Predicting Acidity

The most acidic proton is the one whose conjugate base is most stable. Lower pK_a = stronger acid.

Key Factors

Element: O–H > N–H > C–H in general. Moving down a group increases acidity (S–H > O–H in DMSO).
Resonance: If the conjugate base is resonance-stabilized (e.g., next to C=O, NO₂), acidity increases dramatically.
Induction: Nearby electron-withdrawing groups stabilize the conjugate base through sigma bonds.
Hybridization: sp C–H > sp² C–H > sp³ C–H (more s-character = more acidic).
Aromaticity: Deprotonation that creates an aromatic anion is strongly favored (e.g., cyclopentadiene).

Don't always pick the heteroatom — a C–H alpha to two carbonyls can be more acidic than an alcohol.
Look for alpha positions flanked by multiple activating groups.

Click on the atom bearing the most acidic hydrogen.

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