Basic Strength: Substituent Effects on Aniline

Substituent Effects on Aniline Basicity:

$$\text{Electron-Releasing Groups } (+I, +M) \implies \text{Increase Basicity } (\text{e.g., } -\text{OCH}_3, -\text{CH}_3, -\text{NH}_2)$$

$$\text{Electron-Withdrawing Groups } (-I, -M) \implies \text{Decrease Basicity } (\text{e.g., } -\text{NO}_2, -\text{CN}, -\text{X})$$

Ortho Effect in Aniline:

$$\text{Ortho-Substituted Aniline} < \text{Aniline} \quad (\text{Valid for nearly all ortho-substituents regardless of electronic nature})$$

Concept: Steric hindrance by ortho-substituents prevents protonation or destabilizes the resulting anilinium cation (Steric Inhibition of Resonance / Protonation), drastically reducing basicity.