Electrophilic addition to conjugated systems

The first step in the reaction of HX with an alkene is protonation to yield the more stable cation. If we extend this principle to a conjugated diene, e.g. buta-1,3-diene, then we can see that the preferred carbocation will be produced if protonation occurs 

At first glance, this appears to be a secondary carbocation, bnt on further exanfination one can see that it is also an allylic cation. Allylic carbocations are stabilized by resonance, reselling in dispersal of the positive charge (see Section 6.2.1). From these two resonance forms, we can predict that both carbons 2 and 4 will be electron deficient. Now this has particnlar consequences when we consider subseqnent attack of the nucleophile on to the carbocation. There are two possible centres that may be attacked, resnlting in two different prodncts. The prodncts are the result of either 1,2-addition or 1,4-addition. The addition across the fonr-carbon 

Now the allylic cation has two limiting stractures, one of which is a secondary carbocation and the other a primary carbocation. We wonld expect the secondary carbocation to contribnte more than the primary carbocation, and this is usually reflected in the proportions of the two products actually obtained from the reaction carried out at low temperatures. Why the rider about low temperatures Well, the product ratio is different if the same reaction is carried out at higher temperatures, and typically the 1,2-adduct now predominates. 

Br 80% + Br 20% kinetic control product ratio determined by stability of carbocation 

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In one of these products, HBr has added to one of the two double bonds, and the other double bond is still present in its original position. We call this the product of 1,2-addition. The other product may at first seem unexpected. The hydrogen and bromine have added to carbon-1 and carbon-4 of the original diene, and a new double bond has appeared between carbon-2 and carbon-3. This process, known as 1,4-addition, is quite general for electrophilic additions to conjugated systems. How can we explain it ... 

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