Epoxide opening under base: the mechanism, step by step
A base makes the nucleophile, the nucleophile attacks the less crowded end of the ring, and the strained ring springs open.
The pathway runs in 3 steps. What sits between them is a real structure: it is what the reaction passes through, and on a bad day it is what you isolate instead of the product.
Below is the sequence as it is taught. On the platform it is drawn on whatever structures you supply, with the curly arrows resolved to the actual atoms involved rather than to a general case with R groups on it.
The steps
The base makes the nucleophile
The base takes the hydrogen off the alcohol, leaving an oxygen with a negative charge that is far more eager to attack.
Deprotonation of the alcohol to the alkoxide. It is the alkoxide that opens the ring, not the alcohol.
The alkoxide attacks the less crowded carbon
The negatively charged oxygen attacks one of the two carbons of the three-membered ring. It goes for the less crowded one, and the ring opens as the bond to oxygen breaks.
Backside attack at the less substituted carbon, the strain of the three-membered ring making it a far better electrophile than an ordinary ether.
The alkoxide takes a proton
On work-up the negatively charged oxygen picks up a hydrogen and becomes an alcohol.
Protonation of the alkoxide, from the solvent or on work-up.
What this drawing assumes
- Which end is attacked is decided by how crowded it is, because nothing is pulling the ring open beforehand. Under acid the answer is the other way about, and that is a different pathway.
- The proton transfers are drawn as their own steps and are fast compared with the attack.
Other pathways this class runs by
- Under acid the oxygen is protonated first, the ring is half open before anything attacks, and the nucleophile then goes to the more substituted carbon.
Run it on your own structures
Enter the reactants and the product you expect. If the transformation is one the engine can perform and this pathway reaches that product from those structures, it is drawn on them — with the arrows on the right atoms and the intermediates you would actually pass through. If it does not reach the product, you are told that instead of being shown a drawing that does not apply.
Other mechanisms
Nucleophilic acyl substitution
The nucleophile adds to the carbonyl, the carbon holds four groups for a moment, and the leaving group is pushed out as the double bond comes back.
Bimolecular nucleophilic substitution
One step. The nucleophile comes in on the opposite side from the leaving group, and the carbon turns inside out as the exchange happens.
Imine formation
The amine adds to the carbonyl, the resulting alcohol-amine loses water, and a carbon–nitrogen double bond is left.
Diels-Alder cycloaddition
One step. Six electrons move round a ring at once, two new single bonds form at the ends and the double bond ends up in the middle.
Bimolecular elimination
One step. The base takes a hydrogen from one carbon while the leaving group departs from the next, and a double bond forms between them.
Unimolecular nucleophilic substitution
The leaving group goes first, on its own, leaving a flat carbon with a positive charge. Whatever is around then attacks it from either side.