Why is iodine used in the pinacol rearrangement reaction?
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Answer:
This is a good question. I don't know the mechanism either, but in thinking about it, this is what I propose. Chlorine will react with water to give hypochlorite. Bromine is similar. Hypochlorite can react with alcohols to give a ROCl bond. Hypochlorite would be a better leaving group than hydroxide, although if there is an alpha-hydrogen, it will result in an oxidation with formation of chloride.
Therefore, I will guess that iodine reacts with the OH to give a ROI bond. This will be a better leaving group than hydroxide (which of course is unlikely as it is under acidic conditions). However, this may also be a source of HI, the other product of a reaction with the ROH group. The HI can give a regular acid catalyzed pinacol rearrangement. If that is the case, iodine would be a simple and easy source for a catalytic amount of HI. HI could even protonate the ROI oxygen to make it an even better and neutral leaving group. The actual mechanism could be the effect of one or more of the proposed reactions or another completely different reaction.