Carbocations from triphenylmethyl chloride

There is direct evidence, from ir and nmr spectra, that the fert-butyl cation is quantitatively formed when ferf-butyl chloride reacts with AICI3 in anhydrous liquid HCl. In the case of alkenes, Markovnikov s rule (p. 1019) is followed. Carbocation formation is particularly easy from some reagents, because of the stability of the cations. Triphenylmethyl chloride and 1-chloroadamantane alkylate activated aromatic rings (e.g., phenols, amines) with no catalyst or solvent. Ions as stable as this are less reactive than other carbocations and often attack only active substrates. The tropylium ion, for example, alkylates anisole, but not benzene. It was noted on p. 476 that relatively stable vinylic cations can be generated from certain vinylic compounds. These have been used to introduce vinylic groups into aryl substrates. Lewis acids, such as BF3 or AIEta, can also be used to alkylation of aromatic rings with alkene units. 

In contrast to triphenylmethyl chloride, which is covalent, triphenylmethyl perchlorate is ionic. The carbocation nature of triphenylmethyl perchlorate is definitively supported by an X-ray crystal structure determination. The salt-like structure is confirmed. The three bonds to the central carbon atom are coplanar, but the three phenyl rings are at an angle of 54° to the plane of the central carbon, giving the cation the overall appearance of a propeller-shaped species. The propeller shape is also indicated for triarylmethyl cations in solution, from analysis of their temperature-dependent NMR spectra. The twisting of the aromatic rings with respect to each other is presumed to result from van der Waals repulsions between the ortho hydrogens. 

Probably the first isolation of a triphenylmethyl carbocation salt was by Gomberg and Cone (68) who successfully prepared the perchlorate from the corresponding chloride. A direct synthesis from the carbinol was achieved at about the same time (69), and more recently the preparation of the perchlorate and tetrafluoroborate have been much improved (70). Anderson (7/) succeeded in recording the characteristic visible absorption spectrum of the ion in concentrated acids, and Fairbrother and Wright (72) observed the same absorption when triphenylmethyl bromide was ionised in benzene in the presence of stannic bromide. 

Swain et al. (1953b) noted that a qualitative relationship exists between the stability of a carbocation and its selectivity. For example, the selectivity of a number of carbocations in aqueous solution and in the presence of azide ion was enhanced with increasing carbocation stability the ratio An /Aw, where An and Aw are the specific rate constants with azide ion and water respectively, was found to increase from 3-9 for the t-butyl cation to 170 for the diphenylmethyl cation, to 240 for the 4,4 -dimethyldiphenylmethyl cation, and to 280,000 for the highly stabilized triphenylmethyl cation. Sneen et al. (1966a) observed that this relationship could be quantified. It was found that a plot of log (An /Aw ) against log A (where A is the solvolytic rate constant) for a number of alkyl chlorides gave a linear correlation. Sneen made the first attempt to utilize such a relationship as a mechanistic tool. The selectivity of... 

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