Hunsdieckertyyppiset

Hunsdieckertyyppiset, or Hunsdiecker-type reactions, form a class of decarboxylative halogenations of carboxylic acids. In these reactions a metal carboxylate, commonly silver carboxylate, reacts with a halogen reagent to furnish an alkyl halide with loss of one carbon as carbon dioxide. The method provides a convenient route from carboxylic acids to primary or benzylic alkyl halides.

The proposed mechanism involves formation of an acyl hypohalite intermediate from the carboxylate and halogen, followed by fragmentation to give a carbon-centered radical that captures a halogen atom to form R–X while carbon dioxide is evolved. The reaction is typically radical-chain like, and conditions often favor formation of primary or benzylic halides over more hindered analogs. Variants may use different halogens or carboxylate salts and can alter the outcome in terms of yield and selectivity.

General stoichiometry is commonly represented as 2 RCOOAg + X2 → 2 R–X + 2 CO2 + 2 AgX, though practical descriptions often state RCOOAg + X2 → R–X + CO2 + AgX. The exact balance can depend on the chosen reagents and workup. The reaction is most reliable for less hindered substrates and is widely applied to obtain alkyl halides from carboxylic acids that are difficult to convert by other routes.

Scope and limitations include good yields for primary and benzylic carboxylates; secondary and tertiary substrates may suffer from side reactions. Bromine and iodine are commonly used; chloride is less efficient. Solvent choice and temperature influence success, and the use of silver salts raises cost and waste considerations.

Historically, the reaction was first reported in the 1940s by German chemist Alexander Hunsdiecker. The term Hunsdiecker-type also covers related decarboxylative halogenations and modern refinements aimed at expanding substrate scope and improving safety.