Bromonium ions lifetimes

Interpretation of tiie ratio of capture of competing nucleophiles has led to the estimate that bromonium ions have lifetimes on the order of 10 s in methanol. This lifetime is about 100 times longer than fliat for secondary caibocations. There is also direct evidence for the existence of bromonium ions. The bromonium ion related to propene can be observed by NMR spectroscopy when l-bromo-2-fluoropropane is subjected to superacid conditions. The terminal bromine adopts a bridging position in the resulting cation. 

THE FATES OF BROMONIUM IONS IN SOLUTION LIFETIME WITH MANY DIFFERENT ENDINGS... 

Solvent-dependent lifetimes and ion-pairing of these intermediates can be responsible for the observed variations in the stereo- and chemo-selectivity. Assuming that bromonium ions and carbocations are formed in discrete pathways, the influence of these factors can be readily understood. On the one hand, bridged ions react stereospecifically whatever the medium the... 

Nucleophilic solvents compete with bromide, but anti stereoselectivity is still observed, except when ERG substituents are present. It is proposed that anti stereoselectivity can result not only from a bridged ion intermediate, but also from very fast capture of a carbocation intermediate. Interpretation of the ratio of capture by competing nucleophiles has led to the estimate that the bromonium ion derived from cyclohexene has a lifetime on the order of 10 ° s in methanol, which is about 100 times longer than for secondary carbocations. ... 

There is also kinetic evidence that the addition of bromine to styrene involves an intermediate that is neither a bromonium ion nor a carbocation but a structure in which there is some interaction between the bromine atom and the phenyl-substituted carbon atom. The lifetime of a 1-phenylethyl cation was determined to be but the lifetime estimated by Nagorski... 

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