The nucleophilic substitution reactions of 1,2-phenylene phosphorochloridate (1c) with X-pyridines are investigated kinetically in acetonitrile at $-25.0^{circ}C$. The free energy correlations for substituent X variations in the nucleophiles exhibit biphasic concave upwards with a break point at X = 3-Ph. The pyridinolysis rate of 1c with a cyclic five-membered ring is $2.70{ imes}10^5$ times faster than its acyclic counterpart (1a: phenyl ethyl chlorophosphate) because of great positive value of the entropy of activation of 1c (${Delta}S^{ eq}$ = +26 eu) compared to negative value of 1a (${Delta}S^{ eq}$= -24 eu) over considerably unfavorable enthalpy of activation of 1c (${Delta}H^{ eq}=20.5kcal;mol^{-1}$) compared to 1a (${Delta}H^{ eq}=12.7kcal;mol^{-1}$). 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 on the basis of greater selectivity parameters (${ ho}_X$ = -1.99 and ${eta}_X$ = 0.41) with the strongly basic pyridines compared to those (${ ho}_X$ = -0.42 and ${eta}_X$ = 0.07) with the weakly basic pyridines.