Account for the following, supporting your answer with diagrams orequations wherever possible :

(a) Diazomium salts of aromatic amines are more stable than those of aliphatic amines.


(b) Methylamine in water reacts with ferric chloride to precipitate


hydrated ferric oxide.


OR


Arrange the following in decreasing order of pKb giving reason :


(a) Aniline, p-nitroaniline and p-toluidine


(b) C2H5NH2, (C2H5)2NH, (C2H5)3N in gaseous state



(a) The general formula of diazonium salts of aromatic amines can be represented as Ar-N2+, where Ar is the phenyl group.


The diazonium salts of aromatic amines are stable due to the delocalisation of the positive charge on the benzene ring known as resonance. Due to this exclusive feature of resonance, they become highly stable.



The diazonium salts of aliphatic amines, R-N2+ lack resonance, and hence these salts are highly unstable and therefor they decompose to give nitrogen gas.



(b) Methyl amine is more basic than water.


This is because, the –CH3 group shows, electron pushing effect (+I) on the N atom, due to which the concentration of electrons get increased, such phenomenon is absent for water.


Due to the high electron concentration, the N atom readily donates its electrons to the H atom of H2O (when methyl amine is added in water), as a result, CH3N+H3 and OH- are formed.



Now, we know, when Fe3+ ion reacts with OH- ion, a reddish brown precipitate of Fe(OH)3 is formed.



OR


(a) pKb and basicity are inverse to each other. It means if pKbis higher then basicity is lesser and viceversa.


Basicity of a compound is determined by its tendency to donate more electrons. If a compound is more electron rich, its tendency to donate electrons would be more so its basic strength would be more.


If groups showing electron pushing tendency (alkyl groups), are attached to the N atom, electron concentration would be more. Similarly more is the number of such groups, more is the basic strength.


Again electron withdrawing groups like –NO2 decrease the basic strength.



This is aniline



This is p-toluidine, and here an electron pushing group –CH3 group is added to aniline which makes the pKb value of p-toluidine lesser than aniline (basicity is more).



This p-nitroaniline, an electron withdrawing group, -NO2 is attached to aniline so it pKb value would be more than aniline (lesser basic strength).


So, pKb value in decreasing order:


p-nitroaniline>aniline>p-toluidine.


(b)If groups showing electron pushing tendency (alkyl groups), are attached to the N atom, electron concentration would be more. Similarly more is the number of such groups, more is the basic strength.


So, 3° amines > 2° amines > 1° amine.


But pKbvalues are just the reverse. So the pKb values in descending order :


C2H5NH2> (C2H5)2NH> (C2H5)3N


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