What Is pK_a?

The pK_a measures how easily a compound loses a proton. Lower pK_a = stronger acid. These values are measured in DMSO (not water).

Factors That Increase Acidity

Electronegativity: More electronegative atoms stabilize the conjugate base better (e.g., O–H more acidic than C–H).
Resonance: If the conjugate base is resonance-stabilized, the acid is stronger (e.g., carboxylic acids, 1,3-diketones).
Induction: Electron-withdrawing groups (halogens, carbonyls, nitro, cyano) pull electron density away, stabilizing the conjugate base.
Hybridization: More s-character holds electrons closer → sp > sp2 > sp3 acidity (e.g., terminal alkynes are more acidic than alkenes).
Aromaticity: If deprotonation creates an aromatic anion (e.g., cyclopentadiene → cyclopentadienyl), acidity increases dramatically.

Compare two compounds and pick the one with the lower pK_a (more acidic).

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