Amines

• Amines are the derivatives of ammonia.
• Like ammonia, the nitrogen atom in amines is also sp3 hybridised with an unpaired electron in one of the sp3 hybridised orbitals.
• The shape of amines is also pyramidal.
• Amines are classified into three types – primary (10), secondary (20) and tertiary (30) amines.
• If one hydrogen atom of ammonia is replaced by R (alkyl) or aryl (Ar) group, we get 10 amine.
• The general formula of amines is R - NH2.
• If two hydrogen atoms of ammonia are replaced by two alkyl (R) or aryl (Ar) groups, we get 20 amine.
• The general formula of 20 amine is R2 NH.
• If three hydrogen atoms of ammonia are replaced by R or Ar groups, we get 30 amine.
• The general formula of 30 amine is R3 NH.
• If all the alkyl or aryl groups are the same, that amine is called simple amine and if they are different, it is called mixed amine.
• In common system, amines are named by prefixing alkyl group to amine, i.e., alkyl amine.
• In secondary and tertiary amines, when the alkyl groups are same, the prefix di or tri is used before the name of the alkyl group.
• Lower aliphatic amines are soluble in water because they can form hydrogen bonds with water molecules.
• Solubility of lower aliphatic amines decreases with increase in molar mass of amines due to increase in size of the hydrophobic alkyl part.
• In amine, there is an unshared pair of electrons on nitrogen atom and hence it can donate this electron pair, acting as a Lewis base.
• Alkyl amines are stronger bases than ammonia.
• Aromatic amines are weaker bases than ammonia due to the electron withdrawing nature of the aryl group.
• As the stability of the cation increases, the basicity also increases.
• Higher amines are insoluble in water.
• Tertiary amines do not have intermolecular association due to the absence of hydrogen atom.
• Lower aliphatic amines have a fishy smell.
• Primary and secondary amines are associated through intermolecular hydrogen bonding.
• Due to the electron releasing nature of alkyl group in alkyl amines, it pushes electrons towards nitrogen and thus makes the unshared electron pair more available for sharing with the proton of the acid.
• Basic character of amines can be expressed in terms of Kb and pKb.
• Amines react with acids to form salts, indicating their basic nature.
• Amines are soluble in organic solvents like alcohol, ether and benzene.
• Aliphatic amines are stronger bases than ammonia due to the +I effect of alkyl groups leading to greater electron density on the nitrogen atom.
• The order of boiling points of isomeric amines is as follows: Primary > Secondary > Tertiary.
• The substituted ammonium ion formed from the amine gets stabilized charge by the +I effect of the alkyl group.
• Basic character of an amine depends upon the ease of formation of the cation by accepting a proton from the acid.
• Basicity of amines is related to their structure.
• In IUPAC system, amines are named by replacing of ‘e’ of alkane by the word amine, i.e., alkanamine.
• In the case of secondary and tertiary amines, the alkyl group containing the maximum number of carbon atom is taken as the parent chain and the other alkyl groups are taken as substituents.
• While naming aryl amines according to IUPAC system, the suffix ‘e’ of arene is replaced by ‘amine’, i.e., arenamine.
• The electron pairs are less available for protonation and hence aryl amines are less basic.
• The greater the size of the ion, lesser will be the solvation and the less stabilised is the ion, hence the basicity also decreases.
• The order of basicity of aliphatic amines should be: primary > secondary > tertiary, which is opposite to the inductive effect based order.
• When the alkyl group is small, there is no steric hindrance to H-bonding, while in case the alkyl group is bigger than CH3 group, there will be steric hindrance to H-bonding, hence the size of the alkyl group also determines the order of basic strength.
• Amines react with alkyl halides undergo nucleophilic substitution reaction to form a mixture of secondary and tertiary amines and quaternary ammonium salt, a reaction known as Hofmann’s Ammonolysis.