Question

Account for the following

1. Aniline does not undergo Friedel – Crafts reaction 

2. Diazonium salts of aromatic amines are more stable than those of aliphatic amines 

3. pk_b of aniline is more than that of methy lamine 

4. Gabriel phthalimide synthesis is preferred for synthesising primary amines.

5. Ethylamine is soluble in water whereas aniline is not

6. Amines are more basic than amides 

7. Although amino group is o – and p – directing in aromatic electrophilic substitution reactions, aniline on nitration gives a substantial amount of m – nitroaniline.

Answer 1

1. Aniline does not undergo Friedel – Crafts reaction:

Aniline being a Lewis base reacts with Lewis acid AiCl₃ to form a salt.

Due to the presence of a positive charge on N - atom in the salt the group 

acts as a strongly deactivating group. As a result, it reduces the electron density in the benzene ring and which inhibits the electrophilic substitution reaction. Therefore aniline does not under go Friedel – Crafts reaction.

2. Diazonium salts of aromatic amines are more stable than those of aliphatic amines: 

The diazonium salts of aromatic amines are more stable than those of aliphatic amines due to dispersal of the positive charge on the benzene ring as shown below.

3. pK_b of aniline is more than that of methylamine:

In aniline, the lone pair of electrons on the N – atom is delocalized over the benzene ring. As a result electron density on the nitrogen decreases. In contrast in CH₃NH₂ , +I effect of CH₃ increases the electron density on the N-atom. Therefore, aniline is a weaker base than methylamine and hence its pK value is more than that of methyl amine.

4. Gabriel phthalimide synthesis is preferred for synthesising primary amines: 

Gabriel phthalimide reaction gives pure 10 amine without any contamination of 2° and 3°-amines. Therefore it is preferred for synthesising primary amines.

5. Ethylamine is soluble in water whereas aniline is not:

Ethylamine when added to water forms intermolecular H – bonds with water. And therefore it is soluble in water. But aniline does not form H – bond with water to a very large extent due to the presence of a large hydrophobic – C₆H₅ group. Hence, aniline is insoluble in water.

6. Amines are more basic than amides:

In simple amines, the lone pair of electrons is on nitrogen and hence available for protonation. In amides on the other hand, the electron pair on nitrogen is delocalised to the carboxyl oxygen through resonance and thus it is not available for protonation. So amines are more basic than amides.

7. Although amino group is o – and p – directing in aromatic electrophilic substitution reactions, aniline on nitration gives a substantial amount of m – nitroaniline:

Nitration is usually carried out with a mixture of conc HNO₃ and conc H₂SO₄ . In the presence of these acids, most of aniline gets protonated to form anilinium ion. Therefore, in the presence of acids, the reaction mixture consists of aniline and anilinium ion.

Now – NH₂ , group in aniline is O, P – directing and activating while the – NH₃ group is anilinium ion is meta – directing and deactivating. Whereas nitration of aniline (due to steric hindrance at o – position) mainly gives p-nitroaniline, the nitration of anilinium ion gives m – nitro aniline. In actual practice, approximately a 1 : 1 mixture of P and m – nitroaniline is formed