Perfluoroaromatics.  Tris(polyfluoroaryl) carbocations

“…[38] As Table 1. Calculated gas phase hydroxide and methyl anion affinities ΔH (in kJ mol À1 , at 298 K) [38,43] of substituted tritylium ions (MP2(FC)/6-31+G(2d,p)//B3LYP/6-31G(d,p)) and their pK R+ values [29,[44][45][46][47] Cation a ΔH OH À (Eqn 2) ΔH Me À (Eqn 3) pK R+ (Eqn 5)…”

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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“…While 2 slowly decomposes at room temperature, the (p-F-C 6 H 4 ) 2 CF + cation in 1 simply converts over several days in fluorobenzene to the (p-F-C 6 H 4 ) 3 C + trityl ion, detected by UV-vis spectroscopy some years ago. 17 Cl 11 ]?1.5C 6 H 5 F, 4, were formed readily and the X-ray structures determined. { 11 The structures of the cations in 3 and 4 are generally similar to those of unfluorinated trityl ions and that in 4 is shown in Fig.…”

Section: Communicationmentioning

confidence: 99%

“…While 2 slowly decomposes at room temperature, the (p-F-C 6 H 4 ) 2 CF + cation in 1 simply converts over several days in fluorobenzene to the (p-F-C 6 H 4 ) 3 C + trityl ion, detected by UV-vis spectroscopy some years ago. 17 Thus, all of the C-F bonds of the original trifluoromethyl group have been broken, two of them being converted to C-C bonds. In an analogous reaction using the more highly fluorinated substrate p-CH 3 -C 6 F 4 -CF 3 with Et 3 Si-(CHB 11 Cl 11 ) in fluorobenzene as solvent, the new trityl ion (p-F-C 6 H 4 ) 2 (p-CH 3 -C 6 F 4 )C + is formed.…”

mentioning

confidence: 99%