[chemical substances]
   reactive intermediates
     Class cations [X+]
       Class onium compounds
         Class halonium ions [R2X+ (X=halogen)]
           1 Class fluoronium ions [R2F+]
           2 Class chloronium ions [R2Cl+]
           3 Class bromonium ions [R2Br+]
           4 Class iodonium ions [R2I+]