Computational Study of Reactivity and Transition Structures in Nucleophilic Substitutions on Benzyl Bromides

Abstract

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-

Mechanism for the Oxidation of Sulfides and Sulfoxides with Periodates: Reactivity of the Oxidizing Species

“…The rearrangement of solvent molecules influences, nevertheless, the experimentally derived ∆H ‡ and ∆S ‡ parameters; they decrease/increase together with increasing/decreasing solvation of the TS and may approximately compensate each other's changes according to the equation δ∆G ‡ = δ∆H ‡ -Tδ∆S ‡ . Therefore, in agreement with our earlier observations, [53][54][55][56] although computed and experimentally derived ∆G ‡ values are in acceptable agreement with each other, the corresponding ∆H ‡ and ∆S ‡ data may be different because the rearrangement of the solvent molecules that occurs during the reaction cannot be computed with the applied methods.…”

“…[11] We previously found [53][54][55][56] for several reactions that the computed and experimentally derived ∆G ‡ values are in acceptably good agreement; their deviations were found to be less than 10 kJ mol -1 . The agreement between the experimentally derived and computed ∆G ‡ data may be explained by two reasons.…”

“…[11] The activation parameters for the reactions of substituted methyl phenyl sulfides were correlated with Hammett σ substituent constants [87] [Equation (7), P = G, H or S], as described previously. [53][54][55][56] ∆P ‡ = δ∆P ‡ σ + ∆P o ‡…”

Nucleophilic Aliphatic Substitution

“…80 The charges, bond lengths, and bond angles for the transition states for reaction in the gas phase and in acetone correlated with the Hammett σ and σ + constants, respectively. An electron-withdrawing para-substitutent leads to a tighter transition state in the gas phase.…”

“…The cyclic sulfamidate (79) reacts with a host of neutral and anionic sulfur nucleophiles to give an excellent yield (84-99%) of product (80). 127 1,.0] undec-7-ene (DBU) was added to ionize the neutral nucleophiles.…”