Addition of Hydrogen Halides to Conjugated Dienes

As we saw earlier, dehydrations and dehydrohalogenations are typically regioselective in the direction that leads to the most stable double bond. Conjugated dienes are more stable than isolated dienes and are formed faster via a lower energy transition state. 

Dienes with isolated double bonds can be formed when the structure of the substrate doesn t permit the formation of a conjugated diene. 

We will not discuss the preparation of cumulated dienes. They are prepared less readily than isolated or conjugated dienes and require special methods. 

Our discussion of chemical reactions of alkadienes will be limited to those of conjugated dienes. The reactions of isolated dienes are essentially the same as those of individual alkenes. The reactions of cumulated dienes are—like their preparation—so specialized that their treatment is better suited to an advanced course in organic chemistry. 

Use Learning By Model-ing to view the charge distribu tion in the aiiyiic carbocation shown in the equation. 

PROBLEM 10.9 What dienes containing isolated double bonds are capable of being formed, but are not observed, in the two preceding equations describing elimination in 3-methyl-5-hexen-3-ol and 4-bromo-4-methyl-1-hexene  

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A general method for preparing allylic halides is by addition of hydrogen halides to conjugated dienes. This reaction usually produces a mixture of 1,2- and 1,4-addition products (see Sectioii 13-2) ... 

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