Effect of substituents on activation parameters in aliphatic SN2 reactions. A DFT study

Ferenc Ruff, Ödön Farkas

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Abstract

The activation parameters and optimized structures of the reactants and transition states in the SN2 reactions of substituted pyridines and N,N-dimethylanilines with methyl iodide were computed at the DFT level in different solvents. The measured and calculated ΔG‡/ ΔH‡/ΔS‡ versus σ plots proved to be linear, and their slopes, the δΔG‡, δΔH‡, and δΔS‡ reaction constants, were determined. The least solvent-dependent δΔG‡ reaction constants can be computed with acceptable accuracy. The calculated δΔS‡ data decrease only very slightly with the jointly increasing electron-withdrawing effect of the substituents and tightness of the transition states. The measured δΔS‡ values are influenced mainly by the change of solvation in the reactions, and δΔH‡ is also influenced by the reorganization of the solvent. Consequently, the experimental and calculated δΔS‡ and δΔH‡ reaction constants may deviate considerably from each other. In dipolar aprotic solvents the measured δΔS‡ 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 ΔGO‡, ΔHO‡, and ΔS 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., XC 5H4NCH3+ + I) and for any reaction in protic solvents.

Original languageEnglish
Pages (from-to)3409-3416
Number of pages8
JournalJournal of Organic Chemistry
Volume71
Issue number9
DOIs
Publication statusPublished - Apr 28 2006

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ASJC Scopus subject areas

  • Organic Chemistry

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