Chapter 9 Amines NCERT Solutions Class 12 Chemistry- PDF Download
Page No. 278
9.1. Write IUPAC names of the following compounds and classify them into primary, secondary, and tertiary amines.
(i) (CH3)2 CHNH2
(ii) CH3(CH2)2NH2
(iii) CH3NHCH(CH3)2
(iv) (CH3)3CNH2
(v) C6H5NHCH3
(vi) (CH3CH2)2NCH3
(vii) m-BrC6H4NH2
Solution
(i) Propan-2-amine (1° amine)
(ii) Propan-1-amine (1° amine)
(iii) N-methylpropan-2-amine (2° amine)
(iv) 2-Methylpropan-2-amine (1° amine)
(v) N-Methylbenzamine or N-methylaniline (2° amine)
(vi) N-Ethyl-N-methylethanamine (3° amine)
(vii) 3-Bromobenzenamine or 3-bromoaniline (1° amine)
(viii) N, N-dimethylethanamine (3° amine)
9.2. Give one chemical test to distinguish between the following pairs of compounds:
(i) Methylamine and dimethylamine
(ii) Secondary and tertiary amines
(iii) Ethylamine and aniline
(iv) Aniline and benzylamine
(v) Aniline and N-Methylaniline.
Solution
(i) Methylamine being a 1° amine can be distinguished by the carbylamine test.
Carbylamine test: Aliphatic and aromatic primary amines on heating with chloroform and ethanolic potassium hydroxide form foul-smelling isocyanides or carbylamines. Methylamine (being an aliphatic primary amine) gives a positive carbylamine test, but dimethylamine does not.
(ii) Secondary and tertiary amines can be distinguished by allowing them to react with Hinsberg’s reagent (benzenesulphonyl chloride, C6H5SO2Cl).
Secondary amines react with Hinsberg’s reagent to form a product that is insoluble in an alkali. For example, N, N−diethylamine reacts with Hinsberg’s reagent to form N, N−diethylbenzenesulphonamide, which is insoluble in an alkali. Tertiary amines, however, do not react with Hinsberg’s reagent.
(iii) Ethylamine and aniline can be distinguished using the azo-dye test. A dye is obtained when aromatic amines react with HNO2 (NaNO2 + dil.HCl) at 0-5°C, followed by a reaction with the alkaline solution of 2-naphthol. The dye is usually yellow, red, or orange in colour. Aliphatic amines give a brisk effervescence due (to the evolution of N2 gas) under similar conditions.
Aliphatic amines such as ethylamine give a brisk effervescence due to the evolution of N2 gas under similar conditions.
(iv) Aniline and benzylamine can be distinguished by their reactions with the help of nitrous acid, which is prepared in situ from a mineral acid and sodium nitrite. Benzylamine reacts with nitrous acid to form unstable diazonium salt, which in turn gives alcohol with the evolution of nitrogen gas.
On the other hand, aniline reacts with HNO2 at a low temperature to form stable diazonium salt. Thus, nitrogen gas is not evolved.
(v) Primary amines, on heating with chloroform and ethanolic potassium hydroxide, form foul smelling isocyanides or carbylamines. Aniline, C6H5NH2 is a primary aromatic amine and N-methylaniline, C6H5NH(CH3) is a secondary amine. Hence, aniline will give positive carbylamine test while N-methylaniline will not.
Or,
Aniline and N-methylaniline can be distinguished using the Carbylamine test. Primary amines, on heating with chloroform and ethanolic potassium hydroxide, form foul-smelling isocyanides or carbylamines. Aniline, being an aromatic primary amine, gives positive carbylamine test. However, N-methylaniline, being a secondary amine does not.
9.3. 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) pKb of aniline is more than that of methylamine: Higher the pKb value lower is the basic strength.
In aniline due to resonance, the electrons on the N-atom are delocalised 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.
(ii) Ethylamine is soluble in water whereas aniline is not, Ethylamine when added to water forms intermolecular H-bonds with water. Hence, it is soluble in water.
