Difficulty: Intermediate | Prerequisites: General chemistry acid-base basics, familiarity with electron-dot structures
Organic chemistry uses two acid-base frameworks: Brønsted (proton transfer) and Lewis (electron-pair transfer). The strength of an acid is measured by its pKa, and equilibrium always favours the side with the weaker acid and weaker base. Predicting which way a reaction goes comes down to comparing pKa values and understanding what stabilises a conjugate base.
Brønsted acid
A proton (H⁺) donor. In simple terms, any molecule that can hand off a hydrogen ion to something else.
Brønsted base
A proton (H⁺) acceptor. Think of it as the molecule that grabs the hydrogen ion.
Lewis acid
An electron-pair acceptor. In organic chemistry, this is the electrophile.
Lewis base
An electron-pair donor. In organic chemistry, this is the nucleophile.
Conjugate acid
The species formed when a base gains a proton. Every base has a conjugate acid.
Conjugate base
The species formed when an acid loses a proton. The stability of this conjugate base determines how strong the original acid is.
pKa
The negative log of the acid dissociation constant (Ka). Lower pKa means a stronger acid. pKa = -log Ka.
Keq (equilibrium constant)
The ratio of products to reactants at equilibrium. Keq > 1 means products are favoured; Keq < 1 means reactants are favoured.
Nucleophile
A Lewis base, an electron-rich species that donates an electron pair to form a new bond. Think of it as the "nucleus-loving" species.
Electrophile
A Lewis acid, an electron-poor species that accepts an electron pair. Think of it as the "electron-loving" species.
Brønsted model focuses on proton (H⁺) transfer. The acid donates H⁺; the base accepts H⁺.
Lewis model focuses on electron-pair transfer. The acid accepts an electron pair; the base donates one.
In organic chemistry, Lewis bases are called nucleophiles and Lewis acids are called electrophiles.
Same molecule, different arrow-pushing, different products. The direction of the curved arrows determines whether a reaction follows Brønsted or Lewis logic.
A lower pKa means a stronger acid (it gives up its proton more readily).
A higher pKa means a weaker acid (it holds onto its proton more tightly).
The conjugate base of a strong acid is a weak base, and vice versa.
Strong acid (low pKa) ≈ weak conjugate base
Weak acid (high pKa) ≈ strong conjugate base
Equilibrium always favours the side with the weaker acid and weaker base.
If the reaction arrow points toward the products and Keq > 1, the products are more stable.
If Keq < 1, the reactants are favoured.
You can calculate Keq from pKa values:
log Keq = pKa(product acid) - pKa(reactant acid)
Keq = 10^(pKa of product acid - pKa of reactant acid)
For any acid-base reaction AH + B⁻ ⇌ A⁻ + BH:
Compare the pKa of AH (reactant acid) with the pKa of BH (product acid).
Equilibrium favours the side with the higher pKa (the weaker acid).
Example: NH₃ (pKa ~35 for NH₄⁺) acting as a base with a carboxylic acid (pKa ~5). The equilibrium strongly favours proton transfer to ammonia because the product acid (NH₄⁺) has a much higher pKa.
Charged species are less stable than neutral ones.
Negative charges are best stabilised on electronegative atoms (F, O, N rather than C).
Always look for ways to spread out (delocalise) charges through resonance or induction.
Reference pKa values for common conjugate acids:
CH₄: pKa ~50 (extremely weak acid, CH₃⁻ is an extremely strong base)
NH₃: pKa ~35
H₂O: pKa ~16
HF: pKa ~5
pK_a = -\log K_a \quad \Rightarrow \quad K_a = 10^{-pK_a}\log K_{eq} = pK_a(\text{product acid}) - pK_a(\text{reactant acid})K_{eq} = 10^{(pK_a\text{ product} - pK_a\text{ reactant})} = \frac{K_a(\text{reactant acid})}{K_a(\text{product acid})}\Delta G = -RT \ln(K)where R = 8.314 × 10⁻³ kJ/mol·K and T is in Kelvin.
