Resonance and the Stability of Carbocations

FIGURE 9.19 In the addition of hydrogen chloride to vinyl chloride, it is formation of the more stable, resonance-stabilized carbocation that determines the product. 

In Section 9.2, we looked at the reaction of symmetrically substituted 2,3-dimethyl-2-butene with hydrogen chloride (Fig. 9.2). In the formation of the carbocation, there was no choice to be made—only one cation could be formed. When the less symmetrical alkene 2-methyl-l-butene reacts with hydrogen 

In an unsymmetrical system, it is the more stable carbocation that leads to product  

Unsymmetrical alkenes such as 2-pentene, from which two secondary carbocations of roughly equal stability can be formed, do give two products in comparable amounts (Fig. 9.21). 

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