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Account for the following : (i) pK_b of aniline is more than that of methylamine . (ii) Ethylamine is soluble in water, whereas aniline is not. (iii) Methylamine in water reacts with ferric chloride to precipitate hydrated ferric oxide . (iv) Although amino group is o- and p-directing in aromatic electrophilic substitution reactions, aniline on nitration gives a substantial amount of m-nitroaniline. (v) Aniline does not undergo Friedel-Crafts reaction. |
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Answer» Solution :(i) In aniline, the lone pair of electrons on N atom is delocalized over the benzene ring. As a result, electron DENSITY on the nitrogen decreases. On the other hand , in `CH_3 NH_2 + I ` effect of `CH_3` group increases the electron density on N atom. THEREFORE, aniline is less basic than methylamine and hence `pk_b` of aniline is higher than that of methylamine. (ii) Ethylamine dissolves in water due to intermolecular hydrogen bonding as shown below :  However , because of large hydrophobic part (i.e. hydrocarbon part) of aniline, the extent of hydrogen bonding is less and therefore, aniline is insoluble in water. (III) Methylamine is more basic than water and therefore, accepts a proton from water forming `OH^(-)` ions `CH_3 NH_2 + H - OH to CH_3 - NH_3^+ + OH^-` These `OH^(-)` ions combine with `Fe^(3+)` ions to form brown ppt. of hydrate ferric oxide. `FeCl_3 to Fe^(3+) + 3Cl^(-)` `2Fe^(3+) + 6OH^(-) to underset("Hydrated ferric hydroxide (Brown ppt)")(2Fe(OH)_3 " or " Fe_2 O_3 . 3H_2 O)` (iv) Under strongly acidic conditions of nitrotion (in the presence of a mixture of conc. `NHO_3+ H_2 SO_4`) , aniline gets protonated and is CONVERTED into anilinium ion having - `NH_3^(+)` group. This group is deactivating group and is m-directing . So, the nitration of aniline gives o,p-nitroaniline (mainly p-product) while the nitration of anilinium ion gives m-nitroaniline.  Thus, nitration of aniline gives a substantial amount of m-nitroaniline due to protonation of aniline . (v) Aniline being a Lewis base reacts with Lewis acid such as `AlCl_3` to form a salt. `underset("Lewisbase")(C_6 H_5NH_2) + underset("Lewis acid")(AlCl_3) to underset("salt")(C_6 H_5 NH_2^(+) AlCl_3^(-))` As a result , N of aniline acquires + ve charge and hence it acts as a strong deactivating group for electrophilic substitution reaction. Hence aniline does not undergo Friedel-Crafts reaction. |
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