| Abstract
| - Hydrogen/deuterium exchange was observed by2H-NMR spectroscopy at the CH3 groups of thelong-lived alkanoyl cationsCH3CH2CO+(4), (CH3)2CHCO+(8), and(CH3)3CCO+ (13)when treated with excess DF·SbF5 superacid. The intermediacy of the correspondingprotio(deuterio)acylium dications is suggested toaccountfor the exchanges. Under similar conditions, no exchange wasobserved in the acetyl ion CH3CO+(1) in DF·SbF5,but at the same time its electrophilic reactivity is greatly enhancedin superacids. The acetyl cation in a superacidicmedium also abstracts tertiary hydrogens of isoalkanes to giveprotonated acetaldehyde. Density functional theorycalculations at the B3LYP/6-31G** level were carried out to support thesuggested exchange mechanism as well aslack of exchange in the acetyl cation. On the basis of thecalculated results, two alternative mechanisms are alsosuggested for the Nenitzescu rearrangement of pivaloyl chloride inisobutane with excess AlCl3.
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