Epoxide opening under acid
An acid protonates the epoxide oxygen, and the ring opens at the carbon better able to carry positive charge — the more substituted one.
What it needs, and what it gives
- Reaction class
- Substitution
- An alcohol or water with a strong acid. Addition is anti across the old ring.
- balanced — the co-products are written out, not dropped
- Mechanism
- Epoxide opening under acid, 3 steps
How it is thought to happen
Acid protonates the ring oxygen, which stretches the bond to the carbon that can best hold a positive charge, and the nucleophile attacks that one.
- Acid protonates the ring oxygen
- The nucleophile attacks the more substituted carbon
- A base takes the proton
Try it on your own structures
Enter your reactants in the Reaction Lab. If this transformation matches what you drew, it is applied and the outcome is shown next to the records that back it — with the count of documented reactions doing the same thing on substrates like yours, rather than a confidence score.
Other substitution transformations
Substitution by an anionic nucleophile
An anion displaces the halide from the back, turning the carbon over.
Epoxide opening under base
With no acid to open the ring first, the nucleophile has to attack it directly, so it goes to the carbon that is easier to reach — the less substituted one.
Solvolysis of a crowded halide
A crowded carbon lets its leaving group go before anything arrives. The flat intermediate is then attacked from either face, so any configuration it had is lost.
Alkylation of an amine
An amine attacks an alkyl halide and takes its carbon, becoming a more substituted amine.
Alkylation of a thiol
A thiol or its anion attacks an alkyl halide and takes its carbon, leaving a thioether.
Mitsunobu coupling
A phosphine and an azodicarboxylate turn an alcohol into a leaving group where it stands, and an acidic partner displaces it.