Brønsted acid–base
Acetic acid transfers a proton to water
How the Acetic acid transfers a proton to water works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: Acetic acid (pKa 4.76) + water. Conditions: Aqueous solution, 25 °C. Bonds formed: O–H (H₃O⁺); C=O delocalised in acetate; bonds broken: O–H of acetic acid. Stereochemistry: None (achiral species). Rate and selectivity: Equilibrium: Ka = 1.8 × 10⁻⁵, lies mostly left.
At a glance
Step 1: Base meets acid
Acetic acid's O–H is acidic; water's oxygen carries lone pairs and acts as the base. The molecules orient so a lone pair can reach the proton.
Step 2: Proton transfer
Water's lone pair attacks the acidic proton while the O–H bonding pair stays behind on oxygen, becoming the negative charge of acetate.
Step 3: Acetate and hydronium
Hydronium (H₃O⁺) and acetate (CH₃COO⁻) form. This equilibrium sits mostly to the left — acetic acid is a weak acid — but the electron-pushing logic is the same as for any proton transfer.
Abbreviations and terms
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
- pKa
- Acid-dissociation constant (–log). Lower pKa = stronger acid; used to judge which proton a base removes.
How it compares with related reactions
| Acetic acid transfers a proton to water | Acid-catalyzed amide hydrolysis | Acid-catalyzed epoxide opening with methanol | |
|---|---|---|---|
| Reagents | Acetic acid (pKa 4.76) + water | Amide, H₃O⁺ (HCl or H₂SO₄), H₂O | cat. H₂SO₄ in CH₃OH |
| Conditions | Aqueous solution, 25 °C | 6 M acid, reflux (hours) | MeOH (solvent), 25 °C |
| Stereochemistry | None (achiral species) | No stereocentre created; α-stereocentres retained | Inversion at the carbon attacked (anti opening) |
| Rate / selectivity | Equilibrium: Ka = 1.8 × 10⁻⁵, lies mostly left | Water attack / C–N cleavage rate-limiting; far slower than ester hydrolysis | Protonation first makes the ring a far better electrophile |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Acetic acid transfers a proton to water?
Acetic acid (pKa 4.76) + water. Conditions: Aqueous solution, 25 °C.
Which bonds are formed and which are broken?
Formed: O–H (H₃O⁺); C=O delocalised in acetate. Broken: O–H of acetic acid.
What is the stereochemical outcome?
None (achiral species)
What controls the rate or the selectivity?
Equilibrium: Ka = 1.8 × 10⁻⁵, lies mostly left
Study guides that help with this reaction
- SN1, SN2, E1 or E2? How to decide which mechanism wins
- Nucleophiles, electrophiles and leaving groups: what makes a reaction go