Chemistry Class 12 Chapter 13 Ncert Solutions

NCERT Solutions for Class 12 Science Chemistry Chapter 13 Amines are provided here with simple step-by-step explanations. These solutions for Amines are extremely popular among Class 12 Science students for Chemistry Amines Solutions come handy for quickly completing your homework and preparing for exams. All questions and answers from the NCERT Book of Class 12 Science Chemistry Chapter 13 are provided here for you for free. You will also love the ad-free experience on Meritnation's NCERT Solutions. All NCERT Solutions for class Class 12 Science Chemistry are prepared by experts and are 100% accurate.

Page No 384:

Question 13.1:

Classify the following amines as primary, secondary or tertiary:

(i)

(ii)

(iii) (C₂H₅)₂CHNH₂

(iv) (C₂H₅)₂NH

Answer:

Primary: (i) and (iii)

Secondary: (iv)

Tertiary: (ii)

Page No 384:

Question 13.2:

(i) Write structures of different isomeric amines corresponding to the molecular formula, C₄H₁₁N

(ii) Write IUPAC names of all the isomers.

(iii) What type of isomerism is exhibited by different pairs of amines?

Answer:

(i), (ii) The structures and their IUPAC names of different isomeric amines corresponding to the molecular formula, C₄H₁₁N are given below:

(a) CH₃-CH₂-CH₂-CH₂-NH₂

Butanamine (1⁰)

(b)

Butan-2-amine (1⁰)

(c)

2-Methylpropanamine (1⁰)

(d)

2-Methylpropan-2-amine (1⁰)

(e) CH₃-CH₂-CH₂-NH-CH₃

N-Methylpropanamine (2⁰)

(f) CH₃-CH₂-NH-CH₂-CH₃

N-Ethylethanamine (2⁰)

(g)

N-Methylpropan-2-amine (2⁰)

(h)

N,N-Dimethylethanamine (3°)

(iii) The pairs (a) and (b) and (e) and (g) exhibit position isomerism.

The pairs (a) and (c); (a) and (d); (b) and (c); (b) and (d) exhibit chain isomerism.

The pairs (e) and (f) and (f) and (g) exhibit metamerism.

All primary amines exhibit functional isomerism with secondary and tertiary amines and vice-versa.

Page No 387:

Question 13.3:

How will you convert?

(i) Benzene into aniline

(ii) Benzene into N, N-dimethylaniline

(iii) Cl−(CH₂)₄−Cl into hexan-1, 6-diamine?

Page No 396:

Question 13.4:

Arrange the following in increasing order of their basic strength:

(i) C₂H₅NH₂, C₆H₅NH₂, NH₃, C₆H₅CH₂NH₂ and (C₂H₅)₂NH

(ii) C₂H₅NH₂, (C₂H₅)₂NH, (C₂H₅)₃N, C₆H₅NH₂

(iii) CH₃NH₂, (CH₃)₂NH, (CH₃)₃N, C₆H₅NH₂, C₆H₅CH₂NH₂.

Answer:

(i) Considering the inductive effect of alkyl groups, NH₃, C₂H₅NH_2, and (C₂H₅)₂NH can be arranged in the increasing order of their basic strengths as:

Again, C₆H₅NH₂ has proton acceptability less than NH₃. Thus, we have:

Due to the −I effect of C₆H₅ group, the electron density on the N-atom in C₆H₅CH₂NH₂ is lower than that on the N-atom in C₂H₅NH_2, but more than that in NH₃. Therefore, the given compounds can be arranged in the order of their basic strengths as:

(ii) Considering the inductive effect and the steric hindrance of the alkyl groups, C₂H₅NH₂, (C₂ H₅)₂NH_2, and their basic strengths as follows:

Again, due to the −R effect of C₆H₅ group, the electron density on the N atom in C₆H₅ NH₂ is lower than that on the N atom in C₂H₅NH₂. Therefore, the basicity of C₆H₅NH₂ is lower than that of C₂H₅NH₂. Hence, the given compounds can be arranged in the increasing order of their basic strengths as follows:

(iii) Considering the inductive effect and the steric hindrance of alkyl groups, CH₃NH₂, (CH₃)₂NH, and (CH₃)₃N can be arranged in the increasing order of their basic strengths as:

In C₆H₅NH₂, N is directly attached to the benzene ring. Thus, the lone pair of electrons on the N−atom is delocalized over the benzene ring. In C₆H₅CH₂NH₂, N is not directly attached to the benzene ring. Thus, its lone pair is not delocalized over the benzene ring. Therefore, the electrons on the N atom are more easily available for protonation in C₆H₅CH₂NH₂ than in C₆H₅NH₂ i.e., C₆H₅CH_2­NH₂ is more basic than C₆H₅NH₂.

