When a base takes H⁺ from an acid, one bond forms and another breaks at the same moment. Two curly arrows show it.
Take hydroxide ions, OH⁻, the base in oven cleaner, and ethanoic acid, the acid in vinegar. The hydroxide ion's oxygen has lone pairs and a − charge, so it is the nucleophile. The hydrogen on the acid's O–H bond is δ+, so it is the electrophile.
Two arrows at once
Arrow 1 starts on a lone pair of the hydroxide ion and points to the acid's hydrogen, making a new O–H bond. A hydrogen atom's shell is full with 2 electrons, so it can hold only one bond. As the new bond forms, the old one breaks: arrow 2 starts on the acid's O–H bond and points to its oxygen, which keeps the pair.
Every arrow starts on electrons, on a lone pair or a bond. The H⁺ itself brings no electrons, so arrows point to it.
Charges follow the arrows
The hydroxide's oxygen now shares its lone pair with the hydrogen, so its − charge is gone, and it is a water molecule. The acid's oxygen keeps the whole pair from the O–H bond, so the ethanoate ion carries the − charge. The charge has moved with the electrons, and the total stays the same.
| Quantity | Before | After |
|---|---|---|
| Molecules and ions | OH⁻ + CH₃COOH | H₂O + CH₃COO⁻ |
| Charges | −1 and 0 | 0 and −1 |
| Total | −1 | −1 |