Intrinsic Reactivity of Gaseous Halocarbocations toward Model Aromatic Compounds

Abstract

The gas-phase reactivity of a set of halocarbocations, + CH 2 X (X = Cl, Br, or I), + CHX 1 X 2 (X 1, X 2 = F, Cl, or Br), and + CX 3 (X = F or Cl), with four prototype aromatic compounds (benzene, furan, pyrrole, and pyridine) was investigated via double- and triple-stage mass spectrometry and compared to that of the simplest + CH 3 carbocation. A rich chemistry is observed, and the reaction channels are greatly influenced by the number and type of halogen substituents (X), the strength of the C−X bonds, the nature of the aromatic compound, and the relative stabilities of the carbocation products. [Ar−CH 2 ] +, [Ar−CHX] +, or [Ar−CX 1 X 2 ] + functionalization of the relatively inert aromatic Ar−H bonds is the main reaction channel observed. A structure-specific “methylene by hydride exchange” reaction with toluene and B3LYP/6-311G(d,p) calculations indicate that the benzylium ion and the 2-furanylmethyl cation are formed in the [Ar−CH 2 ] + functionalization of benzene and furan, respectively. Kinetic isotope effects for the [Ar−CHX] + functionalization using naturally occurring halogen isotopes ( 35 Cl/ 37 Cl and 79 Br/ 81 Br) were measured. Using halogen-mixed halocarbocations + CHX 1 X 2, we evaluated the intrinsic competition for either the [Ar−CHX 1 ] + or [Ar−CHX 2 ] + functionalization. In reactions with pyridine, no Ar−H functionalization occurs and either proton transfer, N-addition, or net [CH 2 ] +• transfer due to the loss of X • from the nascent adducts is observed. Structural characterization of product ions was performed by on-line collision-induced dissociation or ion/molecule reactions, or both, and when possible by comparison with authentic ions.