Acyclic Open-Chain Halonium Ions

Alkyl- and Atylhydridohalonium Ions. The self-condensation of alkyl halides in strongly acidic media represents a convenient preparative route to symmetric dialkylhalonium ions 127 [Eq. (4.97)]. This reaction involves hydridohalonium ions 126 as intermediates that subsequently undergo nucleophilic attack by excess alkyl halide. 

Methyl bromide and methyl iodide in HS03F-SbF5-S02ClF at -78°C give both dimethylhalonium ions (127, R = CH3, X = Br, I) as well methylhydridohalonium ions (126, R = CH3, X = Br, I).349 These hydridohalonium ions have been characterized by 13C NMR spectroscopy.349 Methyl chloride does not give methylhydrido-chloronium ion under similar conditions. 

Protonation of methylhalonium ions to form diprotonated dications CH3XH22+ have been calculated (B3LYP/6-31-G level)348 to occur primarily on the C—H bond to form a pentacoordinated carbon with 2e-3c bond. The halogen atoms in dications, as expected, carry more charge than in the corresponding monocations and the dications are less stable than the monocations (49.5-13.Okcal mol-1). 

Protonation of fluorobenzene in the gas phase has been studied by infrared photodissociation (IRPD) spectroscopy by Solca and Dopfer.351 F-protonated fluorobenzene was formed in significant amount when protonation was carried out with CH5+. It was found to be the most stable isomer in the gas phase by quantum mechanical calculations [B3LYP/6-311G(2df,2pd) level] separated by a large energy barrier from the four Wheland intermediates. F-protonated fluorobenzene is best described as a weakly bound ion-dipole complex between the phenyl cation and HF. 

Dialkylhalonium Ions. Dialkylhalonium ions were first observed as stable fluoroantimonates and characterized by Olah and DeMember333 in 1969. Since then, a large number of unsymmetrical and symmetrical halonium ions have been prepared. The alkylating ability as well as intermolecular exchange reactions of dialkylhalonium ions were also studied.107,352 355 

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