Triphenylmethyl cation structure

Thus a number of canonical structures is possible. Like triphenylmethyl cation, diphenylmethyl cation is also very stable. In some cases the carbocations are so stable that their salts have been... 

One of the most stable carbocation structures is the employing all three rings. Trityl chloride ionizes read-triphenylmethyl cation (trityl cation). In this struc- ily, and can capture an available nucleophile, ture, the positive charge is stabilized by resonance... 

The study of carbocations has now passed its centenary since the observation and assignment of the triphenylmethyl cation. Their existence as reactive intermediates in a number of important organic and biological reactions is well established. In some respects, the field is quite mature. Exhaustive studies of solvolysis and electrophilic addition and substitution reactions have been performed, and the role of carbocations, where they are intermediates, is delineated. The stable ion observations have provided important information about their structure, and the rapid rates of their intramolecular rearrangements. Modem computational methods, often in combination with stable ion experiments, provide details of the stmcture of the cations with reasonable precision. The controversial issue of nonclassical ions has more or less been resolved. A significant amount of reactivity data also now exists, in particular reactivity data for carbocations obtained using time-resolved methods under conditions where the cation is normally found as a reactive intermediate. Having said this, there is still an enormous amount of activity in the field. 

As an example of conformational change induced by the introduction of electric charges along a macromolecule, let us consider the polymer (15) which in its side chains carries triarylmethane groups in their leuco-form (15). Upon photoirradiation this neutral group dissociates into triphenylmethyl cation and hydroxyl anion. The polymer chain of the irradiated sample adopts an expanded conformation to alleviate the unfavorable ionic interactions between adjacent cations. These changes are reflected by a marked increase in solution viscosity upon photoirradiation in aqueous or methanolic solutions of a polyacrylamide with side chains of structure (15). The viscosity recovers its initial value when irradiation is discontinued. 

Carbonium ions1 are of the type R1R2R3C+. The triphenylmethyl cation, one of the earliest known, owes its stability primarily to the fact that the positive charge is highly delocalized, as indicated by canonical structures of the type lO-I(a-d). It behaves in some respects like other large univalent cations (Cs+, R4N+, R4As+, etc.) and forms insoluble salts with large anions... 

Comparison of the IR and Raman spectra of the trimethylcarbenium ion with those of its isoelectric analogue — trimethylboron — has conflrmed the planar structure of alkylcarbenium ions. The planar structure of the carbenium fragment —C < was also concluded from the IR spectra of salts of the triphenylmethyl cation (CgH5)3C 449-451) analogues having different substituents at the para positions of the... 

Write two different resonance forms for triphenylmethyl cation. One structure should show the positive charge at an ortho position, the other at a para position. 

In contrast to triphenylmethyl chloride, which has the properties of a covalent compound, triphenylmethyl perchlorate behaves as an ionic compound. The presence of triphenylmethyl cations in this solid has been confirmed by an X-ray crystal structure determination.The central carbon is planar, but the three phenyl rings are at an angle of 54 to the plane of the trigonal carbon so that the overall cation has a propeller-like shape. The temperature-dependent NMR spectrum of the carbocation also indicates that it has this structure in solution.The twisting of the aromatic rings with respect to each other is evidently the result of van der Waals repulsions between the ortho hydrogens. 

Von Baeyer (Nobel Prize, 1905) should be credited for having recognized in 1902 the saltlike character of the compounds formed. He then suggested a correlation between the appearance of color and salt formation—the so-called halochromy. Gomberg (who had just shortly before discovered the related stable triphenylmethyl radical), as well as Walden, contributed to the evolving understanding of the structure of related cationic dyes such as malachite green. 

An interesting X-ray structure of the highly strained, sp hybridized vinyl cation 8 was recently reported by Muller et al. The synthesis of 8 was accomplished by the reaction of alkynylsilane 7 and triphenylmethyl (trityl) cation. 

The site of reaction on an unsaturated organometallic molecule is not restricted to the most probable position of the metallic atom or cation or to a position corresponding to any one resonance structure of the anion. This has been discussed in a previous section with reference to the special case of reaction with a proton. Although the multiple reactivity is particularly noticeable in the case of derivatives of carbonyl compounds, it is not entirely lacking even in the case of the derivatives of unsaturated hydrocarbons. Triphenylmethyl sodium reacts with triphenylsilyl chloride to give not only the substance related to hexaphenylethane but also a substance related to Chichi-babin s hydrocarbon.401 It will be recalled that both the triphenyl-carbonium ion and triphenylmethyl radical did the same sort of thing. 

Another differential reaction is copolymerization. An equi-molar mixture of styrene and methyl methacrylate gives copolymers of different composition depending on the initiator. The radical chains started by benzoyl peroxide are 51 % polystyrene, the cationic chains from stannic chloride or boron trifluoride etherate are 100% polystyrene, and the anionic chains from sodium or potassium are more than 99 % polymethyl methacrylate.444 The radicals attack either monomer indiscriminately, the carbanions prefer methyl methacrylate and the carbonium ions prefer styrene. As can be seen from the data of Table XIV, the reactivity of a radical varies considerably with its structure, and it is worth considering whether this variability would be enough to make a radical derived from sodium or potassium give 99 % polymethyl methacrylate.446 If so, the alkali metal intitiated polymerization would not need to be a carbanionic chain reaction. However, the polymer initiated by triphenylmethyl sodium is also about 99% polymethyl methacrylate, whereas tert-butyl peroxide and >-chlorobenzoyl peroxide give 49 to 51 % styrene in the initial polymer.445... 

The hrst X-ray crystal structure of a carbocation salt was reported in 1965. Triphenylmethyl perchlorate (27) has a planar central carbon. The three phenyl rings are each twisted 30°, so that overall the cation has a propellor shape. Disordered perchlorate anions sit above and below the central carbon, with a C—Cl separation of 4.09 A. 

The crystal structure of the red triphenylmethylsodium TMEDA complex (compound XVI in Fig. 3), published by Weiss and Koster (41), resembles that of the red triphenylmethyllithium-TMEDA complex (42) and can be described as a n complex between a triphenylmethyl carbanion with an sp2-hybridized central carbon atom and a sodium cation coordinated to the bidentade ligand TMEDA. The sodium atom has close contacts to several carbon atoms of the triphenylmethyl ligand, which possesses twisted phenyl groups. An additional short distance exists between sodium and a p-C (phenyl) atom of a neighboring n system. 

The reactions of the chalcogen(IV) halides are quite variable due to their high general reactivity, their remarkable structural and bonding properties, and the specific influence of the inert pair as an electronic chamaeleon on the reaction paths and products. As examples, important reaction types that will be addressed besides the ones mentioned above are (X = chalcogen, Y = halogen, = metal ions or large cations such as tetraphenylphosphonium(arsonium), triphenylmethyl, tetrabutylammonium, etc.)... 

A fluoroborate salt of the ion 43 can be obtained from the acetal 42 by treatment with triphenylmethyl fluoroborate. As indicated in the cycle of reactions, the cation 43 can rearrange successively, by neighboring-group reactions, into subsequent ions all of the ions are structurally identical with 43. The formula scheme indicates that, after each rearrangement step, the stereochemistry is so disposed that a new neighboring-group reaction can follow. The cycle is completed after ten steps, and the starting point is reached. 

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