$\ce{HI/P}$ is used for reduction of alcohols, aldehydes, ketones and acids to alkanes. As for alkyl halides, I have read that only alkyl iodides are reduced, whereas other alkyl halides do not:

$$\ce{CH3CH2I ->[Red P][\pu{1 eq.}~HI] C2H6 + I2}\tag{R1}$$

But I have also been taught that all alkyl halides are reduced to alkanes:

$$\ce{CH3CH2X ->[Red P][\pu{2 eq.}~HI] C2H6 + HX + I2}\tag{R2}$$

Does red phosphorus with hydriodic acid reduces all alkyl halides or just alkyl iodides?


1 Answer 1


I don't think that in general, other alkyl halides react in this way.

When I searched Google Scholar for hydrodechlorination "hydroiodic acid", not much came up. There was this article about the selective dechlorination of 2-chloro,5-bromo substituted pyridines.

I also tried searching in some proprietary databases for reactions where ethyl chloride is reduced to ethane. There were plenty such reactions, but the for all of them, the reported reductants were zinc or hydrogen gas.


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