Difficulty: Introductory | Prerequisites: General chemistry (Lewis structures, electronegativity, periodic trends)
This material underpins every reaction mechanism you will meet in organic chemistry. Acidity and basicity determine which way a reaction runs and which products form; resonance explains why certain molecules are more stable (or more reactive) than others. If you can rank acids by pKa, draw resonance structures without violating the octet rule, and predict which side of an equilibrium is favoured, you have the conceptual toolkit for the rest of the course. Students who skipped gen-chem Lewis structures should review those first.
Acid strength depends on bond length, electronegativity, charge, and conjugate-base stability (especially resonance). Reactions favour the side that forms the weaker acid/base pair, so compare pKa values to predict equilibrium direction. Resonance structures show how electrons delocalise across a molecule; draw them by moving lone pairs or pi bonds, never violating the octet rule.
Bronsted-Lowry acid
A proton (H⁺) donor. In simple terms, it is any species that gives up a hydrogen ion to another molecule.
Bronsted-Lowry base
A proton (H⁺) acceptor, or equivalently, an electron-pair donor. Think of it as the molecule that grabs the proton.
Lewis acid
An electron-pair acceptor. In simple terms, it is anything with an empty or partially empty orbital that can receive electrons.
Lewis base
An electron-pair donor. Think of it as any species with a lone pair available to share.
pKa
The negative log of the acid dissociation constant. A lower pKa means a stronger acid; a higher pKa means a weaker acid. Think of it as a ranking number: the smaller the number, the more readily the molecule gives up its proton.
Conjugate base
The species that remains after an acid donates a proton. In simple terms, it is what is left over once the H⁺ leaves.
Conjugate acid
The species formed when a base accepts a proton.
Adduct
The product formed when a Lewis acid and Lewis base combine without losing any atoms. It forms whenever the octet rule is not violated. Atoms that can expand their octet (P, S, Al, B) readily form adducts.
Resonance structures
Two or more Lewis structures for the same molecule that differ only in the placement of electrons (lone pairs and pi bonds). The real molecule is a weighted average (hybrid) of all valid resonance structures.
Resonance stabilisation
The extra stability a molecule gains because its electrons are delocalised across multiple atoms rather than confined to one. Think of it as spreading charge out so no single atom bears the full burden.
Bond length. A larger ionic radius stretches the bond, making it weaker and easier to break. This is why HI is a stronger acid than HF across the hydrogen halides.
Positive charge on an electronegative atom. Placing a positive formal charge on an electronegative atom makes the attached H far more acidic.
Electronegativity of the atom bearing H. The more electronegative the atom, the more it stabilises the negative charge left behind when H⁺ leaves.
Resonance stabilisation of the conjugate base. If the conjugate base can spread its charge across multiple atoms via resonance, the acid is stronger. Phenol (pKa ~10) is more acidic than ethanol (pKa 17) because phenoxide delocalises its charge into the ring.
Functional Group | Approximate pKa |
|---|---|
SO₃H (sulfonic acid) | -1 |
COOH (carboxylic acid) | 4 |
H₂O (water) | 14 |
Alcohol (R-OH) | 17 |
Alkyne (terminal C-H) | 25 |
NH₃ (ammonia) | 35 |
Alkene (C=C-H) | 43 |
Alkane (C-H) | 50 |
Memorising this ladder lets you predict the direction of any proton-transfer reaction at a glance: equilibrium favours the side with the higher pKa acid (the weaker acid).
Identify the acid and the base. The acid donates H⁺; the base accepts it (or donates electrons).
Ask whether an adduct will form. If neither atom violates the octet rule, a Lewis acid-base adduct forms directly. Watch for atoms that expand their octet: P, S, Al, B.
Compare pKa values. The product side with the larger pKa value of its acid component is thermodynamically favoured. The reaction always runs toward the weaker acid/base pair.
Handle salts carefully. If a reagent is a salt (e.g. NaOH, NaNH₂), dissociate it into its ions before analysing. The anion is typically the reactive base.
Curved arrows move electrons from a source (lone pair or pi bond) to a destination. An arrow cannot originate from and terminate at the same atom.
Electrons always move from a lone pair or a pi bond, never from a sigma bond.
The octet rule cannot be violated for second-row elements (C, N, O, F). Third-row elements (P, S) may expand their octet.
Hybridisation can change between resonance structures. If a lone pair participates in resonance, that atom uses an unhybridised p orbital to hold the lone pair and is effectively sp² (not sp³).
Oxygen and nitrogen may carry a positive formal charge in a resonance structure, but only if they still have a full octet (four bonds for N⁺, three bonds plus a lone pair for O⁺).
A C=O double bond can become C-O⁻ when electrons are donated to oxygen. The reverse (making C=O⁺) does not happen.
Negative charge or lone pair present. Try to withdraw electron density by converting a lone pair into a new pi bond (push electrons toward an adjacent atom).
