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Computational Study of Reactivity and Transition Structures in Nucleophilic Substitutions on Benzyl Bromides
Author(s) -
Ruff Ferenc,
Farkas Ödön,
Kucsman Árpád
Publication year - 2006
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/ejoc.200600543
Subject(s) - chemistry , substituent , nucleophile , reactivity (psychology) , pyridine , medicinal chemistry , solvent , transition state , solvent effects , trigonal bipyramidal molecular geometry , computational chemistry , crystallography , stereochemistry , organic chemistry , crystal structure , catalysis , medicine , alternative medicine , pathology
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 Δ G ‡ /Δ H ‡ /Δ S ‡ vs. σ plots were linear, while in solvents there were breaks in the calculated and measured Δ G ‡ /Δ 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 Δ S ‡ values appeared to be independent of the substituents. In solvents, the calculated Δ G ‡ /Δ H ‡ /Δ 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 substrates bearing e‐w groups were higher than expected. No breaks in the Δ G ‡ /Δ H ‡ /Δ S ‡ vs. σ plots were observed when the substituents on the pyridine nucleophile were changed. The best agreement between the calculated and measured values was obtained for the least solvent‐dependent Δ G ‡ parameter. The experimentally measured Δ S ‡ and Δ H ‡ data were influenced by the rearrangement of the solvent molecules. The calculated structural parameters of the transition states (TSs) varied linearly with the substituent constants, with loose and tight distorted trigonal‐bipyramidal TSs being formed by benzyl bromides bearing e‐d and e‐w groups, respectively. The reactions proceeded by S N 2 mechanisms; only the transition structures were changed with the substituents and the media. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)