the maximum electron density in these structures? Ortho- and para- positions to the –NH group become centres of high electron density. Thus –NH group is ortho and para directing and a powerful activating group. (a) Bromination: Aniline reacts with bromine water at room temperature to give a white precipitate of , , -tribromoaniline.
The main problem encountered during electrophilic substitution reactions of aromatic amines is that of their very high reactivity. Substitution tends to occur at ortho- and para- positions. If we have to prepare monosubstituted aniline derivative, how can the activating effect of –NH group be controlled ? This can be done by protecting the -NH group by acetylation with acetic anhydride, then carrying out the desired substitution followed by hydrolysis of the substituted amide to the substituted amine.
The lone pair of electrons on nitrogen of acetanilide interacts with oxygen atom due to resonance as shown below: Hence, the lone pair of electrons on nitrogen is less available for donation to benzene ring by resonance. Therefore, activating effect of –NHCOCH group is less than that of amino group. (b) Nitration: Direct nitration of aniline yields tarry oxidation products in addition to the nitro derivatives. Moreover, in the strongly acidic medium, aniline is protonated to form the anilinium ion which is meta directing.
That is why besides the ortho and para derivatives, significant amount of meta derivative is also formed. However, by protecting the –NH group by acetylation reaction with acetic anhydride, the nitration reaction can be controlled and the p -nitro derivative can be obtained as the major product. (c) Sulphonation : Aniline reacts with concentrated sulphuric acid to form anilinium hydrogensulphate which on heating with sulphuric acid at -473K produces p-aminobenzene sulphonic acid, commonly known as sulphanilic acid, as the major product. Aniline does not undergo Friedel-Crafts reaction (alkylation and acetylation) due to salt formation with aluminium chloride, the Lewis acid, which is used as a catalyst.
Due to this, nitrogen of aniline acquires positive charge and hence acts as a strong deactivating group for further reaction. II. DIAZONIUM SALTS The