Again, due to the −I effect of C₆H₅ group, the electron density on the N−atom in C₆H₅CH₂NH₂ is lower than that on the N−atom in (CH₃)₃N. Therefore, (CH₃)₃N is more basic than C₆H₅CH₂NH₂. Thus, the given compounds can be arranged in the increasing order of their basic strengths as follows.

Page No 396:

Question 13.5:

Complete the following acid-base reactions and name the products:

(i) CH₃CH₂CH₂NH₂ + HCl

(ii) (C₂H₅)₃N + HCl

Answer:

(i)

(ii) $\underset{\mathrm{Triethylamine}}{{\left({\mathrm{C}}_2{\mathrm{H}}_5\right)}_3\mathrm{N}}+\mathrm{HCl}\to \underset{\mathrm{Triethylammoniumchloride}}{{\left({\mathrm{C}}_2{\mathrm{H}}_5\right)}_3\stackrel{+}{\mathrm{N}}\mathrm{H}\stackrel{-}{\mathrm{Cl}}}$

Page No 396:

Question 13.6:

Write reactions of the final alkylation product of aniline with excess of methyl

iodide in the presence of sodium carbonate solution.

Answer:

Aniline reacts with methyl iodide to produce N, N-dimethylaniline.

With excess methyl iodide, in the presence of Na2CO3 solution, N, N-dimethylaniline produces N, N, N−trimethylanilinium carbonate.

Page No 396:

Question 13.7:

Write chemical reaction of aniline with benzoyl chloride and write the name of

the product obtained.

Answer:

Page No 396:

Question 13.8:

Write structures of different isomers corresponding to the molecular formula,

C₃H₉N. Write IUPAC names of the isomers which will liberate nitrogen gas on

treatment with nitrous acid.

Answer:

The structures of different isomers corresponding to the molecular formula, C₃H₉N are given below:

(a)

Propan-1-amine (1⁰)

(b)

Propan-2-amine (1⁰)

(c)

(d)

N,N-Dimethylmethanamine (3⁰)

1⁰amines, (a) propan-1-amine, and (b) Propan-2-amine will liberate nitrogen gas on treatment with nitrous acid.

Page No 399:

Question 13.9:

Convert

(i) 3-Methylaniline into 3-nitrotoluene.

(ii) Aniline into 1,3,5-tribromobenzene.

Answer:

(i)

(ii)

Page No 400:

Question 13.1:

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

Answer:

(i) 1-Methylethanamine (1⁰ amine)

(ii) Propan-1-amine (1⁰ amine)

(iii) N−Methyl-2-methylethanamine (2⁰ amine)

(iv) 2-Methylpropan-2-amine (1⁰ amine)

(v) N−Methylbenzamine or N-methylaniline (2⁰ amine)

(vi) N-Ethyl-N-methylethanamine (3⁰ amine)

(vii) 3-Bromobenzenamine or 3-bromoaniline (1⁰ amine)

Page No 400:

Question 13.2:

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.

Answer:

(i) Methylamine and dimethylamine 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.

​

(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) 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.

Page No 400:

Question 13.3:

Account for the following:

(i) pK _b 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, 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.

Answer:

(i) p K _b of aniline is more than that of methylamine:

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, pK _b 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 large hydrophobic −C₆H₅ group. Hence, aniline is insoluble in water.

(iii) Methylamine in 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 produces OH⁻ ions by accepting H⁺ ions from water.

Ferric chloride (FeCl₃) dissociates in water to form Fe³⁺ and Cl⁻ ions.

Then, OH⁻ ion reacts with Fe³⁺ ion to form a precipitate of 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-Crafts reaction:

A Friedel-Crafts reaction is carried out in the presence of AlCl₃. But AlCl₃ is acidic in nature, while aniline is a strong base. Thus, aniline reacts with AlCl₃ to form a salt (as shown in the following equation).

