CBSE Class 12th Chemistry Notes: Amines (Part - I)Gurmeet Kaur
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.
• Physical properties
• Basic character
The key notes of the chapter are as follows:
Amines are the organic compounds derived from ammonia (NH3) by replacing its one or more hydrogen atoms by alkyl or aryl group.
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.
• 2° amine: Two hydrogen atoms of NH3 are replaced by alkyl or aryl groups.
• 3° amine: All three hydrogen atoms of NH3 are replaced by alkyl or aryl groups.
Nomenclature of Amines
Common naming system:
• Aliphatic amines are named by prefixing an alkyl group to a mine, i.e., alkylamine.
• Secondary and tertiary amines, having two or more similar groups are named by adding prefix ‘di’ or ‘tri’ before the name of alkyl group.
• Aromatic amines are named as derivatives of the parent member, aniline (C6H5NH2).
IUPAC naming system:
• Aliphatic or aromatic amines are named by replacing ‘e’ in the end of the parent hydrocarbon by ‘amine’.
• 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.
H2N ‒ CH2 ‒ CH2 ‒ NH2
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:
1o > 2o > 3o
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 Kb and pKb 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