But aniline does not undergo H-bonding with water to a very large extent due to the presence of a bulky hydrophobic –C6H5 group. Hence, aniline is insoluble in water.
(iii) Methylamine in water reacts with ferric chloride to precipitate hydrated ferric oxide: Methylamine is more basic than water due to the +I effect of the CH3 group. Therefore, in water, methylamine produces OH– ions by accepting H+ ions from water.
CH3−NH2 + H – OH → CH3−NH3+ + OH−
Then, OH– ion reacts with Fe3+ ion, from dissociated FeCl3 to form a precipitate of hydrated ferric oxide.
2Fe3+ + 6OH− → Fe2O3.3H2O
Fe2O3.3H2O: Hydrated ferric oxide
(iv) Although amino group is o, p-directing in aromatic electrophilic substitution reactions, aniline on nitration gives a substantial amount of m-nitroaniline: Nitration is carried out in an acidic medium. In an acidic medium, aniline is protonated to give anilinium ion (which is meta-directing).
For this reason, aniline on nitration gives a substantial amount of m-nitroaniline.
(v) Aniline does not undergo Friedel-Craft’s reaction: A Friedel-Craft’s reaction is carried out in the presence of AlCl3 but AlCl3 is a Lewis acid while aniline is a strong base. Thus, aniline reacts with AlCl3 to form a salt.
Due to the positive charge on the N-atom, electrophilic substitution in the benzene ring is deactivated. Hence, aniline does not undergo the Friedel-Craft’s reaction.
(vi) Diazonium salts 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 diazonium ion. Hence, diazonium salts of aromatic amines are more stable than those of aliphatic amines.
(vii) Gabriel phthalimide synthesis is preferred for synthesising primary amines: Gabriel phthamide synthesis results in the formation of 1° amine only. 2° or 3° amines are not formed in this synthesis. Thus, a pure 1° amine can be obtained. Therefore, Gabriel phthalimide synthesis is preferred for synthesizing primary amines.
9.4. Arrange the following:
(i) In decreasing order of pKb values:
C2H5NH2, C6H5NHCH3, (C2H5)2NH and C6H5NH2
(ii) In increasing order of basic strength:
C6H5NH2, C6H5N(CH3)2, (C2H5)2 NH and CH3NH2.
(iii) In increasing order of basic strength:
(а) Aniline, p-nitroaniline and p-toluidine
(b) C6H5NH2, C6H5NHCH3, C6H5CH2NH2
(iv) In decreasing order of basic strength in gas phase:
C2H5NH2, (C2H5)2NH, (C2H5)3N and NH3
(v) In increasing order of boiling point:
C2H5OH, (CH3)2NH, C2H5NH2
(vi) In increasing order of solubility in water:
C6H5NH2, (C2H5)2NH, C2H5NH2
Solution
(i) Higher pKb value, lower is the basic strength. In C2H5NH2, only one –C2H5 group is present while in (C2H5)2NH, two –C2H5 groups are present. Thus, the +I effect is more in (C2H5)2NH than in C2H5NH2. Therefore, the electron density over the N-atom is more in (C2H5)2NH than in C2H5NH2. Hence, (C2H5)2NH is more basic than C2H5NH2.
C6H5NHCH3 is more basic than C6H5NH2 and less basic than (C2H5)2NH and C2H5NH2 due to the delocalization of the lone pair in the former two. Hence, the order of increasing basicity of the given compounds is as follows:
C6H5NH2 < C6H5NHCH3 < C2H5NH2 < (C2H5)2NH
Therefore, decreasing pKb values are:
C6H5NH2 > C6H5NHCH3 > C2H5NH2 > (C2H5)2NH
(ii) C6H5N(CH3)2 is more basic than C6H5NH2 due to presence of +I effect of two –CH3 groups in C6H5N(CH3)2. Further, CH3NH2 contains one –CH3 group while (C2H5)2NH contains two –C2H5 groups. Thus, (C2H5)2NH is more basic than C2H5NH2.
Now, C6H5N(CH3)2 is less basic than CH3NH2 because of the –I effect of –C6H5 group.
