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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

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

Grade:11

2 Answers

Gaurav
askIITians Faculty 164 Points
9 years ago
Hello Student
1.Aniline undergoes resonance and as a result, the electrons on the N-atom are delocalized over the benzene ring. Therefore, the electrons on 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 is increased. As a result, aniline is less basic than methylamine. Thus, pKb of aniline is more than that of methylamine.

2.Ethylamine when added to water forms intermolecular H−bonds with water. Hence, it is soluble in water.aniline does not undergo H−bonding with water to a very large extent due to the presence of a large hydrophobic −C6H5 group. Hence, aniline is insoluble in water

3.Due to the +I effect of −CH3 group, methylamine is more basic than water. Therefore, in water, methylamine produces OH− ions by accepting H+ ions from water.
Kushagra Madhukar
askIITians Faculty 628 Points
3 years ago
Dear student,
Please find the attached solution to your problem below.
 
(i)In aniline the N atom is attached to the benzene ring and therefore the lone pair on N is declocalised over the entire
benzene ring. Due to this it cannot accept a proton. In methylamine the electron density on nitrogen is greater than aniline this is due to the presence of methyl group in methylamine. Thats the reason pKb of aniline is more
 
(ii) Ethylamine is capable of forming hydrogen bond with water as it is soluble but aniline the bulkycarbon prevents the formation of effective hydrogen bondinf and not soluble
 
(iii) That is due to the basic strength of CH3NH2. The +I effect of methyl group makes more basicity than water and in water methylamine produce OH- ions
 
(iv) During Nitration process the mixture used (Con HNO3+Con H2SO4) protonates anilinium ion and nitroaniline is formed. The anilium ion acts as a deactivating group and directs the NO2 nucleophile to meta position and meta isomer of nitroaniline is formed
 
(v) During Friedal-Crafts reaction,anhydrous AlCl3 is used as a lewis acid for generation of electrophile from electrophilic reagent. If aniline is used as base there wont be any generation of electrophile takes place and hence reaction does not take place
 
(vi) Diazonium salt of aromatic amine is stable due to the resonance. it carries N atom with a positive charge. such a delocalisation is not possible in aliphatic amines. hence it is less stable
 
(vii) Gabriel pthalmide reaction gives pure primary amines without any contamination in secondary and tertiary amines. hence they are mpore prefered for synthesis of primary amines
 
Hope it helps.
Thanks and regards,
Kushagra

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