The nucleophilic substitution reactions of ethylene phosphorochloridate (2) with X-pyridines are investigated kinetically in acetonitrile at $-20.0^{circ}C$. The free energy correlations for substituent X variations in the nucleophiles exhibit biphasic concave upwards with a break point at X = 3-Ph. Unusual positive ${ ho}_X$ (= +2.49) and negative ${eta}_X$ (= -0.41) values are obtained with the weakly basic pyridines, and rationalized by the isokinetic relationship with isokinetic temperature at $t_{ISOKINETIC}=6.6^{circ}C$. The pyridinolysis rate of 2 with a cyclic five-membered ring is forty thousand times faster than its acyclic counterpart (3: diethyl chlorophosphate) because of great positive value of the entropy of activation of 2 (${Delta}S^{ eq}$ = +49.2 eu) compared to negative value of 3 (${Delta}S^{ eq}$ = -44.1 eu) over considerably unfavorable enthalpy of activation of 2 (${Delta}H^{ eq}=28.4;kcal;mol^{-1}$) compared to 3 (${Delta}H^{ eq}=6.3;kcal;mol^{-1}$). Great enthalpy and positive entropy of activation are ascribed to sterically congested transition state (TS) and solvent structure breaking in the TS. A concerted mechanism involving a change of nucleophilic attacking direction from a frontside attack with the strongly basic pyridines to a backside attack with the weakly basic pyridines is proposed.