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Pure and Applied Chemistry

The Scientific Journal of IUPAC

Ed. by Burrows, Hugh / Stohner, Jürgen

12 Issues per year


IMPACT FACTOR 2017: 5.294

CiteScore 2017: 3.42

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1365-3075
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Volume 89, Issue 6

Issues

Philicity, fugality, and equilibrium constants: when do rate-equilibrium relationships break down?

Herbert Mayr
  • Corresponding author
  • Department Chemie, Ludwig-Maximilians-Universität München, Butenandtstr. 5-13, 81377 München, Germany
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/ Armin R. Ofial
  • Department Chemie, Ludwig-Maximilians-Universität München, Butenandtstr. 5-13, 81377 München, Germany
  • Other articles by this author:
  • De Gruyter OnlineGoogle Scholar
Published Online: 2017-03-18 | DOI: https://doi.org/10.1515/pac-2017-0107

Abstract

Linear free energy relationships, in particular relationships between rate and equilibrium constants, are the basis for our rationalization of organic reactivity. Whereas relationships between the kinetic terms nucleophilicity and nucleofugality and the thermodynamic term basicity have been in the focus of interest for many decades, much less attention has been paid to the relationships between electrophilicity, electrofugality, and Lewis acidity. By using p- and m-substituted benzhydrylium ions (Aryl2CH+) as reference electrophiles, reference electrofuges, and reference Lewis acids of widely varying electron demand and constant steric surroundings of the reaction center, we have developed comprehensive reactivity scales which can be employed for classifying polar organic reactivity and for rationally designing synthetic transformations. It is a general rule that structural variations in electron-surplus species, which increase basicities, also increase nucleophilicities and decrease nucleofugalities, and that structural variations in electron-deficient species, which increase Lewis acidities also increase electrophilicities and decrease electrofugalities. Deviations from this behavior are analyzed, and it is shown that variations in intrinsic barriers are responsible for the counterintuitive observations that structural variation in one of the reactants alters the rates of forward and backward reactions in the same direction. A spectacular example of this phenomenon is found in vinyl cation chemistry: Vinyl cations are not only generated several orders of magnitude more slowly in SN1 reactions than benzhydrylium ions of the same Lewis acidity, but also react much more slowly with nucleophiles.

Keywords: carbocations; free-energy relationships; ICPOC-23; kinetics; structure-reactivity; thermodynamics

Dedicated to the memory of George A. Olah, a dear friend and mentor.

Article note:

A collection of invited papers based on presentations at the 23rd IUPAC Conference on Physical Organic Chemistry (ICPOC-23), Sydney, Australia, 3–8 July, 2016.

References

About the article

Published Online: 2017-03-18

Published in Print: 2017-06-27


Citation Information: Pure and Applied Chemistry, Volume 89, Issue 6, Pages 729–744, ISSN (Online) 1365-3075, ISSN (Print) 0033-4545, DOI: https://doi.org/10.1515/pac-2017-0107.

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©2017 IUPAC & De Gruyter. This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License. For more information, please visit: http://creativecommons.org/licenses/by-nc-nd/4.0/.Get Permission

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[1]
Robert J. Mayer, Nathalie Hampel, Peter Mayer, Armin R. Ofial, and Herbert Mayr
European Journal of Organic Chemistry, 2018
[2]
Sara Meninno, Simone Naddeo, Luca Varricchio, Amedeo Capobianco, and Alessandra Lattanzi
Organic Chemistry Frontiers, 2018

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