Industrial chlorinating agents for alcohols are phosgene, SOCl2, PCl3 and PCl5. Phosgene provides a cheap raw material position and usually excellent yields. However, the handling of phosgene requires enormous safety efforts. In principal the chlorination of alcohols can also be achieved by the reaction with HCl gas. Especially in the case of diols conversion is usually less than 100% and the reaction stops after formation of cyclic or open-chain ethers (Scheme 1)
Scheme 1: Chlorination of alcohols with HCl gas
Surprisingly this completely changes when the reaction is performed in an ionic liquid. Obviously either the nucleophilicity of HCl or the nucleofugicity of the leaving group (water) is drastically increased. Apperently the ether side products are cleaved allowing for a further reaction to the desired bischlorinated product. This type of chemistry is similar to what Eli Lilly has described for the cleavage of aromatic methoxy ethers in an pyridinium hydrochloride melt. Figure 1 shows how selectivities improve when the chlorination of butanediol is performed in an ionic liquid rather than in the organic solvent.
Figure 1: Higher selectivities for the product 1,4-dichlorobutane (DCB) are achieved if the chlorination of the butanediol with HCl gas is performed in an ionic liquid rather than in the pure alcohol. Side products usually are ethers (tetrahydrofurane THF, dichlorobutylether DCBE) or the monochlorinated product 1-chlorobutane-4-ol (CBO).
Butanediol is completely soluble in the ionic liquid. When the reaction proceeds a second organic phase is formed which consists of the reaction product 1,4-dichlorobutane. After complete conversion the organic phase is just separated off. Water which is formed during the reaction is distilled off from the remaining ionic liquid that can be used for the next run without any further work-up. HCl is by far the cheapest chlorinating agent. It is broadly available, can be handled in standard equipment and is consumed in the reaction. With SOCl2 only on chlorine can be used for the chlorination reaction. SO2 and HCl are formed as gaseous by-products. The heavy evolvement of gaseous by-products can be limiting the space-time-yield in large scale production. BASF has developed this phosgene-free chlorination reaction to a pilot plant scale.
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