Triphenylmethylium ions

Three significant hydrocarbon ions, [S" ], [9 ] and [10 ], should be mentioned as additional examples of the formation of stable salts of the hydrocarbon ions. The triphenylmethylium ion [8" ], as a prototypical aromatic carbocation, reacts with the chloride ion to give the chloride, a... 

Besides the carbocations mentioned above, numerous highly stable carboca-tions have been isolated as the salts of inorganic anions. Azulene analogues of triphenylmethylium ion [ll ]-[20 ], [21 j, [22 " ] and [23 j trisubsti-tuted cyclopropenylium ions [l" ] and [24 ]-p6 ] cyclopropyl-substituted tropylium ions [27 ]-[30 ] tropylium ions annelated with bicyclic frameworks [31 ]-[36 ] and [37 ]-[39 ] fulvene-substituted cyclopropenylium [40 ] and tropylium [41 ] ions a tropylium ion condensed with acenaphthylene [42 ] and a cation containing a 147t periphery [43 ] have... 

The paper contains several innovations in the interpretation of polarographic reduction potentials, Eyi. One is the recognition that the Eyi obtained in the presence of base-electrolytes is not that of an isolated, solvent-solvated cation, but of one which is part of an ion-pair or of a higher aggregate. A practically useful innovation is to use the Em of the triphenylmethylium ion as the zero of the potential scale in all solvents. By means of this device one can compare a wide range of Ey2 differences in different solvents, and it is especially useful because that ion is stable in strongly acidic media in which the commonly used marker ferrocene decomposes. 

A useful practical result of our work is that EV2 of the triphenylmethylium ion was found to be a convenient reference potential for strongly acidic solutions as the ion is very stable under such conditions, for which, in fact, there is no other convenient standard. 

In view of the useful interpretations which can be given to the differences in E1/2 values, an obvious step is to select the E1/2 of one convenient ion as a zero or reference point. We follow Taft [14] and Breslow [15] in selecting E1/2 for the triphenylmethylium (trityl) cation, as this ion is easy to procure and is stable under acidic conditions. It seems useful to define a quantity Ef/2 which is 1/2 of the species in question minus EV2 of the triphenylmethylium ion measured under the same conditions of concentration, temperature, solvent, and nature and concentration of base electrolyte, and against the same reference electrode ... 

We have already mentioned that Dorfman and collaborators have developed a versatile technique to observe ort-lived carbenium ions in solution generated by dissociative pulse radiolysis. This novel approach to the characterisation of transient species has also allowed this schod to measure the rate constants of many electrophilic reactions between carbenium ions (the benzylium ion in particular) and various nucleophiles. In the first paper of the series Jones and Dorfman reported the rate constants of the benzylium ion reaction with methanol, ethanol, the bromide and the iodide ions in ethylene chloride at 24 C. Values of about 5 x 10 sec were obtained for the halide ions and of around 10 sec for the alcohols. Later studies confirmed that the reaction of halide ions vrith benzylium, diphenyl-methylium and triphenylmethylium ions is at the limit of diffusion control. Reaction rate constants of these three carbenium ions with amines and alcdiols were also reported in the same paper. More recently, these studies have been extended to include cyclopropylphenylmetiiylium ion as electrophile, ammonia as nucleophile and methylene chloride and trichloroethane as solvents These results are extremely... 

The determination of residual moisture has always been a difficult problem. Penczek and Slomkowski have developped an ingenious method based on the reaction of the triphenylmethylium ion with water. 

These methods involve removal of an anion from carbon the anions may be of classical type or involve hydride or organic groups such as a-exoCN or a-C02H (but not jS-COjH removed by triphenylmethylium ion. Sulfuric acid also can reverse the addition of nucleophiles with C—O, C—N, C —S or cleavage Obtaining a desired... 

Trityl (triphenylmethylium) and tropylium cations are two commercially available carbenium ions. They react with alkenes by either direct addition or by hydride abstraction tropylium ions are usually less reactive than trityl. As shown in Eq. (39), trityl ions initiate styrene polymerization by direct addition. 

In spite of many publications in which equilibria of the type described in Scheme (19) are postulated, accepted or rejected on various grexmds, the cartenium-onium eqpilibrium has not been determined quantitatively. Only recently has this equilibrium been studied in a modd sydem, using tri enylmethylium cation as a model carbenium ion and various linear or cyclic ethars or acetals as nucl )philes At room temperature in CH2CI2, the equflibrium constant wasfound to be K = 1.6 mole 1 and Ke = 6.3 10 mole 1 for THF and 1,3-diox[Pg.17]

The AH0 and ASq values of triphenylmethylium salts are also close to those found for poly-THF and polyoxepane ion-pairs, being equal to - 2.2 kcal mole and - 25.0 e. u., respectively (measured for AsF and SbFg anions in CH2CI2 at 25... 

Heublein a.o. ° have studied stability of various complex anions in decomposition of triphenylmethylium salts, assuming that the rate constant of collapse of ion pair can be used as a measure of anion stability ... 

Total resonance stabilization has been calculated at the MP2/6-31G level for a number of conjugated ions, as shown in Table 3.9. One general trend seen in this data is a decrease of the effect of successive phenyl rings in the benzyl, diphenylmethyl, and triphenylmethylium series. Two factors are involved. The effect of resonance stabilization is decreased because the successive rings must be twisted from planarity. This factor and related steric repulsions attenuate the effect of each successive phenyl group relative to methyl (see Column B). In the allyl series a 1-phenyl substituent adds 39.4 kcal of stabilization, while a 2-phenyl group adds only 11.4. ... 

Treatment of methyl 2-0-benzoyl-3,4-0-benzylidene-P-D-arabinopyranoside with triphenylmethylium tetrafluoroborate in acetonitrile gave an equilibrium mixture of the benzoxonium ions (96) and (97), which reacted with chloride or iodide ions to give(98) and (99) in the ratio of 6 1 (X = Cl) or 1 2 (X = I), signifying differences in the reactivities of (96) and (97) towards different halide ions. The... 

Some carbenium salts that form sterically hindered addition product, that is, triphenylmethylium (trityl) salts may initiate through hydride ion abstraction as shown in Scheme 9 (route Oxocarbenium ions thus formed serve as actual... 

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