Table 1 Rate constants at 250 ºC for the hydrolysis of benzoate esters,
where K1* is the equilibrium constant for the protonation of nonsubstituted
ester
ing to an AAC2 mechanism is dominant. First, the S-shaped
conversion vs. time curve is consistent with autocatalysis. The
acidity of near-critical water seems sufficient to initiate
hydrolysis of the benzoate and formation of the autocatalytic
species benzoic acid. Also, the addition of electron-with-
drawing substituents does not affect reaction rate, as predicted
by the proposed acid-catalyzed mechanism and demonstrated
for many acid-catalyzed hydrolyses in the literature. Thus,
despite the fact that near-critical water is quite different from
water under ambient conditions, hydrolyses in it follow the
mechanistic pathway commonly observed at lower tem-
peratures.
R
X
k2K1*/l2 mol22 h21
Me
Et
Pr
Bu
Bui
Bui
Bui
H
H
H
H
H
Cl
CF3
26.9 ± 2.5
25.7 ± 0.9
10.4 ± 0.5
17.1 ± 0.6
6.7 ± 0.3
7.4 ± 0.6
7.0 ± 0.5
We would like to thank Kris Griffith for synthesizing the
substituted benzoic acid esters and Greg Ladzinske for help
collecting data.
calculated to be 101 ± 13 kJ mol21, indicating that mass transfer
limitations were negligible. Similar activation energies were
obtained for the acid-catalyzed hydrolysis of substituted
benzoic acid esters in low temperature aqueous solvents with a
stoichiometric addition of acid.16
Notes and references
1 A. R. Katritzky, S. M. Allin and M. Siskin, Acc. Chem. Res., 1996, 29,
399.
As expected, longer alkyl chains on the esters yield slower
hydrolyses, and the branched butyl benzoate hydrolyzes more
slowly than the linear butyl benzoate. As the carbonyl carbon
becomes more sterically hindered, nucleophilic attack by water
becomes more difficult, resulting in a slower hydrolysis.
Hydrolyses of substituted isobutyl benzoates provide further
evidence that an acid-catalyzed mechanism predominates under
near-critical conditions. It is clear from Table 1 that substituents
have very little effect on rates of hydrolysis of benzoic acid
esters under these conditions. Because of competing effects in
the first two steps of the AAC2 mechanism (Scheme 2),
substituents exhibit a negligible effect on the rate constants.
This negligible substituent effect is a well-documented charac-
teristic of ester hydrolysis via an acid-catalyzed mechanism in
aqueous solvents at lower temperature.17 Hammett plots of
acid-catalyzed ester hydrolyses have r = 0, compared to r =
2.4 for base-catalyzed hydrolyses of esters.17 In addition to the
reactions reported in Table 1, the hydrolysis of isobutyl p-
hydroxybenzoate was also attempted, but the hydroxy group is
2 J. An, L. Bagnell, T. Cablewski, C.R. Strauss and R.W. Trainor, J. Org.
Chem., 1997, 62, 2505.
3 L. J. Seacock, Jr., D. C. Elliot, E. G. Baker and R. S. Butner, Ind. Eng.
Chem. Res., 1993, 32, 1535.
4 Y. Guissani and B. Guillot, J. Chem. Phys., 1993, 98, 8221.
5 E. T. Ryan, T. Xiang, K. P. Johnston and M. A. Fox, J. Phys. Chem. A,
1997, 101, 1827.
6 R. W. Shaw, T. B. Brill, A. A. Clifford, C. A. Eckert and E. U. Franck,
Chem. Eng. News, 1991, 69, 26.
7 B. Kuhlmann, E. M. Arnett and M. Siskin, J. Org. Chem., 1994, 59,
3098.
8 W. L. Marshall and E. U. Franck, J. Phys. Chem. Ref. Data, 1981, 10,
295.
9 K. Chandler, F. Deng, A. K. Dillow, C. L. Liotta and C. A. Eckert, Ind.
Eng. Chem. Res., 1997, 36, 5175.
10 J. M. L. Penninger, Fuel, 1988, 67, 490.
11 P. Krammer, S. Mittelstädt and H. Vogel, Chem. Eng. Technol., 1999,
22, 126.
12 T. H. Lowry and K. S. Richardson, Mechanism and Theory in Organic
Chemistry, Harper and Row, New York, NY, 1987, pp. 717–723.
13 A. J. Read, J. Solution Chem., 1981, 10, 437.
14 R. M. Kettler, D. J. Wesolowski and D. A. Palmer, J. Solution Chem.,
1995, 24, 385.
15 E. L. Shock, Am. J. Sci., 1995, 295, 496.
16 E. W. Timm and C. N. Hinshelwood, J. Chem. Soc., 1938, 862.
17 H. H. Jaffe, Chem. Rev., 1953, 53, 191.
strongly electron donating (s
decarboxylated.
= 20.38), and the ester
In summary, benzoate esters can be cleaved to benzoic acid
and the corresponding alcohol in near-critical water without the
addition of any acid or base catalyst. Preliminary investigations
indicate that the pathway for acid-catalyzed hydrolysis accord-
Communication 9/06401J
2064
Chem. Commun., 1999, 2063–2064