Broensted versus Lewis acid reactivity of gaseous cations (ethyl, isopropyl, formyl) towards arenes

The reactivity of C2H5+, i-C3H7+, and HCO+ ions, characterized by the simultaneous presence of acidic hydrogen(s) and of a carbon-centered electrophilic site, has been investigated with the aim of obtaining quantitative data on their competitive behavior as Lewis or Bronsted acids toward simple perdeuterated arenes. The reaction of the radiolytically formed cations in gaseous systems at a total pressure of ca. 700 Torr led to ethylated and isopropylated, but not formylated, products in addition to protonated products, i.e., the same substrates incorporating a H-1 atom on the aromatic ring. The NMR analysis of the reaction with toluene yielded H-1 incorporation/alkylation ratios of ca. 3.5 (CH5+/C2H5+ reactant ions) and ca. 0.09 (i-C3H7+ ions), which is suggested to reflect an intrinsic branching of two different reaction pathways, rather than competitive reaction channels of common intermediates. The dual behavior of ethyl cations emerged clearly in their reaction with benzene, where NMR and GC/MS analysis of the products from suitably labeled reactants led to a protonation/alkylation ratio of ca. 1.O. The protonation of toluene by CH5+/C2H5+ shows a low positional selectivity and is followed by 1,2-hydrogen shifts having an estimated free energy of activation of ca. 8 kcal mol-1 at 37°C.