Hunsdiecker reactions silver© oxide

Carboxylic acids (RCOOH) can be converted to norhalides (RBr or RI) by the Hunsdiecker reaction (silver salt and bromine) or by some modification of this (mercuric oxide and bromine, lead tetra-acetate and iodine, or thallium salt and bromine). 

In a modified procedure the free carboxylic acid is treated with a mixture of mercuric oxide and bromine in carbon tetrachloride the otherwise necessary purification of the silver salt is thereby avoided. This procedure has been used in the first synthesis of [1.1.1 ]propellane 10. Bicyclo[l.l.l]pentane-l,3-dicarboxylic acid 8 has been converted to the dibromide 9 by the modified Hunsdiecker reaction. Treatment of 9 with t-butyllithium then resulted in a debromination and formation of the central carbon-carbon bond thus generating the propellane 10." ... 

When iodine is the reagent, the ratio between the reactants is very important and determines the products. A 1 1 ratio of salt to iodine gives the alkyl halide, as above. A 2 1 ratio, however, gives the ester RCOOR. This is called the Simonini reaction and is sometimes used to prepare carboxylic esters. The Simonini reaction can also be carried out with lead salts of acids." A more convenient way to perform the Hunsdiecker reaction is by use of a mixture of the acid and mercuric oxide instead of the salt, since the silver salt must be very pure and dry and such pure silver salts are often not easy to prepare. 

Mercury(II) oxide together with a halogen is an early development of the classic Hunsdiecker reaction (bromodecarboxylation of a carboxylic acid silver salt, see below) which is still in use.20 22 A double Hunsdiecker reaction of cyclobutane-1,1-dicarboxylic acid with red mer-cury(ll) oxide in the presence of bromine gave 1,1-dibromocyclobutane (2) in 46% yield.21 However, a similar reaction performed on spiro[3.3]heptane-2-carboxylic acid afforded 2-bro-mospiro[3.3]heptane (3) in only 16% yield.22... 

Silver(I) carboxylates have been obtained by the addition of equivalent amounts of freshly prepared silver oxide to aqueous solutions of the appropriate acid.247 Their degradation by halogens provides a convenient method for the preparation of alkyl halides (Hunsdiecker reaction) or esters (Simonini reaction).248 Equations (14)-(18) have been proposed to account for the products obtained and were the result of extensive studies. 

The Hunsdiecker reaction has certain disadvantages, mainly because it requires use of the pure dry silver salt, which is often difficult to prepare. With some acids, however, excellent results can be obtained using the acid itself and an excess of red mercuric oxide in place of the silver salt,... 

The classical Hunsdiecker reaction (equation 18), involving the reaction of silver caiboxylates widi halogens, and the various associated side reactions, has been reviewed several tunes. Optimum yields are obtain widi bromine, followed by chlorine. Iodine gives acceptable yields provid diat the correct stoichiometry of 1 1 is used. The reaction is most frequently carried out in tetrachloromediane at reflux. From a practical pmnt of view, one drawback is the difficulty encountered in the preparation of dry silver caiboxylates the reaction of silver oxide on the acyl chloride in tetrachloromediane at reflux has been employed to circumvent diis problem. Evidendy the use of molecular bromine limits die range of functional groups compatible widi die reaction the different reaction pathways followed by the silver salts of electron poor (equation 19) and electron rich (equation 20) aryl carboxyl s illustrate this point well. 

Silver-mediated C—X bond formation was known for many years as the Hunsdiecker reaction, in which substrates bearing carboxylic acids are oxidatively decarboxylated to give alkyl halides in the presence of halogens (142). If olefins are used with silver benzoate and I2, the Prevost reaction occurs to yield diols (143). 

In an earlier chapter, we saw how to reduce the carbon chain by one methylene unit using the Hunsdiecker reaction. The opposite of this, namely the extension of the carbon chain by one unit, may be achieved with the Amdt-Eistert synthesis. In the first step, an acyl halide is treated with diazomethane to form the a-diazo ketone. This is then treated with water and silver oxide. The resultant product is the free acid. If an alcohol is used instead of water, then the related ester is formed. Suggest what is the pathway for this reaction. 

The degradation of carboxylic acids to alkyl halides using mercuric oxide and halogens involves the initial formation of the mercuric salt of the acid, followed by a normal Hunsdiecker reaction of the salt with halogen. The relative insensitivity of the reaction to water is a consequence of the solubility of the mercury salts in the solvent (CC14). There are two limitations tertiary acids are not degraded, and use of iodine as the halogen frequently leads to the ester RCOOR as the major product. The yields in the modified reaction are usually lower than those obtained with the silver salt method.2... 

Hunsdiecker reaction. In the original Hunsdiecker reaction, the silver salt of an acid is prepared in one step and treated with bromine in another. Cristol found that isolation of a metal salt can be dispensed with addition of bromine to a refluxing snspension of red mercuric oxide in a solution of the acid in carbon tetrachloride affords the corresponding alkyl halide in good yield ... 

In the Hunsdiecker reaction, the silver salt of a carboxylic acid, prepared by treating the acid with silver oxide, is treated with a halogen. Bromine is the usual reagent, but iodine may also be used. Carbon dioxide is evolved and the corresponding alkyl halide is obtained, usually in fair to good yield. 

Hunsdiecker reaction Decomposition of a carboxylic acid hy oxidation of its silver. salt. 

If the silver salt of a carboxylic acid is treated with bromine, the salt decarboxylates and a bromoal-kane forms. This process is called the Hunsdiecker reaction. The decarboxylation reaction can also be carried out using the carboxylic acid and mercury(II) oxide. 

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