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Thionyl chloride ($\ce{SOCl2}$) is used with alcohols to produce alkyl chlorides. Similarly, can thionyl bromide ($\ce{SOBr2}$) be used with alcohols to produce alkyl bromides?

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  1. Yes, thionyl bromide can be used with alcohols for synthesis of alkyl bromides.

  2. No, the process is not as efficient. The reasons for the inefficiency include are primarily (a) thionyl bromide is much less stable than thionyl chloride, and (b) that a bromide-tribromide redox equilibrium is far more relevant than a chloride/trichloride equilibrium, which leads to undesired side pathways instead of the desired reaction.

For more information, see this 1955 paper from Frazer & Gerrard.

On the other hand, pyridinium tribromide is readily formed during the addition of thionyl bromide to base hydrobromide, and its intervention weakens the analogy between the thionyl bromide and the thionyl chloride system.

In the same paper, a yield of 75% is reported for the synthesis of 2-bromobutane from butan-2-ol using pyridinium tribromide (which itself would be synthesized from thionyl bromide and pyridinium bromide).

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  • $\begingroup$ I think the reaction with thionyl chloride proceeds in the absence of pyridine too. So, what happens with thionyl bromide in the absence of pyridine? $\endgroup$ – EdmDroid Sep 19 '16 at 16:32
  • $\begingroup$ See also pubs.rsc.org/en/content/articlepdf/1970/J3/J39700000067 and the other papers that come up when searching "thionyl bromide" "alkyl bromide" on Google Scholar. $\endgroup$ – Curt F. Sep 19 '16 at 16:38

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