hydrolysis of 2,4-dinitrophenyl sulfonates is
indeed 10 to 20 times larger than that for the hydrolysis
of the corresponding carboxylates.
F igu r e 2. Brønsted plot for the hydrolysis of aryl 4′-hydroxy-
â-styrenesulfonates. The solid line is calculated from eq 5.
Identity, in increasing order of pK of the leaving substituted
phenoxide is as in Table 1. The dashed line is calculated for
the bimolecular attack of hydroxide ion on neutral aryl
4′-hydroxy-â-styrenesulfonates, and the arrow shows the
calculated changeover point from E1cB to SN2(S) mechanism
(see text).
We are now in a position to make a comparison be-
tween the dissociative hydrolysis of 2,4-dinitrophenyl 4′-
hydroxybenzenesulfonate and 4′-hydroxy-â-styrene-
sulfonate and to evaluate the effect of the interposition
of a vinylene group between the SO2 and OH interacting
groups. The ka value for the hydrolysis of 5 (0.92 s-1
,
had been previously proposed4a by other authors to relate
the reaction constant F0(X) for the alkaline hydrolysis of
X-substituted-phenyl esters of a series of substituted
benzenesulfonic acids (with X, the substituent in the
leaving phenoxide, invariable in the series) to F0, the
reaction constant for the alkaline hydrolysis of (unsub-
stituted) phenyl esters of substituted benzenesulfonic
acids, and σ(X), the Hammett constant of the X-substitu-
ent in the leaving phenoxide, through the constant F*,
which was calculated4a by cross-correlation from kinetic
data relative to the alkaline hydrolysis of aryl esters of
benzenesulfonic acids substituted in both rings. This
way, employing the reported4a values F0 ) 2.24, F* )
-0.61 and σ(X) ) 0.91 for the 2-nitro substituent (and
the usual value of 0.78 for the 4-nitro one) we obtain,
neglecting the temperature-dependence of F*, the Ham-
mett relationship log k/k0 ) 1.21Σσ for the alkaline
hydrolysis of 2,4-dinitrophenyl esters of benzenesulfonic
acids. Taking into account the attenuation factor of 0.54
related to the vinylene group,14 such Hammett correlation
becomes log k/k0 ) 0.65Σσ, and it is now applicable to
the SN2(S) hydrolysis of 2,4-dinitrophenyl esters of
substituted â-styrenesulfonates. If we employ the rate
constant kOH of 2,4-dinitrophenyl â-styrenesulfonate (8.32
M-1 s-1, Table 3) as k0, using the σp substituent constant
(-0.37) for the hydroxyl we finally obtain the calculated
bimolecular rate constant for SN2(S) attack onto 5 (kcalcd
) 4.8 M-1 s-1). It is noteworthy to emphasize that the
value of the measured, apparent second-order rate con-
stant (kapp ) kaKa/Kw) for this ester is about 40000-fold
larger than the calculated value (kcalcd), and this is a
further indication that the actual mechanism is different
from the associative one. Now, if the dashed line with
slope -0.59 (âLG from eq 7) is drawn through the point
indicated as ∆ in Figure 2 (log kcalcd), it will intersect the
solid line at pKLG 7.6, which therefore represents the
break-point for mechanism.
Table 1) is ca. 150-fold larger than that of the corre-
sponding 4′-hydroxybenzenesulfonate (ka ) 6.06 × 10-3
s-1 in water at 60 °C).3d It is generally thought that the
driving forces for the E1cB mechanism are the nucleofu-
gality of the leaving group, the internal nucleophilicity
of the substrate, and the “relative” stability of the
putative intermediate; in the present case the leaving
groups are the same, and therefore the difference in
reactivity has to be ascribed to the other two factors. The
internal nucleophilicity represents the ability of the
conjugate base of the substrate to expel the leaving group,
and it is related to the pKa of the substrate itself. We
have previously reported3d that the ka term for the
dissociative hydrolysis of 2,4-dinitrophenyl esters of
substituted 4-hydroxybenzenesulfonic acids obeys the
Brønsted relationship log ka ) 0.74 × pKa - 6.61 when
the phenolic hydroxy group does not suffer severe steric
constraints. From this correlation, employing the data
pertinent to 2,4-dinitrophenyl 4′-hydroxybenzenesulfonate
(ka ) 6.06 × 10-3 s-1 and pKa ) 6.66)3d and the pKa value
of 5 (7.69, Table 2), we can estimate the ka value expected
for the (hypothetical) 2,4-dinitrophenyl 4′-hydroxy-X-
benzenesulfonate which has exactly the same pKa of ester
5. The resulting value (0.12 s-1) accounts for only ca.
13% of the experimental ratio, thus suggesting that the
increase in reactivity on going from 2,4-dinitrophenyl 4′-
hydroxybenzenesulfonate to 5 should be ascribed mainly
to an increased stability of the intermediate 4 with
respect to 3. As we have previously suggested in the case
of the corresponding carboxylic acid esters,2 such in-
creased stability could be the result of a more extended
delocalization of π electrons due to the presence of an
additional vinylene group.
Exp er im en ta l Section
Gen er a l. Starting reagents and solvents were purified and/
or distilled before use. Buffer materials were of analytical
reagent grade. Water was double distilled and preboiled to
free it from dissolved carbon dioxide. The 1H NMR spectra
were recorded with a Varian Gemini 200 spectrometer (200
MHz) with TMS as internal standard and acetone-d6 or CDCl3
as solvent.
It is interesting to note that this value is not so much
different from that estimated for the alkaline hydrolysis
(14) Williams, A. Chemistry of Enzyme Action; Page, M. I., Ed.;
Elsevier: Amsterdam, 1984; p 127.