Test Index

NCERT Class XII Chemistry
Chapter - Amines
Questions with Solutions

© examsnet.com
Question : 3
Total: 22
Account for the following :
(i) pKb 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.
(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) If the pKb value of any base or compound is higher than that of another, it implies that the former is a weaker base than the latter. In aniline, the N-atom is attached to the benzene ring and therefore the lone pair on N is delocalised overthe entire benzene ring. As a result, it cannot accept a proton or any other electrophile.
This is why it has a lower Kb value (lower basic strength) and high corresponding pKb value.
In methylamine, CH3NH2 , the electron density on nitrogen is greater than that in case of aniline. This is because –CH3 group in methylamine, by virtue of its +I effect, increases electron density on N, which is more available for protonation.
(ii) Any compound capable of forming hydrogen bonds with water, dissolves in it. Ethylamine is able to do the same and hence its solubility.
H5C2−
N...
|
N
|
H
...H−
O
|
H
....H−
H...
|
N
.
:
‌
−C2H5
However, in aniline, the bulky hydrocarbon part – C6H5 prevents the formation of effective hydrogen bonding and therefore it is not soluble.
(iii) The formation of hydrated ferric oxide may be understood by taking into consideration the basic strength of CH3NH2. In presence of CH3NH2, water hydrolyses as
These OH ions react with FeCl3 as:
2FeCl3+6OH→2Fe(OH)3or
Fe2O3.3H2O
hydrated‌feric‌oxide
(iv) During nitration, the nitration mixture used (conc. HNO3 and conc. H2SO4) protonates the NH2 group to produce anilinium ion as
This anilinium ion acts as a deactivating group and directs the incoming nucleophile –
⊕
N
O2 to meta position and the meta-isomer of nitroaniline is obtained.
(v) During Friedel-Crafts reaction, anhydrous AlCl3 is used as a Lewis acid for generation of the electrophile from the electrophilic reagent.
But when this reaction is carried out with aniline, no electrophile generation takes place. The reason being the presence of aniline as a base. Aniline is a Lewis base, reacts with AlCl3 and hence deactivates it. The Lewis acid is therefore no moreavailable for electrophile generation and hence reaction does not take place.
(vi) Diazonium salts carry a N atom with a positive charge. This positive charge is well dispersed in aromatic diazonium salts through resonance as shown below:
Such a charge delocalisation is not possible in aliphatic amines and hence they are less stable.
(vii) Gabriel phthalimide reaction gives pure primary amines without any contamination of secondary and tertiary amines. Therefore, it is preferred for synthesising primary amines.
© examsnet.com
Go to Question:
Prev Question
Next Question