Due to the positive charge on the N-atom, electrophilic substitution in the benzene ring is deactivated. Hence, aniline does not undergo the Friedel-Crafts 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 phthalimide 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.

Page No 400:

Question 13.4:

Arrange the following:

(i) In decreasing order of the pK _b values:

C₂H₅NH₂, C₆H₅NHCH₃, (C₂H₅)₂NH and C₆H₅NH₂

(ii) In increasing order of basic strength:

C₆H₅NH₂, C₆H₅N(CH₃)₂, (C₂H₅)₂NH and CH₃NH₂

(iii) In increasing order of basic strength:

(a) 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.

Answer:

(i) 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.

Also, both C₆H₅NHCH₃ and C₆H₅NH₂ are less basic than (C₂H₅)₂NH and C₂H₅NH₂ due to the delocalization of the lone pair in the former two. Further, among C₆H₅NHCH₃ and C₆H₅NH₂, the former will be more basic due to the +T effect of −CH₃ group. Hence, the order of increasing basicity of the given compounds is as follows:

C₆H₅NH₂ < C₆H₅NHCH₃ < C₂H₅NH₂ < (C₂H₅)₂NH

We know that the higher the basic strength, the lower is the pK _b values.

C₆H₅NH₂ > C₆H₅NHCH₃ > C₂H₅NH₂ > (C₂H₅)₂NH

(ii) C₆H₅N(CH₃)₂ is more basic than C₆H₅NH₂ due to the presence of the +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)₂ NH is more basic than C₂H₅NH₂.

Now, C₆H₅N(CH₃)₂ is less basic than CH₃NH₂ because of the−R effect of −C₆H₅ group.

Hence, the increasing order of the basic strengths of the given compounds is as follows:

C₆H₅NH₂ < C₆H₅N(CH₃)₂ < CH₃NH₂ < (C₂H₅)₂NH

(iii) (a)

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 −R 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 no solvation effect. As a result, the basic strength mainly depends upon the +I effect. The higher the +I effect, the stronger is the base. Also, the greater the number of alkyl groups, the higher is the +I effect. Therefore, the given compounds can be arranged in the decreasing order of their basic strengths in the gas phase as follows:

(C₂H₅)₃N > (C₂H₅)₂NH > C₂H₅NH₂ > NH₃

(v) The boiling points 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. Then, C₂H₅NH₂ undergoes more extensive H-bonding than (CH₃)₂NH. 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.

Now, the given 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. Hence, the solubility in water of C₂H₅NH₂ is more than that of (C₂H₅)₂NH.

Further, the solubility of amines decreases with increase in the molecular mass. This is because the molecular mass of amines increases with an increase in the size of the hydrophobic part. The molecular mass of C₆H₅NH₂ is greater than that of C₂H₅NH₂ and (C₂H₅)₂NH.

Hence, the increasing order of their solubility in water is as follows:

C₆H₅NH₂ < (C₂H₅)₂NH < C₂H₅NH₂

Page No 401:

Question 13.5:

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

Answer:

(i)

(ii)

(iii)

(iv)

(v)

(vi)

(vii)

(viii)

Page No 401:

Question 13.6:

Describe a method for the identification of primary, secondary and tertiary amines. Also write chemical equations of the reactions involved.

Answer:

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 (C ₆ H ₅ SO ₂ Cl). The three types of amines react differently with Hinsberg's reagent. Therefore, they can be easily identified using Hinsberg's reagent.

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.

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.

On the other hand, tertiary amines do not react with Hinsberg's reagent at all.

Page No 401:

Question 13.7:

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.

Answer:

(i) Carbylamine reaction

Carbylamine reaction 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.

For example,

(ii) Diazotisation

Aromatic primary amines react with nitrous acid (prepared in situ from NaNO ₂ 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 NaNO ₂ and HCl at 273−278 K, aniline produces benzenediazonium chloride, with NaCl and H ₂ O as by-products.

(iii) Hoffmann 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.

For example,

(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 (−NH ₂ ) 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 −NH ₂ 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.

Page No 401:

Question 13.8:

Accomplish the following conversions:

(i) Nitrobenzene to benzoic acid

(ii) Benzene to m -bromophenol

(iii) Benzoic acid to aniline

(iv) Aniline to 2,4,6-tribromofluorobenzene

(v) Benzyl chloride to 2-phenylethanamine

(vi) Chlorobenzene to p -chloroaniline

(vii) Aniline to p -bromoaniline

(viii) Benzamide to toluene

(ix) Aniline to benzyl alcohol.

