Basicity of Amines — Cornell Notes (Key Subtopic) — Notes | NoteTube

Question	Answer
What is the gas-phase basicity order?	3° > 2° > 1° > $NH_{3}$ — more alkyl groups = more +I effect = stronger electron donation
What is the aqueous-phase basicity order?	2° > 1° > 3° > $NH_{3}$ — solvation of $R_{3}NH^{+}$ is poor due to steric bulk
Why does solvation affect basicity?	Basicity depends on stability of conjugate acid; better hydration (H-bonding) stabilizes $R_{2}NH_{2}^{+}$ and $RNH_{3}^{+}$ more than $R_{3}NH^{+}$
How do EWG and EDG affect aniline basicity?	EWG (e.g., - $NO_{2}$ at para) → further delocalization → weaker base; EDG (e.g., - $OCH_{3}$ at para) → reduces delocalization → stronger base
Compare pKb of methylamine vs aniline	Methylamine pKb ≈ 3.38 (stronger base); Aniline pKb ≈ 9.38 (much weaker base)
Why can't aromatic amines be as basic as aliphatic?	Resonance structures show N lone pair entering ring → partial double bond C–N; lone pair effectively "trapped" in ring
What is the effect of ortho substitution on aniline basicity?	Steric inhibition of resonance can slightly increase basicity; but EWG at ortho still decreases it

Summary: Amine basicity is governed by two competing factors: electron-donating alkyl groups (+I effect) increase basicity, while solvation of the protonated amine determines the aqueous order. Aniline is fundamentally weaker because resonance withdraws the nitrogen lone pair into the ring, and its extent is modulated by ring substituents.