Decoupling substituent and solvent effects during hydrolysis of substituted anisoles in supercritical water

Abstract

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 I 1.28. The hydrolysisHammett 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.

A Continuum Solvation Model Including Electrostriction: Application to the Anisole Hydrolysis Reaction in Supercritical Water

“…13 The three state points considered were all at T ) 380°C (T r ) 1.01), but included a range of pressures from 23.2 to 49 MPa. The measured rate constant for hydrolysis, k 1 , is shown in Table 2 along with k p , the rate constant for the competing pyrolysis pathways.…”

“…In particular, it has been found that for low pressures homolytic free radical mechanisms dominate, whereas for high pressures more chemically selective heterolytic, ionic mechanisms dominate. 3,13 This variation in reaction process with pressure is known to be tied to the large variation in SCW's density, and thus its dielectric properties, with pressure in this regime. [2][3][4][14][15][16] For example, at 380°C, the dielectric constant of water varies from ) 2 at P ) 21 MPa to ) 14 at P ) 47 MPa.…”

“…18 We test our method by application to a charge separation S N 2 reaction, the hydrolysis of anisole, which has been studied experimentally in SCW. 13,19 The primary manner in which solvents affect solute reaction rates is by altering the relative stabilities of the reacting solute at various positions along its reaction path, thus changing the activation barrier to reaction. As such equilibrium solvation effects alter the rate constant exponentially, in contrast to nonequilibrium solvent effects which alter the prefactor, we consider here only the equilibrium solvent effects.…”

A Compressible Continuum Model Study of the Chloride plus Methyl Chloride Reaction in Supercritical Water

“…12 Subsequently, we applied our compressible continuum (CC) model to this reaction system and found that this model provides a much better description of the pressure dependence of the reaction rate when compared to experiment. 11,37,39 However, the experimental data are extremely limited, providing only the rate constant at three different pressures. 11,40 Thus, no check on the detail of the CC calculationsssolvent density distributions and absolute differentials in solvation energy along the reaction pathscould be made.…”

“…11,37,39 However, the experimental data are extremely limited, providing only the rate constant at three different pressures. 11,40 Thus, no check on the detail of the CC calculationsssolvent density distributions and absolute differentials in solvation energy along the reaction pathscould be made. 37 Johnston and Rossky and co-workers have provided a more detailed demonstration of the failure of the IC model by comparing results from this model against computer simulation.…”