Amines: These are alkyl or aryl derivatives of ammonia and are obtained by replacing one, two or three hydrogen atoms by alkyl/aryl groups.
Nomenclature: In a common system, an aliphatic amine is named by prefixing alkyl group to amine, i.e., alkylamine. In the IUPAC system, amines are named as Alkan amines. In secondary and tertiary amines, when two or more groups are the same, the prefix di or tri is appended before the name of the alkyl group.
Structure: Nitrogen orbitals in amines are sp3 hybridized and the geometry of amines is pyramidal. 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°.
Limitations of Gabriel phthalimide synthesis:
— It is used for the preparation of only 1° amine. tert-Butylamine is a 1° amine, but cannot be prepared by this method. In this case, elimination takes place.
— Aromatic 1° amines cannot be prepared by this method because aryl halides do not undergo nucleophilic substitution reaction with potassium phthalimide under mild conditions.
Lower amines are gases and liquids but higher amines are solids.
Primary and secondary amines have higher boiling points than other organic compounds due to hydrogen bonding.
Primary and secondary amines are soluble in water due to hydrogen bonding between –NH2 and H2O molecules.
Basic character of amines:
— Amines are basic in nature due to the presence of lone pair of electrons on the nitrogen atom.
— Aliphatic amines are stronger bases than ammonia due to +I effect of alkyl groups present in amines.
— Aromatic amines are weaker bases than ammonia due to –I effect of the aryl group.
— Besides the inductive effect, there are other effects like steric effect, solvation effect, resonance effect which effect the basic strength of amines.
— In the gaseous phase, the order of the basicity of amines is 3° amine > 2° amine > 1° amine > NH3.
— In the aqueous phase, despite the inductive effect, the solvation effect and steric hindrance also play an important role. Thus, the order of basicity of amines is
(C2H5)2NH > (C2H5)3N > C2H5NH2 > NH3 and (CH3)2NH > CH3NH2 > (CH3)3N > NH3
Electrophilic substitution reactions of arylamines: Aniline undergoes electrophilic substitution reactions. NH2 group is ortho and para directing and a powerful activating group.