Formation of epoxides from bromohydrins
Description: Halohydrins treated with strong base (such as hydroxide ion) can form epoxides through an intramolecular SN2 reaction.
Notes: The purpose of the base is to make the alcohol a better nucleophile. Many other bases besides NaOH could be used here, such as KOH or NaH.
Notes: Note how the final epoxide product depends on the stereochemistry of the starting halide in examples 1 and 2. In example 3 the epoxide can only form if the hydroxyl and halide are trans to each other. note how in example 4 they are on the same side of the ring, and thus cannot form an epoxide. The last example shows both formation of the chlorohydrin and subsequent epoxide formation.
Mechanism: This reaction is simply an intramolecular SN2 reaction. As with all SN2 reactions it proceeds with inversion of configuration.
The first step is deprotonation of the alcohol to give the alkoxide (Step 1, arrows A and B). This is an excellent nucleophile, and reacts with the adjacent alkyl halide to give the epoxide (Step 2, arrows C and D).
- Note that the configuration at the alkyl bromide is inverted (but not the alcohol).
- Also note that the hydroxide ion preferentially deprotonates the alcohol rather than performs a SN2 on the alkyl halide