A2-Level Amines and Amides
Amines contain a carbon chain bonded to a nitrogen atom that is bonded to either hydrogen or another carbon chain (alkyl).
Amines can be primary, secondary or tertiary (quaternary amines exist as positively charged ions).
A lone pair of electrons on the nitrogen atom enables amines to act as bases (and nucleophiles).
Primary amines are less basic than secondary amines, and secondary amines are less basic than tertiary amines.
This is due to the positive inductive effect of alkyl chains.
Aromatic amines are weak bases as the benzene ring is electron withdrawing.
Amine functional groups derive from ammonia (NH ). They are useful in organic reactions, as they contain a nitrogen atom with a lone pair of electrons that can act as a nucleophile. This means compounds with an amine group can act as nucleophiles. The lone pair of electrons on the nitrogen can also accept protons, making them basic.
Primary, Secondary and Tertiary Amines
Amines can be primary, secondary or tertiary.
A primary amine has an NH group at the end of a carbon chain and the nitrogen is bonded to one carbon atom.
A secondary amine has an NH group and the nitrogen is bonded to two carbon atoms.
A tertiary amine has no hydrogens bonded to the nitrogen and the nitrogen is bonded to three carbon atoms.
Basicity of Amines
How basic an amine is (basicity) can be predicted from its structure.
Alkyl groups are slightly electron donating - this is why primary carbocations are less stable than secondary carbocations. Tertiary carbocations are the most stable as the positive charge is ‘stabilised’ by the slight donation of electrons from all three of the alkyl groups.
Bases accept protons because they have a lone pair of electrons. The more ‘available’ the lone pair is, the easier it is for a proton to be accepted, making the compound more basic.
The slight electron donating effect of alkyl groups increases the basicity of an amine, as it makes the lone pair on the nitrogen atom more available and likely to accept a proton (act as a base).
The more alkyl groups ‘pushing’ electron density towards the nitrogen atom (positive inductive effect), the easier it is for the lone pair of electrons to accept a proton. Primary amines only have one alkyl group, so they have less of the pushing effect than secondary amines. Tertiary amines have three alkyl groups pushing electron density towards the nitrogen atom, which makes tertiary amines the most basic.
An aromatic amine (an amine group on a benzene ring) is less basic than an aliphatic (straight chain) amine, as the benzene ring has an electron withdrawing effect and pulls electron density away from the nitrogen atom. The lone pair is less available for protons to bond to, so the amine is less basic.