CBSE Class 12th Chemistry Notes: Amines (Part - I)

This article provides you the revision notes on Class 12 Chemistry: Chapter- Amines, to give you a quick glance of the chapter. These quick notes are prepared strictly according to the latest CBSE syllabus for Class 12th Chemistry.

o    Amines
     •    Structure
     •    Classification
     •    Nomenclature
     •    Preparation
     •    Physical properties
     •    Basic character

The key notes of the chapter are as follows:

Amines

Amines are the organic compounds derived from ammonia (NH₃) by replacing its one or more hydrogen atoms by alkyl or aryl group.

For example:

   CH₃NH₂                              C₂H₅NH₂                            C₆H₅NH₂

Methyl Amine                         Ethyl Amine                           Aniline

Structure of Amines

Nitrogen atom of amines carries an unshared pair of electrons and is sp³  hybridised with pyramidal shape. Due to the presence of unshared pair of electrons, the angle C–N–E, (where E is C or H) is less than 109.5°.

In aromatic amines, the C‒N bond is slightly stronger due to the partial double bond character which arises as a result of delocalisation of lone pair of N with the benzene ring.

Classification of Amines

On the basis of number of hydrogen atoms replaced in NH₃ molecule, amines are categorized into three types:

•     1° amine: One hydrogen atom of NH₃ is replaced by an alkyl or aryl group.

For example:

CH₃NH₂

Methyl Amine

•     2° amine: Two hydrogen atoms of NH₃ are replaced by alkyl or aryl groups.

For example:

CH₃NHCH₃

Dimethyl Amine

•     3° amine: All three hydrogen atoms of NH₃ are replaced by alkyl or aryl groups.

For example:

Nomenclature of Amines

Common naming system:

•     Aliphatic amines are named by prefixing an alkyl group to a mine, i.e., alkylamine.

For example:

•     Secondary and tertiary amines, having two or more similar groups are named by adding prefix ‘di’ or ‘tri’ before the name of alkyl group.

For example:

•     Aromatic amines are named as derivatives of the parent member, aniline (C₆H₅NH₂).

For example:

IUPAC naming system:

•     Aliphatic or aromatic amines are named by replacing ‘e’ in the end of the parent hydrocarbon by ‘amine’.

For example:

    CH₃NH₂                              C₆H₅NH₂

Methanamine                        Benzenamine

•     Amines containing more than one amino groups at different positions in the parent chain, are named by specifying numbers to the carbon atoms bearing –NH₂ groups along with attaching a suitable prefix such as di, tri, etc. to the amine. The ending ‘e’ of the hydrocarbon is retained.

For example:

H₂N ‒ CH₂ ‒ CH₂ ‒ NH₂

Ethane-1, 2-diamine

Preparation of Amines

1. By reduction of nitro (RNO₂) compounds:

2. By reduction of nitriles (RCN):

3. By reduction of amides (RCONH₂):

4. By ammonolysis of alkyl halides:

5. Hofmann bromamide reaction:

RCONH₂ + Br₂ + 4NaOH → RNH₂ + 2NaBr + Na₂CO₃ + 2 H₂O

This method gives  amine with one carbon less than the parent amide.

6. Gabriel phthalimide synthesis:

Physical Properties of Amines

•    The lower aliphatic amines are gases with fishy smell.

•    Primary amines wit three or more carbon atoms are liquid and higher members are all solids.

•    Lower amines are soluble in water as they can form hydrogen bonds with water, however the solubility decreases with increase in hydrophobic alkyl group.

•    Amines have a higher boiling point than the hydrocarbon of comparable molecular mass. This is due to their ability to associate via intermolecular hydrogen bonding.

•    Boiling points order of various isomeric amines is:
1^o > 2^o > 3^o

Basic strength of amines

Amines act as Lewis bases due to the presence of lone pair of electrons on the nitrogen atom. Basic character of amines can be better expressed in terms of their K_b and pK_b values.

All aliphatic amines are strong bases than NH₃ while aromatic amines are weaker bases than NH₃ due to the electron withdrawing nature of the aryl group.

o    Factors affects the basicity of aliphatic amines are:
      •    Inductive effect
      •    Solvation effect
      •    Steric hinderance

Considering all the above factors the basic strength of methyl substituted amines and ethyl substituted amines in aqueous medium follows the order:

(CH₃)₂NH > CH₃NH₂ > (CH₃)₃N > NH₃

(C₂H₅)₂NH > (C₂H₅)₃N > C₂H₅NH₂ > NH₃