Difficulty: Intermediate | Prerequisites: Chapter 2 kinetics/thermodynamics basics, general chemistry acid-base concepts, periodic table trends
Acid-base chemistry is arguably the single most important framework in organic chemistry. Nearly every reaction mechanism you will encounter involves a proton transfer, a nucleophile donating electrons, or a leaving group departing with electrons. This section teaches you to predict which way an acid-base equilibrium lies by evaluating the stability of the conjugate base. The five stability factors you learn here (electronegativity, size, resonance, hybridisation, induction) will be reused constantly, from predicting pKa values to rationalising why one reaction pathway dominates over another.
A Brønsted acid donates a proton; a Brønsted base accepts one. Equilibrium favours the side with the weaker acid and weaker base (the more stable side). When you do not know pKa values, you evaluate the stability of the conjugate base using five factors: electronegativity, size, resonance, hybridisation, and induction.
Brønsted acid
A proton (H⁺) donor. In a Brønsted acid-base reaction, the acid loses H⁺ to the base. In simple terms, any molecule that gives up a hydrogen ion.
Brønsted base
A proton (H⁺) acceptor. The base uses a lone pair or a bond to grab the proton from the acid. In simple terms, any molecule that takes a hydrogen ion.
Conjugate acid (CA)
The species formed when a base gains a proton.
Conjugate base (CB)
The species formed when an acid loses a proton.
Ka (acid dissociation constant)
A quantitative measure of how strong an acid is. Larger Ka = stronger acid.
pKa
The negative base-10 logarithm of Ka: pKa = −log[Ka]. Smaller pKa = stronger acid. This is the scale you will use most often. Think of it as a convenience: a pKa of 4 is a stronger acid than a pKa of 10.
Equilibrium arrows
Brønsted acid-base reactions use equilibrium arrows (⇌), not single-direction arrows. The equilibrium favours the side with the higher pKa (weaker acid, more stable species).
Curved arrows (electron flow)
Show the movement of electron pairs, drawn from the electron source to the electron sink. In acid-base chemistry, the arrow goes from the base's lone pair to the proton. A common mistake: "magical ABC," where students draw arrows that do not start or end at the correct atoms.
Spectator ion
An ion present in the reaction mixture that does not participate in the acid-base exchange (e.g., Na⁺ in a reaction with NaOH).
Curved arrows show electron-pair flow, always from base to acid
Arrows must be specific: start on a lone pair or bond, end on the atom receiving the electrons
Equilibrium arrows (⇌) are always used; the equilibrium favours the side with the weaker acid (higher pKa)
The side with the higher pKa is more stable overall
Functional groups have approximate pKa values you can look up
Compare the pKa of the acid on the left with the pKa of the conjugate acid on the right
Equilibrium favours the side with the higher pKa
Example: HCl (pKa ≈ −7) + H₂O → H₃O⁺ (pKa ≈ −1.7) + Cl⁻. Equilibrium favours products because H₃O⁺ has a higher pKa than HCl.
Strong acids: HCl, HBr, HI, H₂SO₄, H₃PO₄, HNO₃
Strong bases: NaOH, KOH, LiOH, NaOCH₃, lone pairs of electrons (e.g., on nitrogen or oxygen)
When you do not have pKa values, predict stability by working through these factors. They are listed in a general order of importance, but always start by comparing the charge.
1. Electronegativity
A negative charge is more stable on a more electronegative atom
Example: an alkoxide (charge on O) is more stable than a carbanion (charge on C), because oxygen is more electronegative
2. Size (polarisability)
For atoms in the same column of the periodic table, a larger atom stabilises a negative charge better because the charge is spread over a larger volume
Example: I⁻ is more stable than F⁻ (both halogens, but iodine is much larger)
This factor matters when comparing atoms in the same group; electronegativity dominates when comparing atoms across a row
3. Resonance (charge delocalisation)
The more resonance structures that delocalise the charge, the more stable the anion
Example: carboxylate anion (two equivalent resonance structures delocalising the negative charge over two oxygens) is far more stable than an alkoxide (charge localised on one oxygen)
When comparing molecules, count meaningful resonance forms: more delocalisation = more stable conjugate base = stronger acid
4. Hybridisation
More s-character in the orbital holding the lone pair = closer to the nucleus = more stable negative charge
sp (50% s) > sp² (33% s) > sp³ (25% s) in ability to stabilise a negative charge
pKa values: sp³ C–H ≈ 55, sp² C–H ≈ 45, sp C–H ≈ 25
Example: acetylene (sp, pKa ≈ 25) is a much stronger acid than ethane (sp³, pKa ≈ 55)
5. Induction
Electron-withdrawing groups (e.g., fluorine, CF₃) stabilise a nearby negative charge through the sigma-bond framework by pulling electron density toward themselves
Proximity matters: the closer the withdrawing group, the stronger the stabilising effect
C–H bonds are non-polar and do not contribute inductive stabilisation
When multiple factors compete, the general priority is:
Resonance and electronegativity > size > hybridisation > induction
(Though the notes stress: always compare charge stability first.)
