Basicity of Aliphatic Amines - Definition, Quantifying & Effect

Basicity of Aliphatic Amines Definition

Basicity is a measure of a compound’s ability to accept protons (H⁺).
In aliphatic amines, arises from the lone pair of electrons on the nitrogen atom, which can accept a proton, forming a positively charged ammonium ion.

The basicity of amines is often expressed through their pKb values, which are derived from the base dissociation constant (Kb).
pKb values: A lower pKb indicates a stronger base, while a higher pKb signifies a weaker base.

Its aliphatic amines is influenced by the nature and positioning of substituents on or near the nitrogen atom. Key factors include:

Alkyl Groups:
1. Electron-Donating Effect: Alkyl groups attached to nitrogen exert an inductive effect, pushing electron density towards the nitrogen, which enhances the nitrogen’s ability to donate its lone pair and increases the amine’s basicity.
2. Order of Basicity: The basicity generally increases with the number of alkyl groups in the order: primary (1°) < secondary (2°) < tertiary (3°) amines.
Electron-Withdrawing Groups (EWGs):
- EWGs, such as halogens or nitro groups, pull electron density away from the nitrogen atom, reducing the electron density available for protonation and thus decreasing the amine’s basicity.
Steric Hindrance:
- Bulky substituents near the nitrogen can physically obstruct the approach of protons, making it more difficult for the amine to accept a proton. This steric hindrance reduces the basicity’s, especially in tertiary amines with large alkyl groups.
Hybridization of the Nitrogen Atom:
- sp³ vs. sp² Hybridization: Amines with sp³-hybridized nitrogen are generally more basic than those with sp²-hybridized nitrogen. The higher s-character in sp² hybridization pulls the lone pair closer to the nitrogen, making it less available for protonation, thereby reducing.