Positive charge or empty p orbital present. Try to add electron density by moving a neighbouring pi bond toward the positive centre, or by donating a lone pair from an adjacent atom.
Check the octet. After drawing each new structure, confirm every second-row atom still has eight electrons (or fewer for atoms like carbocations that are allowed to be electron-deficient).
Students often assume a more electronegative atom always makes a stronger acid. Electronegativity matters when comparing atoms in the same row of the periodic table, but across rows, bond length (atomic size) dominates. HI is stronger than HF despite iodine being less electronegative.
Students frequently confuse "resonance stabilised conjugate base" with "stronger base." A resonance-stabilised conjugate base is actually a weaker base, which is why the parent acid is stronger. Phenol is more acidic than ethanol because phenoxide is the weaker base (more stable).
Many students try to move sigma-bond electrons when drawing resonance structures. Only lone pairs and pi bonds move in resonance; sigma bonds stay put.
A common error is drawing a resonance structure where a second-row atom (C, N, O) has more than eight electrons. If your structure gives carbon five bonds, it is wrong.
⚠️ You will almost certainly be asked to predict the direction of an acid-base equilibrium. The method is always: identify acid on each side, compare pKa values, the side with the larger pKa wins.
⚠️ Resonance structure questions are common. Expect at least one problem asking you to draw all valid resonance structures for a given ion or molecule. The grading penalty for octet violations is steep.
⚠️ Questions comparing the acidity of two similar compounds (e.g. phenol vs ethanol, or two substituted carboxylic acids) test whether you can apply the four acidity factors in context.
⚠️ If a reagent is given as a salt (NaOCH₃, KOH), dissociate it first. Forgetting to do so leads to incorrect identification of the reactive species.
True or false: HF is a stronger acid than HI. (False. HI has a longer, weaker bond.)
Fill in the blank: Equilibrium in a proton-transfer reaction favours formation of the ______ acid. (weaker)
True or false: A resonance arrow can move electrons from a sigma bond. (False. Only lone pairs and pi bonds.)
Fill in the blank: If the pKa of acid A is 4 and the pKa of acid B is 17, the equilibrium favours the side containing acid ______. (B, the weaker acid)
True or false: Oxygen can carry a positive formal charge in a valid resonance structure. (True, provided it retains a full octet.)
Q: Rank the following in order of increasing acidity: ethanol, acetic acid, water, ethane.
A: Ethane (pKa ~50) < ethanol (pKa 17) < water (pKa 14) < acetic acid (pKa 4). More acidic means lower pKa.
Q: Phenol is significantly more acidic than cyclohexanol. Explain why.
A: The conjugate base of phenol (phenoxide) is resonance-stabilised: the negative charge delocalises into the aromatic ring across several carbon atoms. Cyclohexanol's conjugate base has no such resonance stabilisation, so it is a stronger (less stable) base, making cyclohexanol a weaker acid.
Q: Draw two valid resonance structures for the acetate ion (CH₃COO⁻) and explain why both oxygen atoms share the negative charge.
A: In one structure the double bond is between C and the left O, with a lone pair and negative charge on the right O. In the other, the double bond and charge swap. The real ion is a hybrid: each oxygen carries half the negative charge, which is why both C-O bonds are the same length.
Q: A reaction mixes NH₃ (pKa 35) with acetic acid (pKa 4). Which direction does equilibrium favour, and why?
A: Equilibrium favours product formation (ammonium ion + acetate). The product side contains the weaker acid (NH₄⁺, pKa ~9) compared with acetic acid (pKa 4). Since equilibrium favours the side with the larger pKa acid, products are heavily favoured.
Q: Why does a lone pair that participates in resonance sit in an unhybridised p orbital rather than a hybridised orbital?
A: For the lone pair to overlap with adjacent p orbitals and participate in pi delocalisation, it must be in a p orbital that is parallel to the rest of the pi system. A hybridised orbital points in the wrong direction and cannot overlap sideways.
Acid-base reasoning feeds directly into Substitution and Elimination (Exam 3): the strength of the nucleophile or base determines whether SN2/E2 or SN1/E1 pathways dominate, and pKa tells you whether a species is more likely to act as a nucleophile or as a base.
Resonance stabilisation reappears in Aromaticity: the 4n+2 rule for aromatic stability is an extension of the same electron-delocalisation logic you learn here. It also governs the stability of allylic and benzylic carbocations and radicals.
In spectroscopy, equivalent vs non-equivalent protons in NMR depend partly on the symmetry resonance imposes on a molecule.
Related Terms / Search Tags
pKa, acid dissociation constant, Bronsted acid, Bronsted base, Lewis acid, Lewis base, conjugate acid, conjugate base, proton transfer, resonance structures, resonance hybrid, delocalisation, electron-pair donor, electron-pair acceptor, adduct, octet rule, expanded octet, formal charge, hydrogen halide acidity, phenol vs ethanol acidity, resonance stabilisation, curved arrow notation