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Effect of the Leaving Group and Solvent Combination on the LFER Reaction Constants
Faculty of Pharmacy and Biochemistry, University of Zagreb, 10000 Zagreb, Ante Kovačića 1, Croatia
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Received: 14 December 2011; in revised form: 3 January 2012 / Accepted: 13 January 2012 / Published: 13 February 2012
Abstract: Fine effects that influence the variations of the reaction constants sf in LFER log k = sf(Nf + Ef) have been summarized here. Increasing solvent polarity in the series of binary mixtures increases the solvolysis rates for the same factor for all benzhydryl derivatives in which the solvation of the leaving group moiety in the transition state is substantial, i.e., log k vs. Ef correlation lines are parallel (same sf). For the substrates in which the demand for solvation of the leaving groups moiety is reduced, (e.g., carbonates) sf parameters decrease as the fraction of the water in a given solvent/water mixture increases (log k vs. Ef plots converge), due to decreasing solvation of the electrofuge moiety toward bigger electrofugality. The abscissa of the intersection of the converging plots might indicate the critical electrofugality above which the solvolysis rates should not depend of the water fraction. Larger reaction constant sf indicate later transition state for structurally related substrates only, while sf parameters for structurally different substrates cannot be compared likely due to different intrinsic barriers. Inversion in relative abilities of leaving groups is possible if they have similar reactivities and are characterized with different reaction constants.
Keywords: reaction constant; nucleofuge specific parameter; nucleofugality; electrofugality; solvation; solvolysis
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Matić, M.; Jurić, S.; Denegri, B.; Kronja, O. Effect of the Leaving Group and Solvent Combination on the LFER Reaction Constants. Int. J. Mol. Sci. 2012, 13, 2012-2024.
Matić M, Jurić S, Denegri B, Kronja O. Effect of the Leaving Group and Solvent Combination on the LFER Reaction Constants. International Journal of Molecular Sciences. 2012; 13(2):2012-2024.
Matić, Mirela; Jurić, Sandra; Denegri, Bernard; Kronja, Olga. 2012. "Effect of the Leaving Group and Solvent Combination on the LFER Reaction Constants." Int. J. Mol. Sci. 13, no. 2: 2012-2024.