Mechanism of Appel Reaction

Mechanism of Appel Reaction

Appel Reaction

The reaction is named after eminent scientist Rolf Appel. Rolf Appel was an inorganic chemist who contributed his important work to the organophosphorus chemistry field. The Appel reaction is a type of organic reaction, which converts alcohol into an alkyl chloride by using chemical materials like triphenylphosphine and carbon tetrachloride. The reason behind use of carbon tetrabromide or bromine as a halide source is to produce alkyl bromides, whereas other materials like carbon tetraiodide, methyl iodide or iodine gives alkyl iodides.

This appel reaction is more or less similar to the one of the important chemicals reaction called Mitsunobu Reaction, where the combination of phosphine, a diazo compound acts as a coupling reagent, and a nucleophile is used to alter the stereochemistry of an alcohol or displace it.


The first step in appel reactions is the formation of phosphonium salt. This salt though to exist as a strong ion pair with which it prevents undergoing alpha-elimination to give dichlorocarbene. In the further step deprotonation of the alcohol, forming chloroform yields an alkoxide.

The nucleophilic dislocation of the chloride by the alkoxide results in the formation of an intermediate compound. With the help of primary and secondary alcohols, the halide can reacts in a SN2 process mechanism forming the alkyl halide product with inverted stereochemistry and triphenylphosphine oxide. Triphenylphosphine oxide which is formed as a byproduct in this reaction is resulted from the strong P=O double bond is a driving force for this reaction.

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