CBSE Class 12 Chemistry Amines Summary Set 01

CBSE Class 12 Chemistry Amines Summary (1). Please refer to the examination notes which you can use for preparing and revising for exams. These notes will help you to revise the concepts quickly and get good marks.

                                                            AMINE

AMINES

Amines are derivatives of ammonia (NH₃), obtained by replacement of 1, 2 or all the 3 hydrogen atoms by alkyl and/or aryl groups.

In nature amines are present in proteins, vitamins, alkaloids and hormones.

Synthetic amines are present in polymers, dyestuffs and drugs.

STRUCTURE OF FUNCTIONAL GROUP:

CLASSIFICATION:

Nomenclature

PREPARATION OF AMINES: [GHAR]

1. Gabriel phthalimide synthesis

2. Hoffmann bromamide degradation reaction

An amide (–CO–NH₂) on reaction with bromine in an aqueous or ethanolic solution of NaOH gives a 1° amine with one carbon less than the amide.

3. Ammonolysis of alkyl halides

The process of cleavage of the C–X bond by ammonia molecule is known as ammonolysis.

The free amine can be obtained from the ammonium salt by treatment with a strong base:

The order of reactivity of halides with amines is RI > RBr > RCl

4. REDUCTION: by LiAlH₄, Na + C₂H₅OH, H₂/Ni

a. Reduction of nitriles:

b. Reduction of amides:

c. Reduction of nitro compounds:

Physical Properties:

1. Lower aliphatic amines are soluble in water because they can form hydrogen bonds with water molecules.
2. Solubility decreases with increase in molar mass of amines due to increase in size of the hydrophobic alkyl part.
3. 1° and 2° amines form intermolecular H-bonding. This H-bonding is more in 1° amines than in 2° amines as there are 2 hydrogen atoms available for H-bond formation in it. 3° amines do not form H-bond. Therefore, the B.Pt. of isomeric amines follows the order: 1° > 2° > 3°

Basic Nature of amines:

The reaction of amines with mineral acids to form ammonium salts shows basic nature of amines. Amines have an unshared pair of electrons on N-atom due to which they act as Lewis base.

R–NH₂ + H₂O ⇌ R–NH₃⁺ + OH⁻

\[ K=\frac{[\mathrm{R{-}NH_3^{+}}]\,[\mathrm{OH^{-}}]}{[\mathrm{R{-}NH_2}]\,[\mathrm{H_2O}]} \]

or \[ K[\mathrm{H_2O}]=K_b=\frac{[\mathrm{R{-}NH_3^{+}}]\,[\mathrm{OH^{-}}]}{[\mathrm{R{-}NH_2}]},\qquad pK_b=-\log K_b \]

Larger the value of K_b or smaller the value of pK_b, stronger is the base.

Structure-basicity relationship of amines:

Basic character of an amine depends upon

1. the ease of formation of the cation by accepting a proton from the acid.
2. Stability of the cation. The more stable the cation relative to the amine, more basic is the amine.

1. Alkanamines (R-NH₂) versus ammonia (NH₃): Due to the electron releasing nature of alkyl group (+I effect), it pushes electrons towards N and thus makes the unshared electron pair more available for sharing with the proton of the acid. Moreover, the substituted ammonium ion formed from the amine gets stabilized due to dispersal of the positive charge by the +I effect of the –R group. Hence, alkyl amines are stronger bases than ammonia.
2. Basicity of amines in the gaseous phase: 3° amine > 2° amine > 1° amine > NH₃.
3. Basicity of amines in the aqueous phase, the substituted ammonium cations get stabilized not only by electron releasing effect of the alkyl group (+I) but also by solvation with water molecules. The greater the size of the ion, lesser will be the solvation and the less stabilised is the ion. The order of stability of ions are as follows:

> >

(left to right: 1°, 2°, 3° ammonium ions)

When the –R group is small, like –CH₃ group, there is no steric hindrance to H-bonding. In case the alkyl group is bigger than –CH₃ group, there will be steric hinderance to H-bonding. Therefore, the change of nature of the alkyl group from –CH₃ to –C₂H₅ results in change of the order of basic strength. The combination of inductive effect, solvation effect and steric hinderance of the –R group decides the basic strength of alkyl amines in the aqueous state. The order of basic strength in case of methyl substituted amines and ethyl substituted amines in aqueous solution is as follows:

(C₂H₅)₂NH > (C₂H₅)₃N > C₂H₅NH₂ > NH₃

(CH₃)₂NH > CH₃NH₂ > (CH₃)₃N > NH₃

4. Aryl amines are weaker base than ammonia: in aniline or other arylamines, the –NH₂ group is attached directly to the benzene ring. The unshared electron pair on nitrogen atom is in conjugation with the benzene ring and thus making it less available for protonation.

5 canonical structures, it is more stable

only 2 canonical structures, it is less stable

Hence, the proton acceptability or the basic nature of aniline or other aromatic amines is less than that of ammonia.

5. Basicity of substituted aniline – electron releasing groups like –OCH₃, –CH₃ increase basic strength whereas electron withdrawing groups like –NO₂, –SO₃H, –COOH, –X decrease it.

CHEMICAL REACTIONS OF AMINES