Epoxide opening under acidic conditions: practice and mechanism
Under acidic conditions the epoxide oxygen is protonated first. The more substituted carbon then carries more positive charge, and methanol attacks exactly there. The OH group ends up on the less substituted carbon.
Example
The acid protonates the epoxide oxygen. In the transition state the more substituted (or benzylic) carbon carries most of the positive charge, so the weak nucleophile methanol attacks there, again from the back.
Mechanism step by step
- 1
The acid protonates the epoxide oxygen, making it a good leaving group.
- 2
The C–O bond to the more substituted carbon weakens and lengthens; a large partial positive charge builds up there (SN1 character).
- 3
Methanol attacks exactly this carbon from the back. After deprotonation the ether is on the more substituted carbon and the OH on the other, both trans.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
- That would be the 1,2-diol from opening with water. Here methanol or methoxide is the nucleophile, so one carbon gets a methyl ether and the other an OH group.
- The epoxide is opened only once: one nucleophile attacks and the epoxide oxygen becomes the OH group. Two methoxy groups do not form.
- Wrong stereochemistry: the nucleophile attacks from the back (SN2-like), opposite the epoxide oxygen. OCH₃ and OH therefore end up trans, not cis.
Often confused with: Epoxide opening under basic conditions
Frequently asked questions
Why is the regiochemistry reversed under acid?
Under acid electronics decide: the more substituted carbon carries positive charge better. Under base sterics decide and attack is at the less hindered carbon.
Does a free carbocation form?
No. Attack occurs on the protonated species from the back, so inversion (the trans product) is retained.
Where does the nucleophile attack under acidic and basic conditions?
Under acid at the more substituted carbon, which carries positive charge best; under base at the less substituted carbon (SN2, less hindered). Both give the trans product.