The activation parameters and optimized structures of the reactants and transition states in the S(N)2 reactions of substituted pyridines and N,N-dimethylanilines with methyl iodide were computed at the DFT level in different solvents. The measured and calculated deltaG/deltaH/deltaS versus sigma plots proved to be linear, and their slopes, the deltadeltaG, deltadeltaH, and deltadeltaS reaction constants, were determined. The least solvent-dependent deltadeltaG reaction constants can be computed with acceptable accuracy. The calculated deltadeltaS data decrease only very slightly with the jointly increasing electron-withdrawing effect of the substituents and tightness of the transition states. The measured deltadeltaS values are influenced mainly by the change of solvation in the reactions, and deltadeltaH is also influenced by the reorganization of the solvent. Consequently, the experimental and calculated deltadeltaS and deltadeltaH reaction constants may deviate considerably from each other. In dipolar aprotic solvents the measured deltadeltaS was less than zero, and in protic solvents it was greater than zero. The ordering of the solvent molecules around the transition state with increasing charge is increased in the former but decreased in the latter media, as compared to the bulk of the solvents. The calculated deltaG(o), deltaH(o), and deltaS(o) parameters of the unsubstituted compounds agree relatively well with the experimental data for reactions of neutral molecules in dipolar aprotic solvents (e.g., XC6H4N(CH3)2 + CH3I). On the other hand, the measured and calculated activation parameters may show considerable deviations for reactions of ions (e.g., XC5H4NCH3+ + I-) and for any reaction in protic solvents.
Keywords: Nucleophilic substitutions / Benzyl bromides / Substituent effect / Solvent effect / Density functional calculations / Activation parameters DFT computations on the mechanisms of nucleophilic substitutions on benzyl bromides were performed and the calculated activation parameters were compared with experimentally acquired data. In vacuo, the presence of electron-withdrawing (e-w) groups on the benzyl bromides accelerated the reactions and the calculated ∆G ‡ /∆H ‡ /∆S ‡ vs. σ plots were linear, while in solvents there were breaks in the calculated and measured ∆G ‡ /∆H ‡ vs. σ + plots of the reactions between the benzyl bromides and Br -, both with electron-donating (e-d) and with e-w substituents accelerating the reactions. The calculated ∆S ‡ values appeared to be independent of the substituents. In solvents, the calculated ∆G ‡ /∆H ‡ /∆S ‡ vs. σ + plots for the reactions between benzyl bromides and pyridine were linear, whereas breaks were observed in the plots of the measured data. These reactions were promoted by e-d substituents, but the measured reactivities of sub-
A DFT study of transition structures and reactivity in solvolyses of tert-butyl chloride, cumyl chlorides, and benzyl chlorides Ferenc Ruff a * and Ö dö n Farkas a DFT computations were performed on the S N 1 and S N 2 solvolyses of substituted cumyl chlorides and benzyl chlorides in ethanol and water, by increasing stepwise the C-Cl distance and by optimization. The total energy increases with the increase in the Cl-C distance in S N 1 reactions, while free energy of activation pass through maximum. To validate the results, the calculated free energies of activation were compared with data obtained by kinetic measurements. The structural parameters of the transition states were correlated with the Hammett substituent constants and compared with the data of hydrolyses of tert-butyl chloride and methyl chloride, which proceed with known mechanisms. Conclusions on the mechanisms of the reactions were driven from the effect of substituents on free energies of activation. Cumyl chlorides substituted with electron-donating (e-d) groups solvolyze with S N 1 mechanism, while the reactions of substrates that bear electron-withdrawing groups proceed with weak nucleophilic assistance of the solvent. Benzyl chlorides hydrolyze through an S N 2 pathway except those derivatives that have strongly e-d groups, where the reaction has S N 1 character, but a weak nucleophilic assistance of the water should also be taken into consideration.
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