- Amines are basic in nature due to the presence of lone pair of electrons on nitrogen atom. (fig.1)
- Aliphatic amines (fig. 2) are stronger bases than ammonia because of +I effect (Inductive effect) of alkyl groups present in amines.
- Aromatic amines (fig. 3) are weaker bases than ammonia due to –I effect of aryl (C6H5 ) group group. Other effects which affect basic nature of amines are steric effect and solvation effect.
Relative basic nature of amines
tertiary amine > secondary amine > primary amine > ammonia (due to +I effect)
Aniline is weaker than ammonia. Why?
The pair of electron on Nitrogen in aniline is delocalised by benzene ring whereas there is no such delocalisation of electron pair in ammonia. Hence aniline is weaker than ammonia.
Inductive Effect
- The polarisation of sigma bond due to the presence of nearby electronegative or electron withdrawing group or atom is called negative inductive effect (-I effect).
- The polarisation of sigma bond due to the presence of nearby electropositive or electron donating group or atom is called positive inductive effect (+I effect).
Steric Effect
- The spatial (stereo = space) arrangement of an atom or group of atoms nearby the reacting site affect its reactivity.
Solvation Effect
- The primary and secondary ammonium salts undergo solvation effects (surrounding of solvent molecules) to a much greater degree than tertiary ammonium salts.
- These solvation effects increase the electron density on the amine nitrogen to a greater degree than the inductive effect of alkyl groups.
