Halogen-Substituted Arenium Ions

To judge by the values of (-1-0.135 -f-0.150 -f 0.114 and —0.073 for I, Br, a and F, respectively only for fluorine the latter effect predominates over the former. Consequently, among the monohalobenzenium ions the most stable is the 4-fluorobenzenium ion. 

Several halogen-containing ions have been generated by the protonation of halomethylbenzenes. 

As pointed out earlier, substituents of the -t- M-type delocalize more effectively the positive charge of benzenium ions if they are para to the proton than in case of their ortho location. Accordingly the fluoro-, chloro- and bromobenzenes are protonated, to judge from the NMR mostly at the para-position. Neverthe- 

23 Protonation of bromomesitylene, together with 3-bromo-2,4,6-trin[iethylbenzemum ion, yielded a small amount of an ion seeming to attach the proton to the carbon bonded with bromine 

In the case of m-fluorotoluene no 2-fluoro-6-methylbenzenium ion has been detected 363) 

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