Answer:

(i)

(ii)

(iii)

(iv)

(v)

(vi)

(vii)

(viii)

(ix)

Page No 401:

Question 13.9:

Give the structures of A, B and C in the following reactions:

(i)

(ii)

(iii)

(iv)

(v)

(vi)

Answer:

(i)

(ii)

(iii)

(iv)

(v)

(vi)

Page No 402:

Question 13.10:

An aromatic compound 'A' on treatment with aqueous ammonia and heating

forms compound 'B' which on heating with Br₂ and KOH forms a compound 'C'

of molecular formula C₆H₇N. Write the structures and IUPAC names of compounds A, B and C.

Answer:

It is given that compound 'C' having the molecular formula, C₆H₇N is formed by heating compound 'B' with Br₂ and KOH. This is a Hoffmann bromamide degradation reaction. Therefore, compound 'B' is an amide and compound 'C' is an amine. The only amine having the molecular formula, C₆H₇N is aniline, (C₆H₅NH₂).

Therefore, compound 'B' (from which 'C' is formed) must be benzamide, (C₆H₅CONH₂).

Further, benzamide is formed by heating compound 'A' with aqueous ammonia. Therefore, compound 'A' must be benzoic acid.

The given reactions can be explained with the help of the following equations:

Page No 402:

Question 13.11:

Complete the following reactions:

(i)

(ii)

(iii)

(iv)

(v)

(vi)

(vii)

Answer:

(i)

(ii)

(iii)

(iv)

(v)

(vi)

(vii)

Page No 402:

Question 13.12:

Why cannot aromatic primary amines be prepared by Gabriel phthalimide synthesis?

Answer:

Gabriel phthalimide synthesis is used for the preparation of aliphatic primary amines. It involves nucleophilic substitution (S_N2) of alkyl halides by the anion formed by the phthalimide.

But aryl halides do not undergo nucleophilic substitution with the anion formed by the phthalimide.

Hence, aromatic primary amines cannot be prepared by this process.

Page No 402:

Question 13.13:

Write the reactions of (i) aromatic and (ii) aliphatic primary amines with nitrous acid.

Answer:

(i) Aromatic amines react with nitrous acid (prepared in situ from NaNO₂ and a mineral acid such as HCl) at 273 − 278 K to form stable aromatic diazonium salts i.e., NaCl and H₂O.

(ii) Aliphatic primary amines react with nitrous acid (prepared in situ from NaNO₂ and a mineral acid such as HCl) to form unstable aliphatic diazonium salts, which further produce alcohol and HCl with the evolution of N₂ gas.

Page No 402:

Question 13.14:

Give plausible explanation for each of the following:

(i) Why are amines less acidic than alcohols of comparable molecular masses?

(ii) Why do primary amines have higher boiling point than tertiary amines?

(iii) Why are aliphatic amines stronger bases than aromatic amines?

Answer:

(i) Amines undergo protonation to give amide ion.

Similarly, alcohol loses a proton to give alkoxide ion.

In an amide ion, the negative charge is on the N-atom whereas in alkoxide ion, the negative charge is on the O-atom. Since O is more electronegative than N, O can accommodate the negative charge more easily than N. As a result, the amide ion is less stable than the alkoxide ion. Hence, amines are less acidic than alcohols of comparable molecular masses.

(ii) In a molecule of tertiary amine, there are no H−atoms whereas in primary amines, two hydrogen atoms are present. Due to the presence of H−atoms, primary amines undergo extensive intermolecular H−bonding.

As a result, extra energy is required to separate the molecules of primary amines. Hence, primary amines have higher boiling points than tertiary amines.

(iii) Due to the −R effect of the benzene ring, the electrons on the N- atom are less available in case of aromatic amines. Therefore, the electrons on the N-atom in aromatic amines cannot be donated easily. This explains why aliphatic amines are stronger bases than aromatic amines.

View NCERT Solutions for all chapters of Class 12

Chemistry Class 12 Chapter 13 Ncert Solutions

Source: https://www.meritnation.com/cbse-class-12-science/chemistry/chemistry-part-ii/amines/ncert-solutions/46_5_217_272

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