Electronic Effects on the Menschutkin Reaction. A Complete Kinetic and Thermodynamic Dissection of Alkyl Transfer to 3- and 4-Substituted Pyridines

Article abstract of DOI:10.1021/ja00538a031

The relationship between kinetic and thermodynamic parameters is explored for quaternization of a series of pyridines (mostly 3- and 4-substituted) with several methylating and ethylating reagents in several solvents.The reaction with methyl iodide in acetonitrile is reversible at temperatures in the neighborhood of 100 deg C so that the effect of substituents on free energy, enthalpy, and entropy for activation of the forward and reverse reactions and for the overall quaternization can be determined.A variety of experimental techniques was used to obtain rates over a range of 10¹³ and to determine enthalpies and entropies of reaction.The results are self-consistent and agree generally with isolated published values for similar systems.The relationship between thermodynamic and activation parameters is examined, and a gross disparity is found between free energy and enthalpy behavior compared with that of the entropies.A consistent picture of the quaternization reaction emerges, based on many studies using a variety of mechanistic probes.The transition state is "early" as far as bond formation to the base in concerned but "late" in terms of bond rupture between the transferring alkyl group and the leaving group with solvent reorganization nearly complete.Quaternization of the 3- and 4-substituted pyridines does not follow the reactivity-selectivity principle, but that of 2-substituted pyridines does.The current practice of assigning detailed bimolecular structures to transition states for substitution, addition, or elimination reactions by application of the Hammond postulate is criticized in view of its inability to handle the dominating role of solvation dynamics and because of the considerable difference in potential energy content (and therefore structure) between the transition states and the reactants or products.

Full text of DOI:10.1021/ja00538a031

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Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.