Preparation from Alkyl Fluorides in Antimony Pentafluoride Solution and Spectroscopic Studies

Propyl, butyl, and pentyl fluorides with antimony pentafluoride gave the isopropyl, ferf-butyl, and ferf-pentyl cations (as their fluoroantimonate salts) 2, 1, and 3, 

The secondary butyl and amyl cations can be observed only at very low temperatures, and they rearrange readily to the more stable tertiary ions. Generally, the most stable tertiary or secondary carbocations are observed from any of the isomeric alkyl fluorides in superacidic solvent systems. 

The 13C NMR shift in the ferf-pentyl cation [C2H5(CH3)2C+] 3 is at 813C 335.4, which is similar to the that of the ferf-butyl cation. The shift difference is much smaller than the 17 ppm found in the case of the related alkanes, although the shift observed is in the same direction. The 13C NMR chemical shifts and coupling constants 7C-h of C3 to Cg alkyl cations 1-13 are shown in Tables 3.2 and 3.3.95 

It is difficult to interpret these large deshieldings in any way other than as a direct proof that (i) the hybridization state of the carbon atom at the carbenium ion center is sp2 and (ii) at the same time, the sp1 center carries a substantial positive charge. 

Data in Tables 3.2 and 3.3 are characterized by substantial chemical shift deshieldings and coupling constants (/c h) that indicate sp2 hybridization. Also subsequently, Myhre and Yannoni50 have obtained 13C NMR spectrum of ferf-butyl cation 1 in the solid state, which agrees very well with the solution data.95 

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