Electrophilic addition to dienes

Earlier in the chapter you saw the epoxidation of a diene to give a monoepoxide only one of the double bonds reacted. This is quite a usual observation dienes are more nucleophilic than isolated alkenes. This is easy to explain by looking at the relative energy of the HOMO of an alkene and a diene—this discussion is on p. 168 of Chapter 7. Dienes are therefore very susceptible to protonation 

Why protonate this double bond and not the other one The cation you get by protonating the other double bond is also allylic, but it cannot.  

If the add is HBr, then nucleophilic attack by Br on the cation follows. The cation is attacked at the less hindered end to give the important compound prenyl bromide. This is very much the sort of reaction you met in Chapter 17—it is the second half of an S jl substitution reaction on an allylic compound. 

Overall, the atoms H and Br are added to the ends of the diene system. The same appears to be the case when dienes are brominated with Br2. 

Changing the conditions slightly gives a different outcome. If the reaction is done at lower temperatures, the bromine just adds across one of the double bonds to give a 1,2-dibromide. 

---

Later in this chapter we ll see how allylic carbocations are involved in electrophilic addition to dienes and how the principles developed in this section apply there as well. 

---