Like nonconjugated dienes, conjugated dienes are subject to attack by electrophiles. In fact, conjugated electrophiles experience relatively greater kinetic reactivity when reacted with electrophiles than nonconjugated dienes do. Upon electrophilic addition, the conjugated diene forms a mixture of two products—the kinetic product and the thermodynamic product—whose ratio is determined by the conditions of reaction. A reaction yielding more thermodynamic product is under thermodynamic control, and likewise, a reaction that yields more kinetic product is under kinetic control.
The electrophilic addition to the conjugated diene follows Markovnikov’s rule. Following the opening of a double bond, a carbocation with two resonance structures capable of addition at its positively charged carbon is formed. The reaction between HCl and 1,3-butadiene demonstrates how the products are formed.
The kinetic product is formed through the intermediate containing the terminal double bond and the thermodynamic product is formed through the intermediate containing the internal double bond. Generally, 1,2 addition yields the kinetic product, while 1,4 addition yields the thermodynamic product.
The kinetic product is formed first and it is favored at low or normal temperatures. However, under high temperatures or simply long reaction times, the thermodynamic product is favored. As we expect, the thermodynamic product is the more stable of the two, since its double bond is internal. Its high activation energy, as read off the reaction coordinate diagram, accounts for its slow rate of formation, as well as how it is favored at higher temperatures, since the higher temperature corresponds with increased energy available to overcome the barrier. Likewise, the low activation energy for the kinetic product accounts for how it is more readily formed. The reason that the kinetic product has a lower activation energy is that the nucleophile attacking the positively charged carbon would be closer to carbon 3 of the kinetic intermediate than it would be to carbon 1 in the thermodynamic intermediate. Additionally, carbon 3 possesses a higher positive partial charge, which makes it more appealing to the nucleophile.
- Sorrell, Thomas, N. Organic Chemistry. New York: University Science Books, 2006.
- Vollhardt, K., Schore, N. Organic Chemistry: Structure and Function. 5th edition. New York, NY: W. H. Freeman and Company, 2007.
- Shore, N. Study Guide and Solutions Manual for Organic Chemistry. 5th edition. New York, NY: W.H. Freeman and Company, 2007.
1. Draw the two relevant resonance structures of the carbocation intermediates in the reaction between 1,3-butadiene and HBr.
2. Draw the products of the reaction in problem #1 and indicate which product is kinetic and which is thermodynamic.
3. State whether this reaction is under thermodynamic or kinetic control.
3. Kinetic Control