Always compare stability of the charge first. Sometimes positive charges need to be compared, not just negative ones. A positive charge is more stable on a less electronegative atom (e.g., ⁺NH₃ is more stable on N than ⁺OH₂ on O, because nitrogen is less electronegative).
Neutral species can be more stable than charged ones. When charges appear on the same side of the equilibrium, the neutral molecule is generally more stable. This can override the five-factor analysis.
Strong acids protonate everything. If one reactant is a strong acid (e.g., H₂SO₄), it will protonate any base present, driving the equilibrium strongly to the product side regardless of subtle stability arguments.
pKa definition:
pKa = −log[Ka]
Equilibrium direction shortcut:
Compare pKa(acid) with pKa(conjugate acid). Equilibrium favours the side with the larger pKa.
General acid-base scheme:
H–A + :B ⇌ H–B⁺ + A⁻ (Acid + Base ⇌ Conjugate Acid + Conjugate Base)
Understanding pKa is how medicinal chemists decide whether a drug molecule will be protonated or deprotonated at physiological pH, which affects whether it can cross a cell membrane. The stability factors are the same ones used to design catalysts, choose solvents, and predict selectivity in industrial synthesis.
Students often think the equilibrium always favours "more products." In acid-base chemistry, the equilibrium favours the more stable (weaker) acid and base pair, which can be on either side of the arrow.
Students frequently draw curved arrows from acid to base. The arrow always starts at the electron source (base), not the proton donor.
Students sometimes assume a bigger pKa means a stronger acid. It is the opposite: smaller pKa = stronger acid.
Students forget that size beats electronegativity when comparing atoms within the same column of the periodic table (e.g., I⁻ vs. F⁻).
⚠️ You will almost certainly be asked to predict the direction of an acid-base equilibrium using pKa values.
⚠️ Expect questions where you are given two molecules and asked "which is the stronger acid?" without pKa values, requiring you to use the five stability factors.
⚠️ Curved-arrow drawing is tested directly. Arrows must start on a lone pair or bond (on the base) and end on the proton or the atom receiving the electrons.
⚠️ Know the common strong acids and bases from memory.
⚠️ "Most acidic hydrogen" questions: identify which H, when removed, produces the most stable conjugate base.
True or false: A pKa of 5 indicates a stronger acid than a pKa of 15.
Fill in the blank: In a Brønsted acid-base reaction, curved arrows are drawn from the ________ to the ________.
True or false: An sp-hybridised carbon holds a negative charge more stably than an sp³-hybridised carbon.
Fill in the blank: The five stability factors in general priority order are electronegativity, ________, resonance, hybridisation, and ________.
True or false: Equilibrium favours the side with the lower pKa.
Answers: 1. True. 2. Base; acid (or: electron source; proton). 3. True (50% s-character vs. 25%). 4. Size; induction. 5. False (equilibrium favours the side with the higher pKa, i.e., the weaker acid).
Q: HBr reacts with water. Which side does the equilibrium favour, given that HBr has a pKa of about −9 and H₃O⁺ has a pKa of about −1.7?
A: Equilibrium favours the products (H₃O⁺ + Br⁻), because the conjugate acid (H₃O⁺, pKa ≈ −1.7) is weaker than HBr (pKa ≈ −9). The side with the higher pKa is favoured.
Q: Without looking up pKa values, predict whether an O–H bond or an S–H bond is more acidic. Explain using stability factors.
A: S–H is more acidic. Sulphur and oxygen are in the same column of the periodic table, but sulphur is larger. The resulting S⁻ anion spreads its charge over a larger volume, making it more stable. Size dominates over electronegativity when comparing within the same group.
Q: Rank the following in order of increasing acidity: CH₄, HF, H₂O.
A: CH₄ < H₂O < HF. Across a row, electronegativity increases from C to O to F, stabilising the conjugate base more effectively.
Q: Acetic acid (CH₃COOH) is a stronger acid than ethanol (CH₃CH₂OH). Explain why using stability factors.
A: The conjugate base of acetic acid (acetate, CH₃COO⁻) delocalises its negative charge across two oxygen atoms via resonance. The conjugate base of ethanol (ethoxide, CH₃CH₂O⁻) has the charge localised on one oxygen. More resonance delocalisation = more stable conjugate base = stronger acid.
Q: A positive charge sits on nitrogen in one molecule and on oxygen in another. Which is more stable, and why?
A: The positive charge on nitrogen is more stable. Nitrogen is less electronegative than oxygen, so it is better able to bear a positive (electron-deficient) charge.
The five stability factors return in every chapter that discusses carbocation stability, leaving-group ability, or nucleophilicity. Acid-base reactions are the first step (or the last step) of most organic mechanisms, so fluency here pays off in substitution (SN1/SN2), elimination (E1/E2), and addition reactions. Resonance and induction also reappear when discussing directing effects in electrophilic aromatic substitution.
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