Heat Capacities of Activation for the Neutral Hydrolysis of Two Acyl-Activated Esters in Water-Rich 2-n-Butoxyethanol-Water Mixtures. Analysis in Terms of the Pseudo-Phase-Separation Model

Article abstract of DOI:10.1021/jo00170a030

The water-catalyzed hydrolysis of p-methoxyphenyl dichloroacetate (1) and 2,2-dichloropropionate (2) in water-rich 2-n-butoxyethanol (2-BE)-H2O exhibits large, positive heat capacities of activation (1, ΔC_p^<*> = 224 +/- 51 cal mol^-1 deg_-1; 2, ΔC_p^<*> = 342 +/- 51 cal mol^-1 deg^-1; temperature 20-48 deg C) at 2 mol percent of 2-BE (n_H2O = 0.98).These positive ΔC_p^<*> values contrast sharply with the negative ΔC_p^<*>'s found for S_N solvolysis reactions in highly aqueous mixed solvents.By use of a pseudothermodynamic approach, the ΔC_p^<*> values as well as positive heat capacities for transfer (ΔC_p^Θ,tr) of two model substrates from water to 2-BE-H2O are rationalized within the framework of a pseudo-phase-separation model.Based on previous thermodynamic and other evidence, this model assumes the onset of microphase separation in 2-BE-H2O at about n_H2O = 0.98 at 25 deg C.It is shown that temperature-dependent partitioning of the acyl-activated esters between a water-rich and a cosolvent-rich microphase provides an adequate explanation for the observed heat capacity data.The observed ΔC_p^<*> for solvolysis of tert-butyl chloride in 2-BE-H2O at n_H2O = 0.98 is also reconcilable with the proposed theory.