PREPARATION OF ETHERS, EPOXIDES, AND THIOETHERS
Ethers can be prepared by the SN2 reaction of an alkyl halide with an alkoxide ion. The reaction works best for primary alkyl halides. Alcohols and alkyl halides can be reacted in the presence of silver oxide to give an ether. Alkenes can be treated with alcohols in the presence of mercuric trifluoroacetate to form ethers by electrophilic addition.
Epoxides can be synthesized from alkenes and meta-chloroperbenzoic acid, or by converting the alkene to a halohydrin and treating the product with base to induce an intramolecular SN2 reaction which displaces the halogen atom. Aldehydes and ketones can be converted to epoxides on treatment with a sulfur ylide.
Thioethers are prepared by the SN2 reaction between an alkyl halide and a thiolate ion. Symmetrical thioethers can be prepared by treating the alkyl halide with KOH and hydrogen sulfide where the latter is not in excess.
The Williamson ether synthesis is the best method of preparing ethers (Fig. 1a).
The procedure involves the SN2 reaction between a metal alkoxide and a primary alkyl halide or tosylate. The alkoxide required for the reaction is prepared by treating an alcohol with a strong base such as sodium hydride. An alternative procedure is to treat the alcohol directly with the alkyl halide in the presence of silver oxide, thus avoiding the need to prepare the alkoxide beforehand (Fig. 1b).
If an unsymmetrical ether is being synthesized, the most hindered alkoxide should be reacted with the simplest alkyl halide, rather than the other way roundZ (Fig. 2). Since this is an SN2 reaction, primary alkyl halides react better than secondary or tertiary alkyl halides.
Alkenes can be converted to ethers by the electrophilic addition of mercuric trifluoroacetate, followed by addition of an alcohol. An organomercuric inter-mediate is obtained which can be reduced with sodium borohydride to give the ether.
Epoxides can be synthesized by treating aldehydes or ketones with sulfur ylides. They can also be prepared from alkenes by reaction with m- chloroperoxybenzoic acid . Alternatively they can be obtained from alkenes in a two-step process (Fig. 4). The first step involves electrophilic addition of a halogen in aqueous solution to form a halohydrin. Treatment of the halohydrin with base then ionizes the alcohol group, which can then act as a nucleophile (Fig. 5). The oxygen uses a lone pair of electrons to form a bond to the neighboring electrophilic carbon, thus displacing the halogen by an intramolecular SN2 reaction.
Thioethers (or sulfides) are prepared by the SN2 reaction of primary or secondary alkyl halides with a thiolate anion (RS-),. The reaction is similar to the Williamson ether synthesis.
Symmetrical thioethers can be prepared by treating an alkyl halide with KOH and an equivalent of hydrogen sulfide. The reaction produces a thiol which is ionized again by KOH and reacts with another molecule of alkyl halide.