Special Issue "Correlation Analysis Applied to Solvolysis Reactions"
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A special issue of International Journal of Molecular Sciences (ISSN 1422-0067). This special issue belongs to the section "Physical Chemistry, Theoretical and Computational Chemistry".
Deadline for manuscript submissions: closed (15 December 2011)
Special Issue Editor
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
Special Issue Information
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
Submission
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Keywords
- solvolysis
- correlation analysis
- linear free energy relationships
- Grunwald-Winstein equation
- multiparameter equations
Published Papers (7 papers)
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Received: 6 January 2011; in revised form: 30 January 2011 / Accepted: 9 February 2011 / Published: 15 February 2011
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Abstract: The specific rates of solvolysis of neopentyl chloroformate (1) have been determined in 21 pure and binary solvents at 45.0 °C. In most solvents the values are essentially identical to those for ethyl and n-propyl chloroformates. However, in aqueous-1,1,1,3,3,3-hexafluoro-2-propanol mixtures (HFIP) rich in fluoroalcohol, 1 solvolyses appreciably faster than the other two substrates. Linear free energy relationship (LFER) comparison of the specific rates of solvolysis of 1 with those for phenyl chloroformate and those for n-propyl chloroformate are helpful in the mechanistic considerations, as is also the treatment in terms of the Extended Grunwald-Winstein equation. It is proposed that the faster reaction for 1 in HFIP rich solvents is due to the influence of a 1,2-methyl shift, leading to a tertiary alkyl cation, outweighing the only weak nucleophilic solvation of the cation possible in these low nucleophilicity solvents.
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Received: 6 July 2011; in revised form: 22 July 2011 / Accepted: 18 July 2011 / Published: 28 July 2011
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Abstract: Rate constants and activations parameters are reported for solvolyses of p-Z-substituted benzoyl chlorides (1, Z = OMe, Me, H, and Cl) in 97% w/w hexafluoroisopropanol-water (97H). Additional kinetic data are reported for solvolyses in acetic and formic acids. Plots of log k vs. σp 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 by an SN2-SN1 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.
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Received: 9 September 2011; in revised form: 21 October 2011 / Accepted: 31 October 2011 / Published: 10 November 2011
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Abstract: The specific rates of solvolysis of isobutyl fluoroformate (1) have been measured at 40.0 °C in 22 pure and binary solvents. These results correlated well with the extended Grunwald-Winstein (G-W) equation, which incorporated the NT solvent nucleophilicity scale and the YCl solvent ionizing power scale. The sensitivities (l and m-values) to changes in solvent nucleophilicity and solvent ionizing power, and the kF/kCl values are very similar to those observed previously for solvolyses of n-octyl fluoroformate, consistent with the additional step of an addition-elimination pathway being rate-determining. The solvent deuterium isotope effect value (kMeOH/kMeOD) for methanolysis of 1 was determined, and for solvolyses in ethanol, methanol, 80% ethanol, and 70% TFE, the values of the enthalpy and the entropy of activation for the solvolysis of 1 were also determined. The results are compared with those reported earlier for isobutyl chloroformate (2) and other alkyl haloformate esters and mechanistic conclusions are drawn.
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Received: 19 October 2011; in revised form: 10 November 2011 / Accepted: 14 November 2011 / Published: 28 November 2011
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Abstract: If a species does not have a finite lifetime in the reaction medium, it cannot be a mechanistic intermediate. This principle was first enunciated by Jencks, as the concept of an enforced mechanism. For instance, neither primary nor secondary carbocations have long enough lifetimes to exist in an aqueous medium, so SN1 reactions involving these substrates are not possible, and an SN2 mechanism is enforced. Only tertiary carbocations and those stabilized by resonance (benzyl cations, acylium ions) are stable enough to be reaction intermediates. More importantly, it is now known that neither H3O+ nor HO– exist as such in dilute aqueous solution. Several recent high-level calculations on large proton clusters are unable to localize the positive charge; it is found to be simply “on the cluster” as a whole. The lifetime of any ionized 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 Grotthuss mechanism of chain transfer along hydrogen bonds, in reality a proton or a hydroxide ion is simply instantly available anywhere it is needed for reaction. Important mechanistic consequences result. Any charged oxygen species (e.g., a tetrahedral intermediate) is also not going to exist long enough to be a reaction intermediate, unless the charge is stabilized in some way, usually by resonance. General acid catalysis is the rule in reactions in concentrated aqueous acids. The Grotthuss mechanism also means that reactions involving neutral water are favored; the solvent is already highly structured, so the entropy involved in bringing several solvent molecules to the reaction center is unimportant. Examples are given.
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Received: 20 December 2011; in revised form: 5 January 2012 / Accepted: 5 January 2012 / Published: 10 January 2012
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Abstract: The specific rates of solvolysis of 2-butyn-1-yl-chloroformate (1) and 2-methoxyphenyl chloroformate (2) are studied at 25.0 °C in a series of binary aqueous-organic mixtures. The rates of reaction obtained are then analyzed using the extended Grunwald-Winstein (G-W) equation and the results are compared to previously published G-W analyses for phenyl chloroformate (3), propargyl chloroformate (4), p-methoxyphenyl choroformate (5), and p-nitrophenyl chloroformate (6). For 1, the results indicate that dual side-by-side addition-elimination and ionization pathways are occurring in some highly ionizing solvents due to the presence of the electron-donating γ-methyl group. For 2, the analyses indicate that the dominant mechanism is a bimolecular one where the formation of a tetrahedral intermediate is rate-determining.
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Received: 14 December 2011; in revised form: 3 January 2012 / Accepted: 13 January 2012 / Published: 13 February 2012
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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.
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Received: 3 February 2012; in revised form: 14 March 2012 / Accepted: 15 March 2012 / Published: 21 March 2012
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Abstract: This study synthesized a europium (Eu3+) complex Eu(DBM)3Cl-MIP (DBM = dibenzoyl methane; Cl-MIP = 2-(2-chlorophenyl)-1-methyl-1H-imidazo[4,5-f][1,10]phenanthroline) dispersed in a benzyl methacrylate (BMA) monomer and treated with ultraviolet (UV) light for polymerization. Spectral results showed that the europium complex containing an antenna, Cl-MIP, which had higher triplet energy into the Eu3+ energy level, was an energetically enhanced europium emission. Typical stacking behaviors of π–π interactions between the ligands and the Eu3+-ion were analyzed using single crystal X-ray diffraction. Regarding the luminescence performance of this europium composite, the ligand/defect emission was suppressed by dispersion in a poly-BMA (PBMA) matrix. The underlying mechanism of the effective enhancement of the pure Eu3+ emission was attributed to the combined effects of structural modifications, defect emissions, and carrier charge transfer. Fluorescence spectra were compared to the composite of optimized Eu3+ emission where they were subsequently chelated to four metal ions via carboxylate groups on the BMA unit. The optical enhanced europium composite clearly demonstrated highly efficient optical responses and is, therefore a promising application as an optical detection material.
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Last update: 26 September 2012
