Comparing Acidity and Basicity in Organic Compounds

The governing principle

The factors that stabilise an anion, in typical order of importance:

1. Resonance — delocalising the charge over several atoms.
2. Induction — electron-withdrawing groups pulling charge away, weakening with distance.
3. The atom bearing the charge — more electronegative atoms hold it better, and larger atoms spread it over more volume.
4. Hybridisation — more s character holds the electron pair closer to the nucleus.
5. Solvation — better solvated anions are more stable, which can reverse gas-phase orders.

Applying it

Carboxylic acids versus alcohols

A carboxylate anion delocalises its charge over two equivalent oxygens; an alkoxide has it on one. The carboxylic acid is therefore far stronger, and resonance is the reason.

Substituted acids

Electron-withdrawing groups near the carboxyl stabilise the anion inductively, increasing acidity. The effect falls off sharply with distance, so a substituent three carbons away has little influence. Comparing a series with the substituent at different positions is a standard question, answered by the distance dependence.

Phenols versus alcohols

A phenoxide delocalises its charge into the ring, which an alkoxide cannot do. Phenols are therefore substantially more acidic, though still much weaker than carboxylic acids since the delocalisation is onto carbon rather than oxygen.

Hybridisation

A terminal alkyne is far more acidic than an alkene or alkane, because the resulting carbanion sits in an sp orbital with fifty percent s character, holding the pair closer to the nucleus.

Basicity

A base is stronger when its lone pair is more available. Anything that delocalises or withdraws that pair reduces basicity.

The amine anomaly

In the gas phase, amine basicity increases steadily with alkyl substitution, since alkyl groups donate electron density. In aqueous solution the order is irregular, because a more substituted ammonium ion has fewer N–H bonds available for hydrogen bonding to water and is therefore less well solvated.

The two effects oppose one another, and the observed order is their net result. Explaining the discrepancy between gas-phase and solution basicity is a favourite question, and the answer must name solvation explicitly.

A working procedure

1. Identify the acidic proton or the basic lone pair.
2. Draw the conjugate base, or consider the lone pair's environment.
3. Check for resonance first — it usually dominates.
4. Then induction, noting distance.
5. Then the atom and its hybridisation.
6. Consider solvation if the comparison involves solution and the other factors are close.

Frequently asked questions

Why is a carboxylic acid stronger than a phenol?

Because the carboxylate delocalises charge over two electronegative oxygens, whereas the phenoxide delocalises onto ring carbons, which hold negative charge less well.

Why is a terminal alkyne acidic?

Because the resulting carbanion occupies an sp orbital with high s character, holding the electron pair close to the nucleus and stabilising it.

Why is an aromatic amine much less basic than an aliphatic one?

Because its lone pair is delocalised into the ring and therefore less available to accept a proton.

Why does amine basicity order differ between gas and solution?

Because solvation stabilises the protonated form, and a more substituted ammonium ion has fewer N–H bonds for hydrogen bonding, so it is less well solvated. The inductive and solvation effects oppose one another.