NCERT Solutions for Class 12 Chemistry Chapter 9 Amines – Free PDF Download

Pk_b value is the negative logarithm of the basicity constant (K_b) .i.e., pK_b = -logK_b

Evidently, smaller the value of pK_b , stronger is the base (strong tendency to donate electrons). Aliphatic amines(R-NH₂) are more basic(tendency to donate electrons) than aromatic amines(C₆H₅NH₂) because of the following reasons:

=>In aliphatic amines, alkyl groups are present. Alkyl groups are electron releasing groups, hence they increase the elecron density of N-atom and thus is easily available to donate electrons. This poperty makes aliphatic amines more basic.

As a result of resonance, the lone pair of electrons on the nitrogen atom gets delocalized over benzene ring. As a result, electron density on the nitrogen decreases and thus is less easily available to donate electrons making it less basic. Hence aliphatic amines(R-NH₂) are more basic than aromatic amines (C₆H₅NH₂).

Now, in C₂H₅NH₂, one ethyl group (alkyl) is present and in (C₂H₅)₂NH₂ two ethyl groups are present. As we know that more alkyl groups are present, more basic will be the amine.

Hence, (C₂H₅)₂NH₂ is more basic than the C₂H₅NH₂

Now in C₆H₅NH₂, due to delocalisation of lone pair of electrons of the N-atom over the benzene ring, makes it less basic than (C₂H₅)₂NH₂ and C₂H₅NH₂

Now, in C₆H₅NHCH_3, due to presence of CH₃ group, makes it more basic than C₆H₅NH₂ but less basic than (C₂H₅)₂NH₂ and C₂H₅NH₂ due to the presence of aromatic ring which is responsible for the delocalisation of lone pair of electrons of N-atom over the benzene ring.

Combining all these facts, the relative basic strength of these four amines decrease in the order:

(C₂H₅)₂NH₂ > C₂H₅NH₂ > C₆H₅NHCH₃ > C₆H₅NH₂

Since, a stronger base has a lower pk_b value, therefore, pK_b values decrease in the reverse order:

C₆H₅NH₂ > C₆H₅NHCH₃ > C₂H₅NH₂ > (C₂H₅)₂NH₂

In (C₂H₅)₂NH₂, two ethyl groups are present and in CH₃NH₂ one methyl group is present. As we know that more alkyl groups are present, more basic will be the amine. Hence, (C₂H₅)₂NH₂ is more basic than the CH₃NH₂

Now in C₆H₅NH₂, due to delocalisation of lone pair of electrons of the N-atom over the benzene ring (decreases the electron density of N-atom) makes it less basic than (C₂H₅)₂NH₂ and CH₃NH₂

Now, in C₆H₅N(CH₃)₂ due to presence of two CH₃ groups (increases the electron density of N- atom) makes it more basic than C₆H₅NH₂ but less basic than (C₂H₅)₂NH₂ and CH₃NH₂ due to the presence of aromatic ring which is responsible for the delocalisation of lone pair of electrons of N-atom over the benzene ring (decreases the electron density of N-atom)

Combining all these facts, the relative basic strength of these four amines increases in the order:

C₆H₅NH₂ < C₆H₅NHCH₃ < CH₃NH₂ < (C₂H₅)₂NH

 

(a) Electron-donating groups such as –CH₃, -OCH₃, -NH₂, etc. increase the basicity and electron- withdrawing groups such as –NO₂, -CN, -SO₃H, -COOH, -X(halogen), etc. decrease the basicity of

Explanation: Electron-donating groups releases electrons, stabilizes the conjugate acid (cation) and thus increases the basic strength. Electron-withdrawing groups withdraws electrons, destabilizes the conjugate acid (cation) and thus decreases the basic strength.

