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

Nov 2, 2016 17:00 IST

    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:
    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

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