Chemistry Unit 4
Amines can be thought of as derivatives of ammonia.
It is where one or more of the hydrogen atoms have been replaced by alkyl or aryl groups.
Naming amines – suffix –amine OR prefix amino-
Primary - general formula RNH2
CH3-NH2 is methylamine
Benzene amine is amino benzene
Secondary – RR’NH
(CH3)2NH is dimethylamine
Tertiary – RR’R’’N
(C2H5)3N is trimethylamine
And as always different substituents are written in alphabetical order
Amines have a lone pair of electrons.
This lone pair may be used to form a bond with a H+ ion, and we say the amine is acting as a base.
The lone pair may also be used to form a bond with an electron-deficient atom, and we say the amine is acting as a nucleophile.
Amines as a base
Amines can accept a proton à they are B.L. bases.
The lone pair on the nitrogen atom allows the nitrogen atom to act as a proton acceptor.
All amines react with strong acids to form alkylammonium salts.
Reactions with acids
Amines react with acids in the same way as ammonia:
NH3 + HCl à NH4Cl
2NH3 + H2SO4 à (NH4)2SO4
Primary amines: R1-NH2 + HCl à R1-NH3Cl
2R1-NH2 + H2SO4 à (R1-NH3)2SO4
Secondary amines: R1R2-NH + HCl à R1R2-NH2Cl
2R1R2-NH + H2SO4 à (R1R2-NH2)2SO4
Tertiary amines: R1R2R3-N + HCl à R1R2R3-NHCl
2R1R2R3-N + H2SO4 à (R1R2R3-NH)2SO4
Comparison of base strengths
The basicity of an amine is dependent 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.
Alkyl groups release electrons away from the alkyl group and towards the nitrogen atom. (Inductive effect)
The inductive effect of the alkyl group increases the electron density on the nitrogen atom. Primary alkylamines are stronger bases than ammonia.
Secondary alkylamines have 2 inductive effects and are therefore stronger bases than primary alkylamines.
Tertiary alkylamines are not stronger bases than secondary alkylamines, as they are less soluble in water.
Aryl groups withdraw electrons from the nitrogen, because the lone pair of…