Electrofugalities of Acceptor‐Substituted Tritylium Ions

Horn, Markus; Metz, Christian; Mayr, Herbert

doi:10.1002/ejoc.201100912

Cited by 24 publications

(39 citation statements)

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“…By combining stopped‐flow techniques with conductivity detection, we have been able to measure ionization processes with first‐order rate constants as fast as 10 3 s −1 . This method is only applicable to mixtures of solvents, however, because the substrates have to be dissolved in an unreactive aprotic solvent, for example, acetonitrile or acetone, which can rapidly be mixed with water or an aqueous solution in the stopped‐flow instrument.…”

Section: Electrofugalities: Rates Of Hydrolyses Of Trityl Derivativesmentioning

confidence: 99%

A comprehensive view on stabilities and reactivities of triarylmethyl cations (tritylium ions)

Horn

Mayr

2012

J of Physical Organic Chem

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By combining conventional photometric and conductimetric methods with stopped‐flow techniques, we were able to investigate ionization rate constants of trityl bromides, chlorides, and carboxylates in aqueous acetonitrile in a reactivity range from 10−5 to 103 s−1. In this way, it became possible to compare ionization processes yielding tritylium ions which differ by 21 units in pKR+, corresponding to 120 kJ mol−1 in free energy. Analysis of such far‐reaching correlations provided new insights into several aspects of organic reactivity. We observed (i) the change of solvolysis mechanisms from SN1 reactions without and with common ion return, (ii) reactions where formation and consumption of the carbocations could be observed visually, (iii) solvolyses where the ionization step is almost complete before the reaction with water becomes significant, and (iv) ionizations that yield solutions of persistent carbocations. The continuous change from reactions with carbocation‐like transition states to reactions with transition states of little carbocation character caused large variations of Winstein–Grunwald m values. Values of m = 0.2 for ionization processes show that low values of m do not necessarily imply SN2 reactions. Although the 4,4′,4″‐trimethoxytritylium ion reacts more than 1000 times faster with water than the 4‐dimethylaminotritylium ion and has a pKR+ value that is more negative by three units, both carbocations are formed with almost equal rates by ionization of the corresponding acetates in aqueous acetonitrile. This comparison again illustrates that electrofugality is not simply the inverse of electrophilicity and presents a challenge for further study of the importance of intrinsic barriers. Copyright © 2012 John Wiley & Sons, Ltd.

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Section: Electrofugalities: Rates Of Hydrolyses Of Trityl Derivativesmentioning

confidence: 99%

A comprehensive view on stabilities and reactivities of triarylmethyl cations (tritylium ions)

Horn

Mayr

2012

J of Physical Organic Chem

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“…The electrofugality scale has also been extended to other electrofuges that are not benzhydrylium derivatives [32,33], whose E f parameter were calculated from the first-order solvolysis rates and the N f parameters of the leaving group used. According to existing electrofugality and nucleofugality scales the solvolysis rates can be predicted in the range of 25 orders of magnitude.…”

Section: Introductionmentioning

confidence: 99%

Effect of the Leaving Group and Solvent Combination on the LFER Reaction Constants

Matić¹,

Jurić²,

Denegri³

et al. 2012

IJMS

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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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“…90 Unexpectedly, Br − common ion return is faster than Cl − return in all the solvents. The ionization rates for these S N 1 reactions were found to correlate with the thermodynamic stability of the tritylium ions in aqueous solution.…”

Section: Medium Effects/solvent Effectsmentioning

confidence: 97%

Nucleophilic Aliphatic Substitution

Westaway¹

2014

Organic Reaction Mechanisms Series

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Electrofugalities of Acceptor‐Substituted Tritylium Ions

Cited by 24 publications

References 48 publications

A comprehensive view on stabilities and reactivities of triarylmethyl cations (tritylium ions)

A comprehensive view on stabilities and reactivities of triarylmethyl cations (tritylium ions)

Effect of the Leaving Group and Solvent Combination on the LFER Reaction Constants

Nucleophilic Aliphatic Substitution

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