Special Issue "Correlation Analysis Applied to Solvolysis Reactions"

Guest Editor
Prof. Dr. Dennis N. Kevill
Distinguished Research Professor Emeritus, Department of Chemistry and Biochemistry, Northern Illinois University, DeKalb, IL 60115, USA
E-Mail: dkevill@niu.edu
Interests: reaction mechanism in organic chemistry; solvolysis reactions; solvent nucleophilicity; mechanism of nucleophilic attack at acyl carbon; phosphorus; sulfur

Dear Colleagues,

Solvolysis reactions involve the solvent also as a reactant. One technique in their study involves correlation analysis using linear free energy relationships. Grunwald and Winstein developed a scale of solvent ionizing power for unimolecular solvolyses and subsequently a second term, involving nucleophilicity, was added for bimolecular reactions. Originally applied to reactions at a saturated carbon, the approach can also be applied to solvolyses at acyl carbon and at heteroatoms, such as sulfur or phosphorus (Kevill, D.N.; D’Souza, M. J.  Sixty years of the Grunwald-Winstein equation: development and recent applications. J. Chem. Res., 2008, 61-66.).

If the interest is more on the nature of the interactions of the substrate with the solvent, rather than reaction mechanism, since there are a number of possibly relevant solvent properties, multiparameter equations will be required. Several books dealing with this area exist. (For example, Shorter, J, Correlation Analysis of Organic Reactivity with Particular Reference to Multiple Regression; Research Studies: New York, 1982).

Contributions in any area of the application of existing correlation equations to new reactions, modification of existing equations, or development of new approaches are welcome.

Prof. Dr. Dennis N. Kevill
Guest Editor

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• solvolysis
• correlation analysis
• linear free energy relationships
• Grunwald-Winstein equation
• multiparameter equations

Feature Paper:

Title: A Greatly Under-Appreciated Fundamental Principle of Physical Organic Chemistry
Author: Robin A. Cox
Affiliation: Former Senior Lecturer, Department of Chemistry, University of Toronto, Ontario M5G1X5, Canada; E-Mail: robin.a.cox@sympatico.ca
Abstract: This principle was first enunciated by Jencks, as the concept of an enforced mechanism: if a species does not have a finite lifetime in the reaction medium, it cannot be a mechanistic intermediate. For instance, it is well known that primary carbocations do not have long enough lifetimes to exist in an aqueous medium, but it is perhaps not so well known that nor do secondary ones, so S_N1 reactions involving these species in aqueous media are not possible, and an S_N2 mechanism is enforced. Only tertiary carbocations (and the lifetimes of some of these are very short), and ones stabilized by resonance (benzyl cations, acylium ions) are stable enough to be reaction intermediates. More importantly, neither H₃O⁺ nor HO^– exist as such in dilute aqueous solution; for instance, several recent high-level calculations on large proton clusters are unable to localize the positive charge, it is simply “on the cluster” as a whole. The lifetime of any protonated water species is exceedingly short, a few molecular vibrations at most; the best experimental study, using modern IR instrumentation, has the most probable hydrated proton structure as H₁₃O₆⁺, but only an estimated quarter of the protons are present even in this form at any given instant. Thanks to the Grötthus mechanism of chain transfer along hydrogen bonds, it is best to think of a proton or a hydroxide ion simply being available anywhere it is needed. This has all kinds of mechanistic consequences in aqueous media. General acid catalysis is the rule, not the exception, in reactions in concentrated aqueous acids; usually, however, it cannot be recognized. Charged tetrahedral intermediates do not exist; neutral ones are formed directly in hydrolysis processes in acid or base. Protonated alcohols do not exist; all of the listed pK_BH+ values for these are wrong, as is the –1.74 value listed for water (numbers around –6 to –8 seem much more probable; the NMR spectral changes observed in alcohols on moving to progressively stronger acid media are exclusively medium effects). There are many other examples.

General Paper:

Type of Paper: Article
Title: Solvolyses of Benzoyl Chlorides in Weakly Nucleophilic Media
Authors: T. William Bentley and H. Carl Harris
Affiliation: Chemistry Unit, Grove Building, Swansea University, Swansea SA2 8PP, Wales, UK; E-Mail: t.w.bentley@swansea.ac.uk (T.W.B.)
Abstract: Rate constants and activations parameters are reported for solvolyses of p-Z-substituted benzoyl chlorides (Z = OMe, Me, H, and Cl) in 97% hexafluoroisopropanol-water (97H). Additional kinetic data are reported for solvolyses in acetic and formic acids. Plots of log k vs. s_p in 97H are consistent with previous research showing that a cationic reaction channel is dominant, even for solvolyses of 1, Z = NO₂. A benzoyl cation intermediate was trapped by Friedel-Crafts reaction with 1,3,5-trimethoxybenzene in hexafluoroisopropanol. The results are explained an S_N2-S_N1 spectrum of mechanisms with variations in nucleophilic solvent assistance. Ab initio calculations of heterolytic bond dissociation energies of various chloro- and fluoro-substituted and other benzoyl chlorides are correlated with log k for solvolyses.
Keywords: solvolysis; substituent effects; solvent effects; acylium cations