Ipso substitution, in nitration

The importance of ipso-substitution in the products from various nitrations of some activated acetophenones has been demonstrated.17 Acetophenone (9a) is, for example, converted into (9b) by 68% nitric acid at 0°C. IV,IV-Dialkylanilines are readily nitrated at the para-position by cerium(IV) ammonium nitrate in acetonitrile at room temperature.18 Care should be taken with this procedure as reaction under closely related conditions proceeds to form presumably carcinogenic tetraalkylbenzidines. 

The synthetic potential of ipso nitration has been previously discussed (see Section 1, Primary Amines) and has made regiospecific nitration possible. The course of the reaction of an ipso nitration product can be directed such that either the isomerically pure o-nitroalkylbenzene or p-nitroalkylbenzene may be obtained from the same starting material (Scheme 25). A rare example of ipso substitution in an aromatic system possessing electron-withdrawing groups has been reported. ... 

A major feature promoting overall ipso substitution will be ease of formation of Y , and we might thus expect to see some such displacement of secondary and tertiary alkyl substituents, because of the relative stability of the resultant carbocations, R . This is found to happen in the nitration (nitrodealkylation) reactions below ... 

Of necessity, only a few of the many papers published in this area have been examined in detail in this review. Clearly, the area of aromatic nitration is still full of surprises38 and ipso-attack of nitrating species on substituted aromatics is proving useful in elucidating many of the subtle mechanistic details37. 

To sum up, the adsorption of p-anisaldehyde on a particule of clay results in a considerable increase ip, the ipso substitution pathway during nitration. 

Exercise 22-30 a. In the nitration of para-cymene by ethanoyl nitrate in ethanoic anhydride, the observed product composition at 0° is 41 % 5 and 6, 41 % 3, 8% 4, and 10% of 4-nitromethylbenzene. Use these results to determine the relative reactivities of the para-cymene ring carbons towards N02 . Give your answer relative to C3 as unity (C3 is the carbon next to the isopropyl group). Determine the relative reactivities based on the data obtained in Equation 22-1. How does neglect of ipso substitution affect calculation of relative reactivities of the ring carbons b. Write a mechanism for the solvolytic conversion of 5 and 6 to 3. 

In the case of 5-bromo-3,4-dimethyl-l-phenyl-l/f, 6//-pyrano[2,3-c]pyrazol-6-one (59), mixed acid nitration results not only in para substitution in the phenyl ring, but also in ipso attack at the 5-position to yield... 

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