While carrying out the nitration of aniline conc.nitric acid is used.As a result nitric acid being a strong oxidising agent results into the formation of ortho and para nitro aniline due to which the yield of the desird product is low nad a mixture of products are obtained.

 

Besides this NH2 group being deactivating group the meta product is also formed to some extent.

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n order to prevent this first aniline is treated with acetic anhydride which forms a complex with NH2 and hence it thereby reduces the deactivating nature of NH2 group and results into the formation of para nitro aniline as a major product.