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Apparently the yield of alkyl chlorides is low as compared to that of alkyl bromides when preparing alkyl halides using the Hunsdiecker reaction.

Why does this occur? The only factor I can see is the lower stability of the free radical of chlorine as compared to that of bromine, as the reaction proceeds by the free-radical mechanism.

Is that what causes the lower yield?

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The Hunsdiecker reaction is a halogenation-decarboxylation reaction of silver salts of carboxylic acids.

Hunsdiecker Reaction

To understand what causes the yield to change, first we have to consider the mechanism of the reaction.

Hunsdiecker mechanism

As seen from the mechanism the homolytic cleavage of the $\ce{O-X}$ bond is the slowest step in the reaction and thus is the rate determining step. So as you predicted it all comes upon the stability of halogen radical. It's commonly known that the bromine radical is fairly stable than the chlorine radical and thus the Hunsdiecker reaction gives better yield with bromine rather than with chlorine.

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