Observation of stable arylalkoxycarbenium ions: a nuclear magnetic resonance study

Abstract

Stable carbenium ions are formed by treatment of acetals of aromatic aldehydes with BF₃ or fluorosulphonic acid. The structure of these ions has been elucidated from their ¹H chemical shifts. Additional evidence is the presence of a common counter ion in the series. Very good correlations were observed between the ¹H chemical shifts and the respective Brown σ⁺ substituent constants emphasizing the dependence of these quantities on the charge density of the ions. The value of σ⁺ for the H–C⁺–OCH₃ group is + 1.074 in the para-position and +0.916 in the meta-position, reflecting the strong electron-withdrawing properties of this group.