Special Topic Rearrangements and Nonrearrangements of Radicals

FIGURE 11.59 Rearrangements occur in the polar addition of hydrogen chloride to 3,3-dimethyl-l-butene. 

FIGURE 11.60 The related radical addition gives no rearranged product. 

FIGURE 11.61 The simple 1,2-shift of a hydrogen atom (H-) is unknown. 

This situation seems mysterious, but a look at the molecular orbitals involved in the transition state for the reaction shows the reason. There is more than one way to approach this problem, but we think an analysis of the transition state for the rearrangement is the easiest. In the transition state for any symmetrical 1,2-shift of hydrogen, the hydrogen atom is half-transferred from one carbon to the other (Rg. 11.62). 

FIGURE 11.62 In the transition state for any symmetrical 1,2-shift, the hydrogen is halfway between the two carbons. Here the symbol or . 

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