Hunsdiecker reaction mechanism

The Hunsdiecker reaction is a free-radical reaction for the synthesis of an alkyl halide. The starting material comes from the reaction of a silver carboxylate with a solution of a halogen in a solvent such as carbon tetrachloride (see Figure 12-44). The overall free-radical mechanism is shown in Figure 12-45. 

Problem 16.20 Suggest a typical free-radical mechanism for the Hunsdiecker reaction which requires the initial formation of an acyl hypobromite. RC—OBr, from the Ag salt and Br,. ... 

The first step is not a free-radical process, and its actual mechanism is not known.451 25 is an acyl hypohalite and is presumed to be an intermediate, though it has never been isolated from the reaction mixture. Among the evidence for the mechanism is that optical activity at R is lost (except when a neighboring bromine atom is present, see p. 682) if R is neopentyl, there is no rearrangement, which would certainly happen with a carbocation and the side products, notably RR, are consistent with a free-radical mechanism. There is evidence that the Simonini reaction involves the same mechanism as the Hunsdiecker reaction but that the alkyl halide formed then reacts with excess RCOOAg (0-24) to give the ester.452 See also 9-13. 

The following reaction, known as the Hunsdiecker reaction, proceeds by a radical chain mechanism. Show the steps in this mechanism. (Hint The weakest bond in the compound, the oxygen-bromine bond, is broken in the initiation step.)... 

Hmtsdiecker Cristol reaction. (I, 657). A detailed procedure for the preparation of l-bromo-3-chlorocyclobutanc by the modified Hunsdiecker reaction has been published. A l-l. three-necked, round-bottomed flask wrapped with aluminum foil to exclude light and equipped with a mechanical stirrer, a reflux condenser, and an addition funnel, is charged with 37 g. (0.17 mole) of red mercuric oxide and 330 ml. 

The Hunsdiecker reaction, in which the silver salt of a benzoic acid is thermally decarboxylated in the presence of bromine, gives moderate yields of aryl bromides. The mechanism is uncertain, but may involve the generation of aryl radicals (Scheme 9.5). 

In the Hunsdiecker reaction, the silver salt of a carboxylic acid (RC02Ag) is treated with Br2 to give an alkyl bromide RBr with one fewer C atoms. The reaction does not work well with aromatic acids, suggesting that a free-radical mechanism is involved. The carboxylate and bromine react to give an acyl hy-pobromite, which decomposes by a free-radical chain mechanism. 

Write a radical chain mechanism for the following reaction (a reaction called the Hunsdiecker reaction). 

The mechanism of a green Hunsdiecker reaction of cinnamic acids that produce (ii)-2-halostyrenes has been studied theoretically. The Hammett correlations and DFT calculations have shown that the rate-determining step is the electrophilic addition of a chlorine atom to the double bond. 

This fast, easily performed variant to the Hunsdiecker procedure, applies successfully to primary, secondary, or tertiary esters, even unsaturated ones (p. 353). From citronellic acid, despite the presence of a double bond in a position permitting cyclization, the latter reaction was not detected. Mechanistically, the sonochemical reaction differs from the photochemical analogue, during which the initiation occurs from the cleavage of the thiohydroxamic ester itself. The initiation probably takes place in the bubble then the halogen atom couples with the alkyl radical in the solution, possibly by a chain mechanism. 

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