Replacement of hydrogen by halogen in ethers

Ethers in which hydrogen atoms are replaced by halogen can be obtained by direct action of chlorine or bromine on the unhalogenated compounds. Other methods that start from aldehydes, hemiacetals, or acetals (see pages 232 and 

240) are equally important but constitute, at least formally, a replacement of a C-O bond by a C-halogen bond. For reviews of methods of preparing halogen-ated ethers see Summers507 and Gross and Hoft.508 

Direct replacement of hydrogen in ethers by halogen is of preparative importance only in the case of chloro ethers. The chlorination of diethyl ether, which is a radical reaction,509 was first investigated a long time ago for early reviews see Lieben510 and Jacobsen.511 At room temperature the chlorine enters diethyl ether successively in the 1-, 1,2-, 1,2,2-, and 1,2,2,2-positions presumably as a result of successive removals of HC1 and additions of Cl2. The removal of HC1 is largely avoided on chlorination at —25° to —30° under irradiation, and 57% of the theoretical amount of l,l -dichlorodiethyl ether is then obtained if the starting material is dry.512 

If less chlorine is used, this method gives 1-chloroethyl ethyl ether as main product, which is more easily separated from by-products by distillation than in the case of Henry synthesis (see page 232). Similarly chlorination of tetra-hydrofuran under UV-irradiation at —30° to —40° gives 2-chloro- and 2,5-dichloro-tetrahydrofuran,513 and that of dioxan in CC14 at —5° to —10° gives 2,5-dichlorodioxan.514 However, no definite product was isolated on low-temperature chlorination of diisopropyl ether.515 

In the preparation of 1,2-dichloroethyl ether from diethyl ether516 care must be taken to cool the mixture in ice-water in the early stages and to introduce the chlorine slowly, since accumulation of chlorine easily leads to a violent reaction in which the mixture catches fire. As the reaction proceeds, the mixture becomes saturated with HC1, whereby the flammability is reduced. However, unless the stirring is rapid, the solution can become supersaturated with HC1 which is then later released with violence. Chlorination of diethyl ether (800 g), first with ice-cooling (35 h to a density of 0.785), then at temperatures rising gradually to room temperature (a further 47 h to a density of 0.96), gave 24.2% of 1,2-dichloroethyl ethyl ether. 

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