Cahours-Hofmann reaction

What is Cahours-Hofmann reaction?

Cahours and Hofmann first reported this reaction in 1857. Cahours-Hofmann reaction involves the degradation or breakdown of quaternary phosphonium hydroxides into hydrocarbons and trialkyl (or triaryl) phosphine oxides in the presence of bases like NaOH.

Cahours-Hofmann reaction
Cahours-Hofmann reaction

Quaternary phosphonium hydroxides are formed by the reaction of a base and quaternary phosphonium halides, which are derived from the interaction of alkyl halides and trialkyl (or triaryl) phosphine. Trialkyl phosphine can be easily prepared by reacting phosphorus trichloride with zinc alkyls. The reaction is believed to occur via the attack of a hydroxide ion on a transient pentacoordinate adduct that results from the combination of a phosphonium cation and a second hydroxide ion. The decomposition or degradation of quaternary phosphonium hydroxide occurs in the following order, depending on the type of alkyl or aryl groups attached to the phosphorus atom:

CH2=CH-CH2, C6H5CH2 > C6H5 > CH3 > C6H5CH2CH2 > C2H5 > C3H7

Tetrahydroxylmethyl phosphonium chloride also decomposes in aqueous base to yield trihydroxylmethyl phosphine oxide, hydrogen gas, and formaldehyde. Unlike the formation of trialkyl phosphine oxide, trialkyl phosphine sulfide can be directly produced from the reaction of trialkyl phosphine and sulfur.

References

Cahours, A. and Hofmann, A.W. (1857), Untersuchungen über die Phosphorbasen. [Studies on the phosphorus bases.] Justus Liebigs Ann. Chem., 104: 1-39. https://doi.org/10.1002/jlac.18571040102

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