Source: "The Lucky 13" by Dr. Noel M. Paul, The Ohio State University
Tags: pKa, acid strength, base strength, conjugate base, dissociation, organic chemistry, Lucky 13, functional group acidity
Difficulty: Intermediate | Prerequisites: Basic understanding of bonding, electronegativity, and functional group identification.
Acid strength is one of the most fundamental concepts in organic chemistry. Nearly every reaction mechanism you encounter, from substitution to elimination to carbonyl chemistry, depends on knowing which protons are acidic and which are not. The Lucky 13 chart ranks thirteen common functional group families by their approximate pKa values, giving you a single reference for predicting proton transfers. If you can internalise this ranking, you will be able to predict the direction of any acid–base equilibrium on sight.
Stronger acids have lower pKa values and dissociate more readily; weaker acids have higher pKa values. The Lucky 13 ranks functional groups from inorganic acids (pKa around −5) to alkanes (pKa around 55). The ranking is governed by how stable the conjugate base is once the proton leaves.
pKa
The negative logarithm of the acid dissociation constant (Ka). A lower pKa means a stronger acid. Each unit change represents a tenfold difference in acid strength. In simple terms, pKa is a number that tells you how easily a molecule gives up a proton. The smaller the number, the more willingly it lets go.
Acid strength
A measure of how readily a compound donates a proton (H⁺) to a base. Strong acids dissociate fully or nearly fully in solution; weak acids dissociate only partially. Think of it as how eager a molecule is to part with its hydrogen.
Conjugate base (A:⁻)
The species that remains after an acid has donated its proton. Written as A:⁻ in the Lucky 13 chart. The stability of this species determines how strong the parent acid is. In simple terms, it is whatever is left behind once the proton has gone.
Dissociation
The process by which an acid releases a proton into solution. A bond breaks, the proton departs, and a conjugate base forms.
H–A bond
The bond between a hydrogen atom and the rest of the acid molecule. Acid strength correlates inversely with H–A bond stability: less stable bonds dissociate more easily.
The chart arranges thirteen functional group families in order of decreasing acid strength (increasing pKa). The approximate pKa values are:
Inorganic acids (HCl, HBr, HI, H₂SO₄, HNO₃): pKa ≈ −5
Oxonium ions (protonated alcohols, protonated water): pKa ≈ 0
Carboxylic acids (R–COOH): pKa ≈ 5
Ammonium ions (R–NH₃⁺): pKa ≈ 10
1,3-Dicarbonyl compounds (beta-diketones, beta-keto esters): pKa ≈ 10
Alcohols (R–OH): pKa ≈ 15
Carbonyl alpha-hydrogens (ketones, aldehydes at the alpha position): pKa ≈ 20
Alkynes (terminal R–C≡C–H): pKa ≈ 25
Diallyl / dibenzyl compounds: pKa ≈ 35
Amines (R–NH₂, R₂NH): pKa ≈ 35
Allyl / benzyl compounds: pKa ≈ 40
Alkenes (vinyl C–H): pKa ≈ 45
Alkanes (R–H, sp³ C–H): pKa ≈ 55
The left side of the chart holds the strongest acids (lowest pKa). These have unstable H–A bonds that break easily. The right side holds the weakest acids (highest pKa), whose H–A bonds are stable and resist dissociation.
The conjugate base relationship runs in the opposite direction. The conjugate bases of the strongest acids are the weakest (most stable, least reactive) bases. The conjugate bases of the weakest acids are the strongest (most reactive, least stable) bases.
Acid strength is not really about the acid itself. It is about the conjugate base. If the conjugate base is stable after losing the proton, the acid is strong. If the conjugate base is unstable and desperate to reclaim a proton, the acid is weak.
This single idea, conjugate base stability, is the thread that connects every entry on the chart.
Buffer solutions in biological systems rely on compounds with pKa values near physiological pH. Understanding where carboxylic acids and amines sit on this scale explains why amino acids behave as buffers and why drug molecules are designed with specific ionisation profiles.
Students often assume that a "strong acid" in organic chemistry means the same as in general chemistry (full dissociation in water). In organic chemistry, "strong" and "weak" are relative terms across the full pKa range, and many reactions occur in non-aqueous solvents.
Confusing pKa with pH is common. pKa is a property of a specific compound; pH is a property of a solution. They are related through the Henderson–Hasselbalch equation but are not interchangeable.
Some students think a high pKa means the compound cannot act as an acid at all. It can, but only with a sufficiently strong base to deprotonate it. Alkanes (pKa ≈ 55) can be deprotonated by superbases like n-butyllithium.
Memorising the exact numbers is less important than knowing the order. Exam questions typically ask you to compare two species, not to recall that alcohols are precisely 15.
⚠️ Predicting the direction of proton transfer is one of the most common exam tasks. Equilibrium favours the side with the weaker acid (higher pKa) and the more stable conjugate base.
⚠️ You will be asked to rank compounds by acidity. The Lucky 13 gives you the framework; the stability factors (covered in Part 2) give you the reasoning.
⚠️ Understanding this scale is prerequisite for mechanisms involving enolates, Grignard reagents, elimination reactions, and nucleophilic additions.
True or false: A compound with a pKa of 10 is a stronger acid than one with a pKa of 25.
Fill in the blank: The conjugate base of a strong acid is a ______ base.
True or false: Alkanes are the strongest acids on the Lucky 13 chart.
Fill in the blank: Acid strength is ultimately determined by the stability of the ______.
True or false: Each pKa unit represents a tenfold difference in acid strength.
Answers: 1. True. 2. Weak (stable). 3. False, they are the weakest. 4. Conjugate base. 5. True.
Q: An alcohol (pKa ≈ 15) is mixed with the conjugate base of an alkane (pKa ≈ 55). Will a proton transfer occur, and in which direction?
A: Yes. The alkane conjugate base is extremely strong and will deprotonate the alcohol. Equilibrium favours formation of the weaker acid (alkane, pKa 55) and the more stable conjugate base (alkoxide).
Q: Rank the following in order of increasing acid strength: amines, carboxylic acids, alkynes, inorganic acids.
A: Amines (pKa ≈ 35) < alkynes (pKa ≈ 25) < carboxylic acids (pKa ≈ 5) < inorganic acids (pKa ≈ −5). Acid strength increases as pKa decreases.
Q: Why does acid–base equilibrium favour the side with the higher-pKa acid?
A: The higher-pKa acid is weaker, meaning its conjugate base is more stable. Reactions proceed towards the more thermodynamically stable products, so equilibrium favours formation of the more stable (weaker) conjugate base.
Q: A student claims that because water has a pKa of about 15.7, it is a weaker acid than a carboxylic acid. Is this correct?
A: Yes. Water (similar range to alcohols on the chart, pKa ≈ 15) is a weaker acid than a carboxylic acid (pKa ≈ 5). The carboxylate conjugate base is stabilised by resonance, making the carboxylic acid roughly 10¹⁰ times more acidic.
This material connects directly to nucleophilicity and leaving-group ability: weak bases tend to be good leaving groups. It also underpins carbonyl chemistry, where the formation of enolates depends on the pKa of alpha-hydrogens (≈ 20) relative to the base used. If you go on to study biochemistry, enzyme active sites exploit precisely these pKa differences to shuttle protons during catalysis.
pKa scale, acid dissociation constant, Ka, proton donor, Brønsted acid, Brønsted base, conjugate acid, conjugate base, proton transfer, acid–base equilibrium, Lucky 13 chart, functional group acidity ranking, Henderson–Hasselbalch, deprotonation, organic acid strength comparison