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Basicity of Organic Compounds

January 28, 2013

In general Amines may be regarded as derivatives of ammonia, in which one or more hydrogen atoms have been replaced by alkyl groups. Thus they may be

    • Primary
    • Secondary or
    • Tertiary

Depending on whether one, two, or all the three hydrogen atoms of ammonia molecules are replaced by alkyl group.

NH3 (ammonia), for primary amine it is denoted as (–NH2), for secondary amine it is denoted as (NH), and for tertiary amine it is specified as (–N-).

Thus the secondary and tertiary amines may be further classified as simple or mixed depending on the fact, whether the alkyl group is identical or different. The important amines in this group include aniline, Methylamine, biogenic amines, amino acids. Amines are usually named after the alkyl groups attached to the nitrogen atom.

Suppose if the compounds containing nitrogen group gets attached with the carbonyl group of the structure R-C (=O) NR2 results in the formation of amides which have different physical and chemical properties respectively.

Basicity of Organic Compounds:classes of Amines: Aliphatic Amines and Aromatic Amines

Classes of Amines: Amines may be usually classified as aliphatic amines and Aromatic amines.

Aliphatic amines: They are primary amines which when formed in the replacement of one of the hydrogen atom in the ammonia by an alkyl group. The most important primary amines in this class includes Methylamine, ethanolamine etc.

Whereas in the case of secondary amines two alkyl substituent gets attached with the nitrogen with the replacement of hydrogen. E.g.Dimethylamine, methyl ethanolamine etc.

In the case of tertiary amines all the three hydrogen atoms is replaced with the alkyl groups. E.g. trimethylamine, methyl ethyl Propyl amine etc.

Aromatic amines: in this case the nitrogen atom present in the amines gets attached with the aromatic rings in the anilines. Thus the alkaline character of the aromatic ring decreases based on the substituent added. But the presence of the amine group increases the activity of the aromatic ring due to the electron donating effect.

Basicity of Organic Compounds:general Physical Characteristics of Basicity of Organic Compounds

General physical characteristics of basicity of Organic compounds

    • lower members are combustible gases at ordinary temperature, the members from C3 to C11 are volatile liquids, while still higher members are solids.
    • The lower members have fishy ammoniacal odor, which vanishes in higher members. The lower members are much soluble in water, but the solubility decreases with increase in molecular weight.
    • The boiling point also increases with increase in molecular weight. The aqueous solutions of amines are alkaline to litmus and conduct electricity.
  • Like ammonia, amines are polar compounds and can form intermolecular hydrogen bonding, though not strong as in the case of alcohols and carboxylic acids.

Spectroscopic properties of basicity of organic compounds

  • In U.V. spectra saturated aliphatic amines show a weak absorption band in the region 210-230 mμ. In I.R. spectra amines exhibit C-N stretching absorption in the region 1220-1020 cm-1, N-H stretching bands in the region 3500-3400 cm-1 (for primary amines), and 3350-3300 cms-1 (secondary amines).
  • The absorption frequencies are lowered significantly in case of hydrogen bonding, but the effect is generally smaller than that of hydroxyl group.
  • In N.M.R. spectra the chemical shift for protons attached to nitrogen is generally between 0.3 to 2.2δand a single sharp absorption band is obtained.

General chemical characteristics of basicity of organic compounds:

The basic character of amines is due to the presence of unshared electron pair on nitrogen atom, which accepts proton. Due to positive inductive effect of alkyl groups the electron density at nitrogen atom increases and makes the unshared electron pair more available for protonation. Thus it is expected that the basicity of organic compounds i.e amines should be in the order tertiary>secondary>primary.

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