Epoxide opening with a Grignard reagent: practice and mechanism
The Grignard attacks the epoxide at the less hindered carbon (SN2-like, as under basic conditions). The methyl group goes to the former CH₂ and the OH group to the other epoxide carbon.
Example
The Grignard reagent attacks the epoxide SN2-like at the less substituted carbon and opens the ring. Acidic workup gives an alcohol: the new C–C bond is at the former epoxide CH₂ and the OH group sits on the other epoxide carbon.
Mechanism step by step
- 1
Mg²⁺ coordinates to the epoxide oxygen and weakens the C–O bonds.
- 2
The methyl group attacks the less substituted carbon from the back (SN2) and the ring opens.
- 3
Acidic workup protonates the magnesium alkoxide to the alcohol.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
- Grignard reagents are strong nucleophiles and attack the less substituted carbon, not the more substituted one.
- The nucleophile is the methyl group, not water. Water only comes in during workup and just protonates the alkoxide.
Often confused with: Epoxide opening under basic conditions · Epoxide opening under acidic conditions · Grignard reaction
Frequently asked questions
How many carbons are added?
The chain grows by the two epoxide carbons plus the Grignard group. With ethylene oxide R–MgBr becomes the alcohol R–CH₂–CH₂–OH.
How does this differ from reaction with aldehydes?
With an aldehyde the OH group is on the carbon that carries the new bond. With an epoxide it is one carbon further away (β to the new C–C bond).