pKₐ Values and Acidity Trends

pKₐ (−log Kₐ) quantifies acid strength: lower pKₐ → stronger acid. Each pKₐ unit is a 10× change, so functional groups span huge ranges. Here are key benchmarks and the structural factors that stabilize conjugate bases.

• Benchmark pKₐ Values
• Factors That Increase Acidity
• Summary

Benchmark pKₐ Values

Using the same visual format as the conjugate-pairs article, here are common acids with pKₐ and their conjugate bases.

#	Acid	Structure	pKₐ (water)	Conjugate base	Conjugate base (name)
1	Acetic acid / Ethanoic acid		4.76		Acetate
2	Phenol / Phenol		≈10.0		Phenoxide
3	Methylammonium / Methanaminium		≈10.6		Methylamine
4	Ethanethiol / Ethane-1-thiol		≈10.6		Ethanethiolate
5	Ethanol / Ethan-1-ol		≈16		Ethoxide

Bases and Conjugate-Acid pKₐ Benchmarks

#	Base	Structure	Conjugate acid pKₐ (water)	Conjugate acid	Conjugate acid (name)
1	Acetate		4.76		Acetic acid
2	Pyridine		≈5.2		Pyridinium
3	Ammonia		≈9.25		Ammonium
4	Methylamine		≈10.6		Methylammonium
5	Ethoxide		15.9		Ethanol

Factors That Increase Acidity (Stabilize Conjugate Base)

• Atom (electronegativity/size): More electronegative or larger atoms better stabilize charge.
• Resonance: Delocalizing negative charge (e.g., carboxylates) greatly boosts acidity.
• Inductive effects: Electron-withdrawing groups pull electron density and stabilize the anion.
• Hybridization: More s-character (sp > sp² > sp³) stabilizes the anion and raises acidity.

Summary

• pKₐ is logarithmic; small shifts mean big strength changes.
• Acidity tracks with conjugate-base stability: atom effects, resonance, induction, and hybridization all matter.
• Use benchmarks to triage acidity (strong acids ~ −7, carboxylic ~5, alcohols ~16, amines ~38, alkanes ~50).