||Chapter 16: Ethers, Epoxides
Reactions of Epoxides
Reaction type: Nucleophilic
- Reactive nucleophiles react with
epoxides in an SN2 type of reaction.
- Reactive nucleophiles are usually
anions so the reaction conditions are essentially basic.
- Examples of such nucleophilic
systems are : RMgX, RLi, RC≡CM, LiAlH4,
- The leaving group is the oxygen
atom of the epoxide in the form of the alkoxide which is converted to the
alcohol on an acidic work-up.
- For non-symmetrical epoxides,
there is a question of regioselectivity :
The Nu attacks the more substituted
C of the epoxide at 180o to the C-O bond that breaks.
The Nu attacks the least hindered
end of the epoxide at 180o to the C-O bond that breaks.
- Experimental results show that
scenario 2 is observed by these stronger nucleophiles such as Grignard
- Recall that SN2
reactions of alkyl halides follow the order CH3- >
1o > 2o (i.e. favours the less
- Reactive nucleophiles attack
the least hindered end of the epoxide in an SN2 type
fashion at 180o to the leaving group bond.
- This results in the formation
of the more substituted alcohol.
- Under basic conditions, epoxides
open in an "SN2 like" fashion with the nucleophile attacking
the less substituted end.