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Kinetics and mechanism of the reaction between substituted benzoyl chlorides (1) and ammonium thiocyanate (2) were investigated theoretically and experimentally using the DFT method at M062x/6‐311G++(2d,2p) level of theory and UV-vis spectrophotometry technique, respectively. The reaction followed second‐order kinetics according to the effect of concentration on the reaction rate. The solvent effect demonstrates that media with the lower dielectric constant is in favour of the reaction rate. On the basis of  the Eyring plot, activation parameters were determined in a lower dielectric constant solvent such as 1,4 dioxane, the low value of ∆G (58.7 kJ mol-1) in this solvent relation to polar solvent, help to increase the reaction rate. In fact, unfavourable ∆S value (-188.18 J mol-1 K-1) can be compensated by the favourable ∆H value (lower, 4.01 kJ mol-1). In this case, the reaction is entropy controlled, while in the polar solvent (acetonitrile), the unfavourable ∆H value (higher, 45.6 kJ mol-1) can be compensated by the favourable ∆S value (-80.9 J mol-1 K-1), so the reaction is enthalpy-controlled. Different substituents examined on the reaction rate in both methods. The rate constant was in favour of strong para electron-withdrawing substituent (EWS) groups (i.e. NO2) on benzoyl chloride. A comparison of theoretical and experimental rate constant values in both methods indicated differences between data. This is expected, because of the real liquid phase (for experimental results) has a great difference from the unlike liquid phase (for theoretical data). Hammet study, exhibited that the large value of ρ=1.94 imply that TS structure is constructed with negative charges; hence, EWS plays a significant role in stabilizing TS character for increasing the reaction rate. The result of this study confirmed that the reactions in the presence of various para-substituted benzoyl chlorides have the same kinetics role. Also, the effect of leaving group was studied on the reaction between (1) and (2), theoretically. The result showed that the reaction rate in the presence of benzoyl bromide has been increased approximately 25 times more in the gas phase and also 170 times more in a liquid phase, compared to benzoyl chloride. A linear dependence of ΔH versus ΔS approved based on the isokinetic and Exner equations, so the reaction exhibited the same kinetics role in the different solvents.

Ammonium thiocyanate, benzoyl isothiocyanate, density functional theory, isokinetic, kinetics and mechanism, UV-vis spectrophotometry technique.

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