A Catalyzed Addition to Alkenes Hydration

We all have to get used to this ambiguity—it is part of the human condition. We can be quite certain we are right in our mechanistic ideas, and the more time that goes by, and the more tests our ideas pass, the more confident we can be that we are basically right. However, there have been some big surprises in science, as conventional wisdom is reversed every once in a while, and there will be more. To be honest, we sort of like it when that happens. 

All which direction does HX add problems can be approached—and solved— in the same way. Always draw out the two possible carbocations formed by initial protonation of the double bond. For an unsymmetrical (ends different) double bond, there are always two and only two possibilities. Next evaluate those two carbocations—ask yourself which of them is more stable It is that one that will lead to the major product. The answer to Problem 3.30 is typical. Use it as a prototype, a template for answering all similar questions. 

FIGURE 3.80 The highly desirable addition of water to an alkene. 

Let s work through the mechanism for the hydration of alkenes. The first step is protonation of the alkene by the catalyst, H30 . As in the reaction of HCl, this addition leads to a carbocation intermediate (Fig. 3.81). Now there is no halide 

FIGURE 3.81 The acid-catalyzed addition of water to 2,3-dimethyl-2-butene —a hydration reaction. 

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