Ipso attack in nitration

In particular, di- and polyalkyl benzenes having alkyl groups ortho or para to each other, so that mutual activation toward electrophilic attack can occur, may undergo from 25% to as much as 80% ipso attack in nitration reactions. (1,1) As pointed out by Professor Myhre, the ipso nitroarenium ions thus formed can undergo a variety of further processes depending on the nature of the substrate and reaction conditions. (4) One of these reactions is shown below. 

The accumulating weight of indirect evidence indicates that initial positional selectivity of most nitrating agents is nearly the same in a variety of media. ( ) One approach to measuring the extent of ipso attack (in nitration), then, is to perform the nitration in nucleophilic medium under these conditions, the ipso nitroarenium ion is trapped by para nucleophilic addition (below). The amount of 1,4-diene formed is taken to represent the minimum amount of ipso attack occurring. Diene can be assayed... 

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