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Decoupling Substituent and Solvent Effects during Hydrolysis of Substituted Anisoles in Supercritical Water Michael T. Klein,* Yves G. Mentha,+and Lori A. Torry Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716
The kinetics of hydrolysis in supercritical water of a series of substituted anisoles to methanol and the corresponding substituted phenol probed the coupling of solvent and substituent effects. For supercritical water concentrations [H20] 5 3 mol/L, the substituent effects exhibited compelling correlation according to the classic Hammett formalism. For [H20]> 5 mol/L, however, the Hammett correlation had an internal minimum at cr = 0, indicative of rate acceleration by both electronwithdrawing and electron-donating substituents. The solvent and substituent effects were thus defined to be “decoupled” for [H20]5 3 mol/L, where the dielectric constant t I1.28. The hydrolysis Hammett reaction constant increased with decreasing water density. The reaction constant was also of opposite sign to reported values for pyrolysis of substituted benzyl phenyl ethers, suggesting that the practical significance of the present work is the ability to design a substituent-specific solvent effect for selective hydrolysis or pyrolysis.
Introduction The reactions of heteroatom-containing alkyl aromatics in supercritical water expose interesting new opportunities for kinetics analysis. First, supercritical water hydrolyzes saturated carbon-heteroatom linkages (Townsend et al., 1988) by a non-free-radicalmechanism that likely involves SN2polar chemistry (Klein et al., 1990). Second, supercritical water provides a reaction medium that exerts solvent effects on the hydrolysis chemistry (Townsend et al., 1988; Klein et al., 1990; Penninger and Kolmaschate, 1989). For example, observed overall second-order hydrolysis rate constants are dependent upon the easily adjusted supercritical water density (Townsend et al., 1988) and, therefore, medium dielectric constant. The details underlying the foregoing observations are not yet well understood. In particular, the solvent effect is ill-defined, in part because the solvent is a reactant. It is difficult, for example, to make a classic determination of reaction order by variation of initial water concentration because the supercritical water density and therefore solvent property and effect also vary with initial water ‘Present address: Firmenich SA, CH-1211 Geneva 8, Switzerland.
0888-5885/92/2631-0182$03.00/0
Table I. Substituted Anisoles Used To Probe Solvent and Substituent Effects during Hydrolysis in Supercritical Water
X H H H H CHO CF3
Y H Me OH CHO H H
reactant anisole p-methylanisole p-hydroxyanisole p-anisaldehyde m-anisaldehyde m-(trifluoromethy1)anisole
concentration (Uematsu and Franck, 1980, Franck, 1978). We therefore sought to probe the chemistry and kinetics of hydrolysis in supercritical water through variation of the structure of the hydrocarbon reactant. This would provide complementary information to that obtained by variation of the solvent. To this end, we report on the hydrolysis in supercritical water of a series of para- and meta-substituted anisoles. The hydrolysis of 0-hydroxyanisole(guaiacol)to methanol and catechol is at the methyl group and involves the ca0 1992 American Chemical Society
Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992 183 Table 11. Representative Products and Yields (mol %) from Reaction of Substituted Anisoles Neat and in Supercritical Water at 380 f 2 "C substituent (X) Pr
tlh component X-CGHI-OCH, X-CEHd-OH CH30H
0.0 2.0
2.0 0.6
P-CH3 0.0 2.0 7.7 6.2
55 6 0
58 34 37
77 9 0
p-OH
C6H6
PhCH3 PhOCHB PhOH othersb mol balance' Redundant.
7 2 2 3 66
1 93
95
H 0.0 7.7
2.0 6.2
82 8 0
62 33
4
2
2
a a
a a
99
2 96
2 99
6 90 92
m-CHO 0.0 2.0 0.25 0.3