In p-nitroaniline, NO₂ group is present. As we know that NO₂ group is an electron withdrawing group which decreases the basic strength of amine. In p-toluidine, CH₃ group is present and as we know that CH₃ group is an electron donating (releasing) group which increases the basic strength of amine. Hence, p-toluidine is more basic than p-nitroaniline

Now in C₆H₅NH₂ (aniline), due to delocalisation of lone pair of electrons of the N-atom over the benzene ring (decreases the electron density of N-atom) makes it less basic than p-toluidine but more basic than p-nitroaniline (NO₂ group is present which decreases the density of aniline)

Combining all these facts, the relative basic strength of these three amines increases in the order:

p-nitroaniline< aniline < p-toluidine

 

(b) C₆H₅NH₂, C₆H₅NHCH₃, C₆H₅CH₂NH₂

In C₆H₅NH₂ and C₆H₅NHCH₃, the N-atom is directly attached to the aromatic ring. Hence, they both show resonance in which delocalization of lone pair of electrons N-atom takes place. As a result the electron density of N-atom decreases. Hence both are weaker bases. However in C₆H₅NHCH₃, CH₃ group (electron withdrawing) is present which increases the overall density of electrons. Hence, C₆H₅NHCH₃ is more basic than C₆H₅NH₂

In C₆H₅CH₂NH₂, the N-atom is not directly attached to the aromatic ring. As a result, it does not show resonance. There is no effect on the electron density of lone pair of electrons of N-atom. Hence, it is more basic than C₆H₅NH₂ and C₆H₅NHCH₃

Combining all these facts, the relative basic strength of these three amines increases in the order:

C₆H₅NH₂ < C₆H₅NHCH₃ < C₆H₅CH₂NH₂

Amines in gas phase or in non-aqueous solvents, there is no solvation effect (getting influenc

(i) CH₃CH₂CH₂NH₂ + HCl → CH₃CH₂CH₂N⁺H₃Cl^-

The final product is (N-propyl ammonium chloride.)

(ii) (C₂H₅)₃N + HCl → (C₂H₅)₃N⁺HCl^-

The final product is (Tri ethyl ammonium chloride)

1- Alkyl group contribute inductive effect which increases the basic strength of

NH₃2H5NH2< (C2H5)2NH.

Then C₆H₅NH₂ is having –I effect that reduces strength. And C₆H₅CH₂NH₂ increases the basic strength but not as much as C₂H₅ group.

Hence final order will be C₆H₅NH₂<3

2- By taking into consideration –R effect and steric hindrance of groups we can arrange them in the order

C₆H₅NH₂< C₂H₅NH₂< (C2H5)₃N< (C₂H₅)₂NH.

Because (C₂H₅)₃N has a lot of steric hindrances that reduces the basic strength.

3- In C₆H₅NH₂, N is directly attached to the ring that causes delocalization of electrons of the benzene ring. Whereas in case of C₆H₅CH₂NH₂ it is not directly connected to benzene ring Hence it has more basic strength.

Due to –I effect of (CH₃)₃ ^– a group it has more basic strength than C₆H₅CH₂NH₂. Hence final order will be

C₆H₅NH₂< C₆H₅CH₂NH₂< (CH₃)₃N< CH₃NH₂< (CH₃)₂NH

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.

Therefore aromatic primary amines cannot be formed by gabriel phthalimide process.

1- C₆H₅NH₂ + CHCl₃ + KOH

It is a carbylamine reaction in which a isocyanide compound is formed along with side products of potassium chloride.Basically the name of reaction is given is due to formation of a foul smelling compound called as isocyanide.

2- C₆H₅N₂Cl + H₃PO₂ + H₂O

Benzenediazonium chloride is a very reactive compound which oxidises hypophosphorous acid to hypophosphoric acid and the reactant is reduced to benzene.

3- C₆H₅NH₂ + H₂SO₄ (conc.)

Aniline undergoes sulphonation to anillium hydrogensulphate.

4- C₆H₅N₂Cl + C₂H₅OH

Aniline is very activating group which undergoes reaction to give ortho and para product. But in acidic medium aniline accquires positive charge on N atom which is metadirecting group and hence the product.

5- C₆H₅NH₂ + Br₂ (aq)

It is clubbing reaction of a very reactive compound undergoing addition to give benzene.

6- C₆H₅NH₂ + (CH₃CO)₂O

As mentioned aniline is avery activating group, so to reduce its activity it is reacted with anhydride to give n-phenylethanamine.

7- C₆H₅N₂Cl

Hydrofluroburic acid reacts qith benzenediazonium chloride along with sodium nitrite to give a reduced product as Nitrobenzene.

