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Account for the following

(i) pKb of aniline is more than that of methyl amine. 

(ii) Ethylamine is soluble in water whereas aniline is not

(iii) Methyl amine in water reacts with ferric chloride to precipitate hydrated ferric oxide

(iv) Although amino group is o- and p- directing in aromatic electrophilic substitution, aniline on nitration gives a substantial amount of m-nitroaniline

(v) Aniline does not undergo Friedel Crafts reaction

(vi) Diazonium salts of aromatic amines are more stable than those of aliphatic amines

(vii) Gabriel phthalimide synthesis is preferred for synthesising primary amines


Solution

(i) In aniline, the electron pair on nitrogen atom is involved in conjugation with ring and is less available for protonation than in methyl amine. Therefore, pKb value of aniline is more than that of methylamine and aniline is less basic, (As higher the pKb value, less is the basicity).

(ii) Ethylamine is soluble in water due to hydrogen bonding.

In aniline due to bulky hydrocarbon part, the extent of hydrogen bonding is less and it is not soluble in water.

(iii) Methylamine being more basic than water, accepts a proton from water and OH ions are produced.

(iv) In presence of conc. HNO3 and conc. H2SO4, most of aniline gets protonated to form anilinium ion. Now aniline and anilinium ions both are present in the reaction mixture. In aniline NH2 group is o- and p-directing but anilinium ion is m-directing. Because of more amount of anilinium ion, a lot of m-nitroaniline is formed.

(v) Aniline being a Lewis base forms a complex with AlCl3, a Lewis acid. As a result, nitrogen of aniline becomes positively charged and acts a strong deactivating group for electrophilic substitution reaction. Consequently, aniline does not undergo Friedel Crafts reaction.

C6H5NH2Aniline(Lewis base)+AlCl3(Lewis acid)[C6H5NH2]+[AlCl3]Complex

(vi) Diazonium salts of aromatic amines are more stable than those of aliphatic amines because these are resonance stabilised while no such resonance stabilisation is possible in the corresponding diazonium salts of aliphatic amines.

(vii) Gabriel phthalimide synthesis produces primary amines only without the traces of secondary or tertiary amines. So, this method is preferred for the synthesis of primary amines.


Chemistry
NCERT Textbook
Standard XII

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