Conjugated diene allylic carbocations from

Conjugare is a Latin verb meaning to link or yoke together and allylic carbocations allylic free radicals and conjugated dienes are all examples of conjugated systems In this chapter we 11 see how conjugation permits two functional units within a molecule to display a kind of reactivity that is qualitatively different from that of either unit alone... 

Conjugated dienes undergo several reactions not observed for nonconjugated dienes. One is the 1,4-addition of electrophiles. When a conjugated diene is treated with an electrophile such as HCl, 1,2- and 1,4-addition products are formed. Both are formed from the same resonance-stabilized allylic carbocation intermediate and are produced in varying amounts depending on the reaction conditions. The L,2 adduct is usually formed faster and is said to be the product of kinetic control. The 1,4 adduct is usually more stable and is said to be the product of thermodynamic control. 

Electrophilic additions to conjugated dienes usually involve allylic cations as intermediates. Unlike simple carbocations, an allylic cation can react with a nucleophile at either of its positive centers. Let s consider the addition of HBr to buta-1,3-diene, an electrophilic addition that produces a mixture of two constitutional isomers. One product, 3-bromobut-l-ene, results from Markovnikov addition across one of the double bonds. In the other product, 1 -bromobut-2-ene, the double bond shifts to the C2—C3 position. 

The intermediate carbocation formed from the conjugated diene is allylic and is more stable than the isolated carbocation from the isolated diene. Since the transition state for the rate-controlling first step leading to the lower-enthalpy allylic R+ also has a lower enthalpy, A// for this reaction is smaller and the reaction is faster. It is noteworthy that although the conjugated diene is more stable, it nevertheless reacts faster. 

The key step is hydride shift from a 2° carbocation to a 2° allylic, resonance-stabilized carbocation, which can subsequently lose a proton to form a conjugated diene. 

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