Ch16: HO-C-C-X => epoxide

Chapter 16: Ethers, Epoxides and Sulfides

Using Halohydrins to Synthesise Epoxides
(review of Chapter 6 and 8)

preparation of a 1,2-halohydrin

intramolecular S<sub>N</sub>2 to give an epoxide

Reaction type: Electrophilic Addition then Nucleophilic Substitution

Summary

Overall transformation :C=C to an epoxide
Halohydrins can be formed by the addition reaction X₂ / H₂O or HOX to alkenes.
In the presence of a base, ring closure occurs via an intramolecular S_N2 reaction.
Suitable base could be hydroxide, HO^- or maybe Na metal.

QUESTIONS

	What named reaction is the ring closure step most like ?	ANSWER
	If the ring closure is an S_N2 reaction then what arrangement of the ^-O-C-C-X system is required in the reactive conformation ?	ANSWER
	What is the stereochemistry of the halohydrin formation ?	ANSWER

Related reactions

MECHANISM OF HALOHYDRIN TO EPOXIDE
Step 1: An acid/base reaction. The base deprotonates the alcohol forming an alkoxide intermediate that has enhanced nucleophilicity.
Step 2: An intramolecular S_N2 reaction where the alkoxide nucleophile attacks the electrophilic C displacing the leaving group, the halide ion. The nucleophile has to attack anti to the C-X bond.

© Dr. Ian Hunt, Department of Chemistry