Rearrangements during HX addition to alkenes

When HCl adds to isopropylethylene (3-methyl-l-hutene), a quite surprising thing happens. We do get some of the product we expect, 2-chloro-3-methylbutane, hut the major compound formed is 2-chloro-2-methylhutane (Fig. 9.44). 

FIGURE 9.44 Reaction of HCl with 3-methyl-l-butene gives a new product in addition to the expected 

The obvious question is. Why Will our mechanism for addition have to be severely changed to accommodate these new data In fact, we will be able to graft a small modification onto our general mechanism for addition reactions and avoid drastic changes. The way to attack a problem of this kind—explaining the formation of an unusual product—is to write the mechanism for the reaction as we have developed it so far and then see if we can find a way to rationalize the new product. In this case, protonation of 3-methyl-l-butene can give either a secondary 

FIGURE 9.45 No real choice here the more stable secondary carbocation is formed. 

FIGURE 9.46 In the reaction between 3-methyl-1-butene and HCl the major product is formed through a rearrangement mechanism (path b) involving the migration of hydrogen with its pair of electrons (hydride, 

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