How to Use a pKa Table

1. Lower pKa means a stronger acid and a more stable conjugate base.

The pKa is the negative logarithm of the acid dissociation constant, so every unit is a factor of ten in acidity. HCl, with a pKa of about −7, is roughly ten trillion times more willing to give up its proton than ethanol at pKa 16. When you scan a table from top to bottom, you are watching acids get progressively weaker. The payoff is on the right-hand side of each row: the weaker the acid, the stronger and less stable its conjugate base. A strong acid like HCl produces a happy, stable chloride ion, while a weak acid like ethanol produces a reactive, high-energy ethoxide.

2. Every row pairs an acid on the left with its conjugate base on the right.

A pKa table is really a table of conjugate acid–base pairs. Reading across a row, the neutral (or protonated) acid loses one proton to become the conjugate base directly to its right. This is the single most useful habit to build: whenever you look up a pKa, immediately picture the species that forms after the proton leaves. Acetic acid becomes acetate; water becomes hydroxide; ammonium becomes ammonia; a terminal alkyne becomes an acetylide. The pKa number belongs to the acid, but it tells you everything about the reactivity of the base.

3. A reaction favors the side with the weaker acid (the higher pKa).

This is the rule that turns a pKa table into a prediction machine. In any acid–base reaction the proton moves from the stronger acid to the stronger base, generating the weaker acid and the weaker base. To decide which way an equilibrium lies, find two pKa values: the acid you start with, and the acid you would form on the product side. Equilibrium settles toward whichever acid has the higher pKa. The size of the gap sets the position: the equilibrium constant is roughly 10 raised to the difference in pKa, so K_eq ≈ 10^{(pKa of acid formed − pKa of acid consumed)}. A difference of just a few units already means the reaction runs essentially to completion.

Take acetic acid reacting with hydroxide. Acetic acid (pKa 4.76) is the stronger acid, so it hands its proton to hydroxide, producing acetate and water (pKa 15.7). Water is far weaker, so products win: ΔpKa ≈ 15.7 − 4.76 ≈ 11, meaning K_eq ≈ 10¹¹. This reaction goes.

4. To fully deprotonate a compound, pick a base whose conjugate acid has a higher pKa.

Turning the rule around gives a recipe for choosing reagents. If you want to remove a proton completely, you need the reverse reaction to be uphill for re-protonation — that is, the acid you form (the base's conjugate acid) must be weaker than the compound you are deprotonating. In table language: the base's conjugate acid must have a higher pKa than your substrate. A terminal alkyne has pKa ~25. Sodium hydroxide will not touch it, because water (pKaH 15.7) is a stronger acid than the alkyne — the equilibrium sits ~10⁹ on the wrong side. Sodium amide (NaNH₂) works: its conjugate acid is ammonia at pKa ~38, comfortably higher than 25, so deprotonation is favorable by about 13 pKa units.

5. A representative table gives you the anchors you actually need.

You do not have to memorize hundreds of values. Learn a dozen reference points spread across the scale and you can place almost any organic acid by interpolation. Keep these in your head:

6. Estimate an unknown pKa from the acidity factors.

When a compound is not in your table, reason from what stabilizes the conjugate base. Use the “CARDIN” ideas: the more stable the anion, the stronger the acid. Compare the atom holding the charge (electronegativity and size: an O–H is more acidic than an N–H, which beats a C–H), look for resonance (a carboxylic acid at ~4 crushes an alcohol at ~16 because acetate spreads its charge over two oxygens), count inductive electron-withdrawing groups nearby, and consider hybridization (sp carbon in an alkyne, pKa 25, holds the lone pair far better than sp³ carbon in an alkane, pKa 50). Phenol sits at 10 rather than 16 precisely because its phenoxide is resonance-stabilized into the ring.

7. Summary

A pKa table is a ranked list of acids — lower pKa, stronger acid, more stable conjugate base — with each acid paired to the conjugate base it forms. To predict a reaction, look up two pKa values (the acid consumed and the acid produced) and let equilibrium favor the higher-pKa, weaker acid; the gap gives K_eq ≈ 10^ΔpKa. To deprotonate completely, choose a base whose conjugate acid has a higher pKa than your target proton — that is why NaNH₂ (pKaH 38) deprotonates a terminal alkyne (25) but NaOH (pKaH 15.7) cannot. For anything off the table, estimate from atom, resonance, induction, and hybridization. Master these four moves and the table stops being a chart to memorize and becomes a tool you reason with.