Amines are weak bases; the lone pair of electrons on the nitrogen atom can behave as a proton acceptor. The strength of amines varies, but all react with strong acids to make alkylammonium salts.
Amines have a lone pair of electrons which can form bonds with:
- An H
+ ion- hence act as Bronsted-Lowry base
- An electron- deficient C, hence acting as a nucleophile
Reaction of amines with acids
Amines react with acids in the same way as ammonia:
NH3+ HCl ⇨ NH4Cl
2NH3+ H2SO4 ⇨ (NH4)2SO4
R1-NH2 + HCl ⇨ R1-NH3Cl
2R1-NH2+ H2SO4 ⇨ (R1-NH3)2SO4
R1R2-NH + HCl ⇨ R1R2-NH2Cl
2R1R2-NH + H2SO4 ⇨ (R1R2-NH2)2SO4
R1R2R3-N + HCl ⇨ R1R2R3-NHCl
2 R1R2R3-N+ H2SO4 ⇨ (R1R2R3-NH)2SO4
Eg: CH3NH2 + HCl ⇨ CH3CH2NH3+Cl-
The charges on the ions are often shown, eg R1R2-NH2+Cl-
Comparison of base strength of different amines
The basicity of ammonia and amines depends on the availability of the lone pair of electrons on the nitrogen atom. The more available the lone pair, the stronger the base.
The availability of the lone pair in turn depends on how much electron density is on the nitrogen atom, and this depends on the adjacent atoms.
Primary amines have one alkyl group attached to the N atom:
The hydrogen atom is slightly more electropositive than the carbon atom. This means that the carbon atom has a slight surplus of electrons which the nitrogen atom can attract towards itself. This effect is known as a “positive inductive effect” and it means that the N atom has an excess of electron density:
The greater the electron density on the N atom, the more likely the lone pair is to be released for bonding. Primary amines are thus stronger bases than ammonia, as the alkyl groups push electron density onto the N atom, making the lone pair more available for bonding.
It follows that secondary amines, which have two electron-pushing alkyl groups, are stronger bases than primary amines and that tertiary amines are stronger bases than secondary amines.
Reaction of alkylammonium salts with bases
The alkylammonium salts are slightly acidic and can be converted back into amines on addition of alkalis:
Primary ammonium salts: R1-NH3Cl + NaOH ⇨ R1-NH2 + NaCl + H2O
Secondary ammonium salts: R1R2-NH2Cl +NaOH ⇨ R1R2-NH +NaCl + H2O
Tertiary ammonium salts: R1R2R3-NHCl+ NaOH ⇨ R1R2R3-N+ NaCl + H2O
Quaternary ammonium salts are not acidic as the N atom is not bonded to any hydrogen atoms.
Drafted by Eunice (Chemistry)