Hence, the decreasing order of the basic strengths of the given compounds is as follows:
(C2H5)2NH > CH3NH2 > C6H5N(CH3)2 > C6H5NH2
(iii) (a) p-toluidine > Aniline > p-nitroaniline
In p-toluidine, the presence of electron-donating –CH3 group increases the electron density on the N-atom. Thus, p-toluidine is more basic than aniline.
On the other hand, the presence of electron withdrawing –NO2 group decreases the electron density over the N-atom in p-nitroaniline. Thus, p-nitroaniline is less basic than aniline. Hence, the increasing order of the basic strengths of the given compounds is as follows:
p-Nitroaniline < Aniline < p-Toluidine.
(b) C6H5NHCH3 is more basic than C6H5NH2 due to the presence of electron-donating –CH3 group in C6H5NHCH3. Again, in C6H5NHCH3, –C6H5 group is directly attached to the N-atom. However, it is not so in C6H5CH2NH2. Thus, in C6H5NHCH3, the – I effect of –C6H5 group decreases the electron density over the N-atom. Therefore, C6H5CH2NH2 is more basic than C6H5NHCH3. Hence, the increasing order of the basic strengths of the given compounds is as follows:
C6H5NH2 < C6H5NHCH3 < C6H5CH2NH2
(iv) In the gas phase, there is a no solvation effect. Hence, the basic strength only depends upon the + I effect. Also, greater the number of alkyl groups, the higher is the + I effect. Hence, decreasing basic strength follows order:
(C2H5)3N > (C2H5)2NH > C2H5NH2 > NH3
(v) The boiling point of compounds depend on the extent of H-bonding present in that compound. The more extensive the H-bonding in the compound, the higher is the boiling point. (CH3)2NH contains only one H-atom whereas C2H5NH2 contains two H-atoms.
Hence, the boiling point of C2H5NH2 is higher than that of (CH3)2NH. Further, O is more electronegative than N. Thus, C2H5OH forms stronger H-bonds than C2H5NH2. As a result, the boiling point of C2H5OH is higher than that of C2H5NH2 and (CH3)2NH. Therefore, compounds can be arranged in the increasing order of their boiling points as follows:
(CH3)2NH < C2H5NH2 < C2H5OH
(vi) The more extensive the H-bonding, the higher is the solubility. C2H5NH2 contains two H-atoms whereas (C2H5)2NH contains only one H-atom. Thus, C2H5NH2 undergoes more extensive H-bonding than (C2H5)2NH. Aniline has a bulky hydrophobic part. Hence, less solubility of C6H5NH2 than C2H5NH2 and (C2H5)2NH. Hence, the increasing order of their solubility in water is as follows:
C6H5NH2 < (C2H5)2NH < C2H5NH2
9.5. How will you convert:
(i) Ethanoic acid into methanamine
(ii) Hexanenitrile into 1-aminopentane
(iii) Methanol to ethanoic acid.
(iv) Ethanamine into methanamine
(v) Ethanoic acid into propanoic acid
(vi) Methanamine into ethanamine
(vii) Nitromethane into dimethylamine
(viii) Propanoic acid into ethanoic acid
Solution
(i) Converting Ethanoic acid into methanamine
(ii) Converting Hexanenitrile into 1-aminopentane
(iii) Converting Methanol to ethanoic acid.
(iv) Converting Ethanamine into methanamine
(v) Converting Ethanoic acid into propanoic acid
(vi) Converting Methanamine into ethanamine
(vii) Converting Nitromethane into dimethylamine
(viii) Converting Propanoic acid into ethanoic acid
9.6. Describe the method for the identification of primary, secondary and tertiary amines. Also write chemical equations of the reactions involved.
Solution
Primary, secondary and tertiary amines can be identified and distinguished by Hinsberg’s test. In this test, the amines are allowed to react with Hinsberg’s reagent, benzenesulphonyl chloride (C6H5SO2Cl). The three types of amines react differently with Hinsberg’s reagent. Therefore, they can be easily identified using Hinsberg’s reagent.
