Appel reaction

The Appel reaction is an organic reaction that converts an alcohol into an alkyl chloride using triphenylphosphine and carbon tetrachloride ^[1].

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The Appel reaction is a mild method of halide introduction, and is successful on primary, secondary, and most tertiary alcohols. The use of carbon tetrabromide or bromine as a halide source will yield alkyl bromides, while using methyl iodide or iodine will give alkyl iodides.

Reaction mechanism

The initial step of the Appel reaction is the formation of the phosphonium salt pair 2. Deprotonation of the alcohol, forming chloroform 3, yields an alkoxide ion pair 4. The nucleophilic displacement of the chloride by the alkoxide yields intermediate 5. With primary and secondary alcohols, the chloride anion reacts in a SN2 process forming the desired alkyl chloride 6 and triphenylphosphine oxide 7. Tertiary alcohols form the products 6 and 7 via a SN1 mechanism.

The driving force behind this and similar reactions is provided by the formation of solid triphenylphosphine oxide, which phase separates from the reaction mixture.

The yields are almost quantitative.

An example is the chlorination of geraniol to geranyl chloride ^[2].

References

1. ^  R. Appel, Angew. Chem. 1975, 87(24), 863-874.
2. ^  Jose G. Calzada and John Hooz Organic Syntheses, Coll. Vol. 6, p.634 (1988); Vol. 54, p.63 (1974) Article.

See also

• Mitsunobu reaction
• Organophosphorus chemistry