Mixed or Variable Mechanisms

Dehydration of p-hydroxyketones is both a common reaction (we will see in Chapters 17 and 20 why these molecules are easy to prepare) and a very straightforward one. The product obtained is invariably the conjugated one, whatever the mechanism. Acid-catalyzed reactions are common they are usually El with tertiary alcohols and E2 with primary alcohols. Secondary alcohols such as 10.18 may react by either or both mechanisms. 

Although our discussion has been dominated by reactions in which HX, where X is a halogen or OH, is eliminated, other examples are possible. In elimination from 10.19, we might have wanted to eliminate HBr to give the unsaturated acid, but something quite different happens. The proton that is most easily and rapidly removed, by many orders of magnitude, is that on the carboxyl group. So what is lost is carbon dioxide and bromide ion. 

Iodide ion may be used to dehalogenate vicinal dibromides as shown in the following  

Given this observation, predict the product of the reaction shown as follows  

We will first need to rotate about the single bond in this molecule to put it into the right conformation to do the elimination. The elimination process is then identical to that shown earlier. 

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