(i) Primary amines react with benzenesulphonyl chloride to form N-alkylbenzenesulphonyl amide which is soluble in alkali.
Due to the presence of a strong electron-withdrawing sulphonyl group in the sulphonamide, the H-atom attached to nitrogen can be easily released as proton. So, it is acidic and dissolves in alkali.
(ii) Secondary amines react with Hinsberg’s reagent to give a sulphonamide which is insoluble in alkali.
There is no H-atom attached to the N-atom in the sulphonamide. Therefore, it is not acidic and insoluble in alkali.
(iii) On the other hand, tertiary amines do not react with Hinsberg’s reagent at all.
9.7. Write short notes on the following:
(i) Carbylamine reaction
(ii) Diazotisation
(iii) Hofmann’s bromamide reaction
(iv) Coupling reaction
(v) Ammonolysis
(vi) Acetylation
(vii) Gabriel phthalimide synthesis
Solution
(i) Carbylamine reaction: Also called as isocyanide reaction which is used as a test for the identification of primary amines. When aliphatic and aromatic primary amines are heated with chloroform and ethanolic potassium hydroxide, carbylamines (or isocyanides) are formed. These carbylamines have very unpleasant odours. Secondary and tertiary amines do not respond to this test.
(ii) Diazotisation: Aromatic primary amines react with
nitrous acid (prepared in situ from NaNO2 and a mineral acid such as
HCl) at low temperatures (273-278 K) to form diazonium salts. This conversion
of aromatic primary amines into diazonium salts is known as diazotization.
For example, on treatment with NaNO2 and HCl at 273−278 K,
aniline produces benzenediazonium chloride, with NaCl and H2O as
by-products.
(iii) Hoffmann’s bromamide reaction: When an amide is treated with bromine in an aqueous or ethanolic solution of sodium hydroxide, a primary amine with one carbon atom less than the original amide is produced. This degradation reaction is known as Hoffmann bromamide reaction. This reaction involves the migration of an alkyl or aryl group from the carbonyl carbon atom of the amide to the nitrogen atom. Carboxylic group leaves as metal carbonate by-product.
(iv) Coupling reaction: The reaction of joining two aromatic rings through the −N=N−bond is known as coupling reaction. Arenediazonium salts such as benzene diazonium salts react with phenol or aromatic amines to form coloured azo compounds.
It can be observed that, the para-positions of phenol and aniline are coupled with the diazonium salt. This reaction proceeds through electrophilic substitution.
(v) Ammonolysis: When an alkyl or benzyl halide is allowed to react with an ethanolic solution of ammonia, it undergoes nucleophilic substitution reaction in which the halogen atom is replaced by an amino (– NH2) group. This process of cleavage of the carbon-halogen bond is known as ammonolysis.
When this substituted ammonium salt is treated with a strong base such as sodium hydroxide, amine is obtained.
Though primary amine is produced as the major product, this process produces a mixture of primary, secondary and tertiary amines and also a quaternary ammonium salt as shown.
(vi) Acetylation: Acetylation (or ethanoylation) is the process of introducing an acetyl group into a molecule.
Aliphatic and aromatic primary and secondary amines undergo acetylation reaction by nucleophilic substitution when treated with acid chlorides, anhydrides or esters. This reaction involves the replacement of the hydrogen atom of –NH2 or –NH group by the acetyl group, which in turn leads to the production of amides. To shift the equilibrium to the right hand side, the HCl formed during the reaction is removed as soon as it is formed. This reaction is carried out in the presence of a base (such as pyridine) which is stronger than the amine.
When amines react with benzoyl chloride, the reaction is also known as benzoylation. For example,
(vii) Gabriel phthalimide synthesis: Gabriel phthalimide synthesis is a very useful method for the preparation of aliphatic primary amines. It involves the treatment of phthalimide with ethanolic potassium hydroxide to form potassium salt of phthalimide. This salt is further heated with alkyl halide, followed by alkaline hydrolysis to yield the corresponding primary amine.