(ii) Tertiary, because the Nitrogen atom is attached to the 3 carbon atoms.
(iii) Primary, because the nitrogen atom is attached to the only 1 carbon atom.
(iv) Secondary, because the nitrogen atom is attached to the only 1 carbon.

(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 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 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 (? 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.

(i) Aromatic amines react with nitrous acid (prepared in situ from NaNO₂and a mineral acid such as HCl) at 273 - 278 K to form stable aromatic diazonium salts e., NaCl and water.

(ii) Aliphatic primary amines react with nitrous acid (prepared in situ from NaNO₂and a mineral acid such as HCl) to form unstable aliphatic diazonium salts, which further producealcohola and acid called as hydrochloric acid and evolution of nitrogen

(iii) Aliphatic amines are stronger bases than aromatic amines due to following reasons:

(a) The lone pair of electrons of the nitrogen atom of aromatic amines is involved in

conjugation with the π−bond pairs of the ring as follows

(b) Anilinium ion obtained by accepting a proton is less stabilized by resonance. Due to the −R effect of the benzene ring, the electrons on the N- atom are less available in the case of aromatic amines. Therefore, the electrons on the N-atom in aromatic amines cannot be donated

So, aniline is a weaker base than alkyl amines, in which +1 effect increases the electron density on the nitrogen atom.

(i) Ethyl iodide reacts with NaCN gives a substitution reactions to give propanitrile upon partial hydrolysis gives B, upon reaction with sodium hydroxide gives C.

(ii) Benzenediazoniumchloride gives nucleophilic substitution reactions gives A, upon hydrolysis the CN ion is replaced by OH ion which is less better leaving group gives B upon heating with ammonia gives C.

(iii) Ethylbromide gives nucleophilic substitution reactions gives B, upon reduction gives B followed by reacting with nitrous acid, i.e oxidation gives propanol.

(iv) Nitrobenzene upon reduction with iron/acid gives A, reacting with sodium nitrite gives benzenediazonium chloride, followed by complete Hydrolysis gives phenol.

(v) Acetic acid upon heating with ammmonia gives A, which is an amide. This amide reacts with NaOBr which extracts the carbonyl carbon giving B, followed by reacting with sodium nitrite gives methanol.

(vi) Nitrobenzene upon reduction with iron/acid mixture gives A, followed by oxidation with nitrous acid gives B and reacting with phenol undergoes addition reaction to give C.

Benzene into aniline

When Benzene is treated with HNO₃/H₂SO₄ it forms nitrobenzene. When Nitrobenzene reduced with Sn/HCL it forms Aniline because Sn/HCl is a reducing Mixture.

When Benzene is reacted with nitrating mixture it forms nitrobenzene. When it Reduced H₂/Pd in ethanol or Sn/HCl, it forms Aniline. When Aniline reacts 2 times with CH₃Cl It forms N, N- dimethylaniline.

When 1,4-dichlorobutane reacts with NaCN it forms Di cyanide compound, After Hydrogenation it forms the Hexane 1,6-Diamine.

(i) & (ii) There are total 8 geometrical isomers of the given compound.

(iii) a) Pairs 1,2,6,7 Exhibit Position isomerism; means the change in position of the substituent.

b) Pairs 1,3 and 1,4 and 2,3 and 2,4 exhibit chain isomerism i.e. in this type of isomerism the different structures can be produced by changing the chain of the

c) Pairs 5,6 and 5,7 exhibit metamerism; e. different group on either side of the central atom.

d) All Primary amines exhibit functional isomers. All secondary amines share functional isomerism and same for The functional isomerism means same functional group.

(i) Aromatic amines react with nitrous acid (prepared in situ fromNaNO₂ and a mineral acid such as HCl) at 273 - 278 K to form stable aromatic diazonium salts i.e., NaCl and water. This reaction is widely used for preparation of variety of compounds.

(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 is very reactive, which further produce alcohol and HCl with the evolution of nitrogen gas.

It is given that compound 'C' having the molecular formula, C₆H₇N is formed by heating compound 'B' with Br₂and KOH. This is a Hoffmann bromamide degradation reaction (in which isocyanide compound is formed). Therefore, compound 'B' is an amide and compound 'C' is an amine. The only amine having the molecular formula, C₆H₇N is aniline. Therefore, compound 'B' (from which 'C' is formed) must be benzamide.

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: