The ring-opening reactions of propylene oxide with chloroacetic acids

The association constants, k_d, of dichloroacetic acid (DCA) with ethers (tetrahydrofuran, tetrahydropyran, dibutyl ether, 1,4-dioxane) were measured in CCl₄ from 20°–40° using near IR spectra. A linear relationship between log₁₀K_D and log₁₀K_p was found, where K_p was the association constant of phenol with ethers. On the basis of their relationship the association constants of DCA with propylene oxide (PO) were estimated from those of phenol with PO. Furthermore, the ring-openings of PO with chloroacetic acids (trichloroacetic acid (TCA) or DCA) were studied kinetically in toluene. It was found that the PO-TCA reactions obeyed the second-order kinetics dependent on the first order both in the PO concn and in the TCA concn, and that the PO-DCA reactions obeyed the third-order kinetics on the first order in the PO concn and on the second order in the DCA concn. The elementary reaction rate constants were estimated on the basis of measured rate constants and estimated equilibrium constants and activation enthalpies were obtained from their temp dependence to be 13.5 $\text{kcal}\text{mol}$ in the PO-TCA reaction and 19.7 $\text{kcal}\text{mol}$ in the PO-DCA reaction. The mechanism was based on the activation enthalpy of elementary reaction and it was concluded that the main factor governing the reaction was the extent of activation of the protonated oxirane (the extent of polarization of the C-O bond in the oxirane ring).