HX Addition Reactions Hydration

If HBr and HCl add to double bonds, it should not be surprising that other acidic HX compounds undergo similar addition reactions. Hydrogen iodide is a simple example, and HI addition rather closely resembles the reactions we have already studied in this chapter. Markovnikov s rule is followed, for example (Fig. 9.32). 

Water (HOH) is an HX reagent, but, as we saw on p. 139 it is not a strong enough acid to undergo addition to simple alkenes at a useful rate. If one mixes water and a typical alkene, no reaction occurs. The difference between HOH and HX is 

A catalyst provides a lower energy path but it does not alter the energy of starting material and product, A and B rather, it changes the energy of the transition state, or transition states, in the reaction 

FIGURE 9.33 A catalyst does not change the energy of either starting material or product but does provide a lower energy path between them. 

FIGURE 9.35 Three possible regiochemical results from the hydration of 2-methylpropene. 

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