Hofmann-Sand reaction

What is Hofmann-Sand reaction?

The reaction between mercuric salts (such as acetates, halides, nitrates, or sulfates) and compounds containing a carbon-carbon double bond in aqueous or alcoholic solutions was first reported by Hofmann and Sand in 1900. Hofmann-Sand reaction can result in the formation of addition products or coordination complexes, or both. The solvents used can also participate in the reaction, causing hydrolysis or alcoholysis to produce products containing hydroxyl or alkoxyl groups.

Hofmann-Sand reaction - general reaction scheme
Hofmann-Sand reaction
Hofmann-Sand reaction - general reaction scheme
Hofmann-Sand reaction

The compounds formed in this reaction can decompose to regenerate the original olefins in the presence of hydrochloric acid, typically in only 5 % concentration, except for the product of dibenzoyl derivative and mercurated 1,2-dihydrobenzofurans. For example, 1-chloromercurimethyl-1,2-dihydrobenzofuran is stable in boiling acetic acid and does not decompose after standing in 15 % hydrochloric acid or 50 % sulfuric acid for several days at ordinary temperatures. The overall outcome of the reaction depends on several factors, such as the structure of the olefin, the acidity of the solution, the anion in the mercuric salt, and the temperature and concentration of the solution.

Allyl alcohol typically produces an internal ether product in this reaction. The trans-olefin usually reacts more slowly than the cis-isomers, and unsymmetrical olefins give products with the mercury atom attaching to the carbon of fewer substituents, and the existing negative group may even hinder or prevent the reaction.

Mercuric nitrate and acetate are the best salts for general use, and the reaction usually completes when the solution of the reactants no longer gives a precipitate of mercuric oxide with caustic alkali. The Hofmann-Sand reaction occurs most readily in methanol solution, where even trans-olefins with negative groups attached would react.

The mercuric salt-olefin adducts are crystalline substances with relatively high thermal stability, and both cis- and trans-1-methoxy-2-(chlormercuri)-cyclohexanes can be distilled under vacuum without decomposition. The mercurate salt-olefin adducts are typical organic compounds that can further react with a variety of compounds.

Some of the reactions that the mercurate salt-olefin adducts undergo include conversion into iodohydrin by treatment with iodine in KI solution, reduction to alcohol by sodium amalgam, conversion into ether compounds in 5 % H2SO4, regeneration of the olefin in aqueous solution of cyanides and thiocyanates, conversion to sulfide from hydrogen sulfide, reaction with alkyl halide to form alkyl alcohol along with the evolution of the original olefin, liberation of the olefin when reacting with Grignard reagent along with the isomerization of cis-olefin to trans-olefin, and formation of aldehyde when reacting with diazonium salt.

The utility of Hofmann-Sand reaction extends to the determination of olefinic configuration and the conversion of cis-olefins to trans-olefins.

References

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