Question -
Answer -
(i) pKb of aniline is more than that of methylamine.
(ii) Ethylamineis soluble in water whereas aniline is not.
(iii) Methylamine in water reacts with ferric chloride toprecipitate hydrated ferric oxide.
(iv) Althoughamino group is o, p− directing in aromaticelectrophilic substitution reactions, aniline on nitration gives a substantialamount of m-nitroaniline.
(v) Anilinedoes not undergo Friedel-Crafts reaction.
(vi) Diazoniumsalts of aromatic amines are more stable than those of aliphatic amines.
(vii) Gabriel phthalimide synthesis is preferred forsynthesising primary amines.
Answer
(i) pKb of aniline is more than that of methylamine:
Aniline undergoes resonance and as a result, the electronson the N-atom are delocalized over the benzene ring. Therefore, the electronson the N-atom are less available to donate.
On the other hand, in case of methylamine(due to the +I effect of methyl group), the electron density on the N-atom isincreased. As a result, aniline is less basic than methylamine. Thus, pKb ofaniline is more than that of methylamine.
(ii) Ethylamineis soluble in water whereas aniline is not:
Ethylamine when added to water forms intermolecularH−bonds with water. Hence, it is soluble in water
But aniline does not undergo H−bonding withwater to a very large extent due to the presence of a large hydrophobic −C6H5 group.Hence, aniline is insoluble in water.
(iii) Methylaminein water reacts with ferric chloride to precipitate hydrated ferric oxide:
Due to the +I effect of −CH3 group,methylamine is more basic than water. Therefore, in water, methylamine producesOH− ions by accepting H+ ionsfrom water.
Ferric chloride (FeCl3)dissociates in water to form Fe3+ and Cl− ions.
Then, OH− ion reacts withFe3+ ion to form a precipitate of hydrated ferric oxide.
(iv) Althoughamino group is o,p− directing in aromatic electrophilicsubstitution reactions, aniline on nitration gives a substantial amountof m-nitroaniline:
Nitration is carried out in an acidic medium. In anacidic medium, aniline is protonated to give anilinium ion (which is meta-directing).
For this reason, aniline on nitration gives a substantialamount of m-nitroaniline.
(v) Anilinedoes not undergo Friedel-Crafts reaction:
A Friedel-Crafts reaction is carried out inthe presence of AlCl3. But AlCl3 is acidic innature, while aniline is a strong base. Thus, aniline reacts with AlCl3 toform a salt (as shown in the following equation).
Due to the positive charge on the N-atom, electrophilicsubstitution in the benzene ring is deactivated. Hence, aniline does notundergo the Friedel-Crafts reaction.
(vi) Diazoniumsalts of aromatic amines are more stable than those of aliphatic amines:
The diazonium ion undergoes resonance as shown below:
This resonance accounts for the stability of the diazoniumion. Hence, diazonium salts of aromatic amines are more stable than those ofaliphatic amines.
(vii) Gabrielphthalimide synthesis is preferred for synthesising primary amines:
Gabriel phthalimide synthesis results in the formation of1° amine only. 2° or 3° amines are not formed in this synthesis. Thus, a pure1° amine can be obtained. Therefore, Gabriel phthalimide synthesis is preferredfor synthesizing primary amines.