Electrophilic Attack on Conjugated Dienes Kinetic and Thermodynamic Control

ELECTROPHILIC ATTACK ON CONJUGATED DIENES KINETIC AND THERMODYNAMIC CONTROL 

Does the structure of conjugated dienes affect their reactivity Although more stable thermodynamically than dienes with isolated double bonds, conjugated dienes are actually more reactive kinetically in the presence of electrophiles and other reagents. 1,3-Butadiene, for example, readily adds 1 mol of cold hydrogen bromide. Two isomeric addition products are formed 3-bromo-1-butene and l-bromo-2-butene. 

The generation of the first product is readily understood in terms of ordinary alkene chemistry. It is the result of a Markovnikov addition to one of the double bonds. But what about the second product  

The presence of l-bromo-2-butene is explained by the reaction mechanism. Initial protonation at Cl gives the thermodynamically most favored allylic cation. 

This cation can be trapped by bromide in two possible ways to form the two observed products At the terminal carbon, it yields l-bromo-2-butene and, at the internal carbon, it furnishes 3-bromo-l-butene. The l-bromo-2-butene is said to result from 1,4-addition of hydrogen bromide to butadiene, because reaction has taken place at Cl and C4 of the original diene. The other product arises by the normal 1,2-addition. 

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