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Basic Strength of Amines | chemca

Basic Strength of Amines | chemca
Organic Chemistry

Basic Strength of Amines

Factors affecting basicity in Aliphatic and Aromatic Amines.

By chemca Team • Updated Jan 2026

Amines behave as Lewis bases due to the presence of an unshared pair of electrons on the nitrogen atom. They accept protons to form ammonium ions.
Basicity Measurement: Stronger bases have higher $K_b$ values and lower $pK_b$ values.

1. Aliphatic Amines

Gaseous vs. Aqueous Phase

A. Gaseous Phase:

Only the Inductive Effect (+I) of alkyl groups matters. More alkyl groups $\rightarrow$ higher electron density on N $\rightarrow$ stronger base.

$$ 3^\circ > 2^\circ > 1^\circ > NH_3 $$

B. Aqueous Phase (The Real Challenge):

In water, basicity is determined by the combined effect of:

  • +I Effect: Favors $3^\circ$.
  • Solvation Effect: The conjugate acid ($R-NH_3^+$) is stabilized by H-bonding with water. Lesser steric hindrance implies better solvation. Favors $1^\circ$.
  • Steric Hindrance: Bulky groups hinder proton attack. Favors $1^\circ$.
Experimental Orders (Memorize This!):
1. Methyl Substituted ($R = CH_3$):
$$ (CH_3)_2NH > CH_3NH_2 > (CH_3)_3N > NH_3 $$ $$ 2^\circ > 1^\circ > 3^\circ > NH_3 $$
2. Ethyl Substituted ($R = C_2H_5$):
$$ (C_2H_5)_2NH > (C_2H_5)_3N > C_2H_5NH_2 > NH_3 $$ $$ 2^\circ > 3^\circ > 1^\circ > NH_3 $$

2. Aromatic Amines (Aniline)

Why is Aniline a Weaker Base?

Aniline is much less basic than Ammonia and Aliphatic amines.

  • Resonance: The lone pair on Nitrogen is delocalized into the benzene ring (+R effect), making it less available for protonation.
  • Stability of Ions: The Anilinium ion (conjugate acid) has only 2 resonating structures, while Aniline has 5. Thus, Aniline is more stable than its ion, shifting equilibrium backwards.
$$ R-NH_2 > NH_3 > C_6H_5NH_2 $$

3. Substituted Anilines

Effect of Groups

A. Electron Releasing Groups (ERG):
Groups like $-CH_3, -OCH_3, -NH_2$ increase electron density on the ring and Nitrogen.
$\rightarrow$ Increases Basicity.
B. Electron Withdrawing Groups (EWG):
Groups like $-NO_2, -CN, -X, -COOH$ withdraw electrons.
$\rightarrow$ Decreases Basicity.
$$ p\text{-Toluidine} > \text{Aniline} > p\text{-Chloroaniline} > p\text{-Nitroaniline} $$

4. The Ortho Effect

Steric Inhibition

Ortho-substituted anilines are weaker bases than aniline, regardless of whether the substituent is electron-donating or electron-withdrawing.

Reason: Steric hindrance and electronic factors combine to destabilize the conjugate acid (anilinium ion), making protonation difficult.
Order for Toluidines ($CH_3$ group):
$$ p\text{-Toluidine} > m\text{-Toluidine} > \text{Aniline} > o\text{-Toluidine} $$
Order for Nitroanilines ($NO_2$ group):
$$ \text{Aniline} > m\text{-Nitro} > p\text{-Nitro} > o\text{-Nitro} $$

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