The free energy equation links thermodynamics to equilibrium. A negative ΔG means products are favoured (Keq > 1).
"A strong base is always a strong acid in reverse." No. A strong base has a weak conjugate acid, and a strong acid has a weak conjugate base. They are inversely related, not equivalent.
Confusing Brønsted and Lewis definitions. Brønsted is about proton transfer only. Lewis is about electron pairs. Every Brønsted acid-base reaction is also a Lewis acid-base reaction, but the reverse is not true (Lewis acid-base reactions do not always involve proton transfer).
Assuming Keq > 1 means the reaction goes to completion. Keq > 1 means products are favoured at equilibrium. The reaction is still reversible. A Keq of 10 is mildly product-favoured; a Keq of 10¹⁰ is essentially irreversible.
Forgetting that pKa refers to the acid, not the base. When asked about base strength, you still compare the pKa values of the conjugate acids. A stronger base has a conjugate acid with a higher pKa.
⚠️ You will be asked to predict the direction of acid-base equilibria using pKa values. Memorise the Keq formula from pKa.
⚠️ Know the difference between Brønsted and Lewis acid-base reactions. Expect questions showing the same molecule reacting differently depending on which arrow-pushing model applies.
⚠️ Be able to identify the conjugate acid and conjugate base in any reaction.
⚠️ Curved arrows indicate electron movement, not atom movement. Arrows go from the electron source (base/nucleophile) to the electron sink (acid/electrophile).
⚠️ The concept "nature hates charges" underpins many later topics, including why reactions proceed through certain intermediates.
True or False: A Brønsted acid donates an electron pair. (False, it donates a proton.)
Fill in the blank: A lower pKa value indicates a ______ acid. (stronger)
True or False: Keq < 1 means the products are favoured. (False, the reactants are favoured.)
Fill in the blank: The conjugate base of a strong acid is a ______ base. (weak)
True or False: Every Brønsted acid-base reaction is also a Lewis acid-base reaction. (True.)
Q: What is the difference between a Brønsted base and a Lewis base?
A: A Brønsted base accepts a proton (H⁺). A Lewis base donates an electron pair. All Brønsted bases are Lewis bases, but a Lewis base does not have to accept a proton to qualify.
Q: Given that the pKa of acetic acid is ~5 and the pKa of NH₄⁺ is ~10, predict the direction of equilibrium for the reaction CH₃COOH + NH₃ ⇌ CH₃COO⁻ + NH₄⁺.
A: log Keq = pKa(product acid) - pKa(reactant acid) = 10 - 5 = 5. Keq = 10⁵. The equilibrium strongly favours the products.
Q: Why does equilibrium favour the weaker acid and weaker base?
A: The weaker acid and weaker base are, by definition, more stable than their conjugate counterparts. Systems tend toward lower energy, so equilibrium settles on the side with the more stable (less reactive) species.
Q: A reaction has Keq = 10⁻⁵⁰. What can you say about the pKa values of the acids on each side?
A: The reactant acid has a much higher pKa than the product acid. The reaction is overwhelmingly reactant-favoured, meaning the reactant acid is far weaker than the product acid.
Q: Explain why NH₃ acts as a nucleophile when it reacts with CH₃Br.
A: NH₃ has a lone pair on nitrogen, making it an electron-pair donor (Lewis base). When it attacks the electrophilic carbon in CH₃Br, it acts as a nucleophile, forming a new N-C bond.
This material connects directly to nucleophilic substitution (SN1/SN2) and elimination reactions in later chapters, where you will need to identify nucleophiles, electrophiles, and leaving-group ability based on conjugate-base stability.
pKa values reappear constantly in carbonyl chemistry (Chapters 19-22), where enolate formation depends on the acidity of alpha-hydrogens.
The Lewis acid-base framework is the foundation for understanding catalysis, coordination chemistry, and reaction mechanisms throughout the rest of organic chemistry.
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