text: In presence of a base halohydrins undergo internal S2 reaction to form epoxides. Industrially, the base is calcium hydroxide, whereas in the laboratory, potassium hydroxide is often used.
  This reaction is the reverse of the formation reaction from an epoxide and can be considered a variant of the Williamson ether synthesis. Most of the world's supply of propylene oxide arises via this route.
  Such reactions can form the basis of more complicated processes, for example epoxide formation is one of the key steps in the Darzens reaction.