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The Appel reaction is an organic reaction that converts an alcohol into an alkyl chloride using triphenylphosphine and carbon tetrachloride. The use of carbon tetrabromide or bromine as a halide source will yield alkyl bromides, whereas using carbon tetraiodide, methyl iodide or iodine gives . The reaction is credited to and named after Rolf Appel, it had however been described earlier. The use of this reaction is becoming less common, due to carbon tetrachloride being restricted under the Montreal protocol.
Drawbacks to the reaction are the use of toxic halogenating agents and the coproduction of organophosphorus product that must be separated from the organic product. (1979). 9780121543501, Academic Press. ISBN 9780121543501 The phosphorus reagent can be used in catalysis quantities. The corresponding alkyl bromide can also be synthesised by addition of lithium bromide as a source of bromide ions. A more sustainable version of the Appel reaction has been reported, which uses a catalytic amount of phosphine that is regenerated with oxalyl chloride.
The driving force behind this and similar reactions is the formation of the strong PO double bond. The reaction is somewhat similar to the Mitsunobu reaction, where the combination of an organophosphine as an oxide acceptor, an azo compound as a hydrogen acceptor reagent, and a nucleophile are used to convert alcohols to esters and other applications like this.
Illustrative use of the Appel reaction is the chlorination of geraniol to geranyl chloride.
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