Tert-Amyl cation

Figure 6.1, H NMR spectra of (a) the tert-butyl cation [trimethylcarbenium ion, (CH jj-jC ] [b) the tert-amyl cation [dimethyletbylcarbenium ion, (CH3)3C -C2H 5] and (c) the isopropyl cation [dimethylcarbenium ion, (CH3)2C Hj (60 MHz, in SbF5 S02CIF solution, —60°C).

Dimethylpropane91 (neopentane) is transformed to a mixture of ethyldi-methylcarboxonium ion (21) and dimethylmethylcarboxonium ion (15). At —78°C 21 is formed exclusively. At temperatures higher than -20°C, 15 becomes the predominant product. Formation of 21 can be best explained by a reaction path that involves insertion of protonated ozone into the C—H a bond, formation of the tert-amyl cation through rearrangement, and its quenching by ozone ... 

Fundamental rate data for carbonium ion rearrangements may be obtained by dynamic NMR studies. Analysis of the temperature-dependence of the NMR spectrum of tert-amyl cation in SbF5-FS02Cl revealed that the methyl protons became equivalent to each other. The mechanism proposed ... 

Reactions of this kind formally involve rearrangements of intermediate carbonium ions by a 1,2 migration of alkyl. In this case the carbonium ion initially formed is an unstable primary ion. The rearrangement leads to its conversion to a tertiary carbonium ion in which three — / alkyl groups are attached to the cationic center. The products are then derived from the resulting tert-amyl cation rather than neopentyl. 

Besides the tert-butyl cation, we also succeeded in preparing and studying the related isopropyl and the terf-amyl cations. The isopropyl cation was of particular relevance. 

Eigure 6.2. IR spectra of tert-huty [top], isopropyl [center], and tert-amyl [bottom) cations. T = transmission. 

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