Definition:
Basicity is a measure of a compound's ability to accept protons (H⁺).
For aliphatic amines, this property is primarily due to the presence of a lone pair of electrons on the nitrogen atom, enabling the formation of a positively charged ammonium ion upon protonation.
Quantifying Basicity:
The basicity of amines is often expressed through their pKb values, which are derived from the base dissociation constant (Kb).
A lower pKb value indicates a stronger base, while a higher pKb value signifies a weaker base.
Effect of Substituents on Basicity
The basicity of aliphatic amines is influenced by various factors, including the nature and positioning of substituents on or near the nitrogen atom.
These effects can be grouped into four main categories:
1. Alkyl Groups:
1. Electron-donating Effect:
Alkyl groups attached to nitrogen exert an inductive effect, pushing electron density towards the nitrogen.
This enhances the nitrogen’s ability to donate its lone pair, increasing the amine’s basicity.
2. Order of Basicity:
Generally, basicity increases with the number of alkyl groups, following the order: primary (1°) < secondary (2°) < tertiary (3°) amines.
2. Electron-withdrawing Groups (EWGs):
EWGs, such as halogens or nitro groups, pull electron density away from the nitrogen atom.
This diminishes the electron density available for protonation, thereby decreasing the amine's basicity.
3. Steric Hindrance:
Bulky substituents near the nitrogen atom can physically obstruct the approach of protons, making it harder for the amine to accept a proton.
This steric hindrance generally leads to a decrease in basicity, notably affecting tertiary amines with large alkyl groups.
4. Hybridization of the Nitrogen Atom:
sp³ vs. sp² Hybridization:
Amines where the nitrogen is sp³-hybridized tend to be more basic than those with an sp²-hybridized nitrogen.
The higher s-character in sp² hybridization pulls the lone pair closer to the nitrogen, making it less available for protonation and thus, reducing basicity.