Structure and Reactivity of Free Arylium Ions. 2. Reactions of Decay-Formed Isomeric XC6H4+ Ions with Gaseous and Liquid Nucleophiles

Labeled arylium ions XC6H4+ (X = NOz, CN, C1, Br, OH, OCH3) (ortho, 40%; meta, 40%; para, 20%) from the decay of uniformly multitritiated arenes XC6H5 have been allowed to react with methanol in the liquid and gaseous phase (P = 5-65 Torr) and with methyl halides in the gas phase (P = 10-760 Torr) at room temperature. The isomeric composition of the products from 02NCsH4+, characterized by a pronounced depletion of the ortho isomers, is regarded as due to effective singlet-carbenic to biradicalic state crossing in o-O2NC&+, favored by proximity between a lone pair of the NO2 substituent and the formally vacant orbital. The same process is prevented in o-NCC6H4+ by unfavorable arrangement of the corresponding orbitals. Nuclear decay Of XCsH5 (X = C1, Br, OCH3) in liquid CH30H generates the corresponding methoxy derivatives in proportions reflecting those of their corresponding XC6H4+ precursors (ortho:meta:para = 2:2:1). The specific solvation mode in o-HOC&+ allows instead its extremely fast conversion to phenoxenium ion C6H50+, prior to addition to CH30H. The nature and the isomeric composition of the labeled products formed in gaseous CH30H (5-65 Torr) and methyl halides (10-760 Torr) point to Xc&+ ions (X = C1, Br) with no tendency to undergo ring-hydrogen migration. In HOC6H4’ ions, instead, intramolecular 1,2-hydrogen transfers readily take place whose phenomenological rate constants have been evaluated. In CH30C6H4’ ions, the same process is obscured by a rapid l,4-hydride ion transfer in o-CH30C6H4+ from the methyl moiety to the vacant ring orbital. The effects of the X substituent upon the behavior of arylium ions in both gaseous and liquid media have been compared with the data of previous related studies and are discussed in light of theoretical predictions.