Kinetic vs. Thermodynamic reaction of HBr with 1,3-butadiene
Kinetic vs. Thermodynamic reaction of HBr with 1,3-butadiene
The reaction of 1,3-butadiene with HBr is an interesting reaction to allow us to begin to consider kinetic and thermodynamic reaction conditions and how they influence the products of a reaction. A pi bond containing negative electrons attacks the proton of the HBr acid as Br- leaves as a good leaving group. The hydrogen atom attached to carbon C1 so the carbocation can form in the allylic position. This position is more stable because resonance forms can be drawn. When considering the two resonance forms, the secondary carbocation is more stable than the primary carbocation. We could also say the secondary carbocation is lower in energy than the primary carbocation.
When the bromide attacks the carbocation, it can attack the carbocation of either resonance form. If it attacks the carbocation of the first resonance form, the H and Br end up on carbon atoms C1 and C2. This is called 1,2-addition. If the bromide attacks the carbocation of the second resonance form, the H and Br end up on carbon atoms C1 and C4. This is called 1,4-addition.
When comparing the stability of the two alkene final products, we find that the product of 1,4-addition is more stable than the 1,2-addition product according to Uncle Saytzeff because it contains a more substituted alkene. The 1,4-addition product has the double bond on the interior of the molecule with 2 carbon groups attached to it compared to the 1,2-addition product with the double bond on the end of the molecule with only one carbon group attached.
The reaction energy diagram for this would look something like this. Carefully analyze this to make sure you understand it. Notice that for the final products, the one for 1,2-addition is higher in energy than the product for 1,4-addition. This is because the 1,4-addition product is more stable because of Saytzeff’s rule. Notice that the transition state going to the more stable 1,4-addition product is higher in energy than the transition state heading to the 1,2-addition product. This is because the higher energy transition state going to the 1,4-addition product more closely resembles the higher energy, less stable carbocation intermediate.
1,2 and 1,4-addition of HBr to 1,3-butadiene
The 1,2-addition product is formed faster in this reaction. This is because the transition state leading to the 1,2-addition product is lower in energy than the other transition state. Therefore, we call the 1,2-addition product the kinetic product. The 1,4-addition product is more stable than the 1,2-addition product. This is because the 1,4-addition product is lower in energy than the 1,2-addition product. The more stable product is called the thermodynamic product. Many times, the kinetic and thermodynamic products of a reaction are the same. This is one example of a reaction where the kinetic and thermodynamic products are different. What if we wanted more of the kinetic product in a reaction, or more of the thermodynamic product, is there a way to do that? Yes there is.
If we want more of the kinetic product, the reaction is performed under kinetic conditions or under kinetic control. This means the reaction is run at very cold temperatures. Usually, the reaction is run at -78°C. When dry ice is added to acetone it makes a slurry that is at -78°C. This is a very common cold temperature used in the chemistry lab. How does running the reaction at very cold temperatures help make more of the kinetic product? At very cold reaction temperatures, there is less energy in the reactants. If we’re looking at the reaction coordinate diagram, we could see that there is enough energy to get over the lower energy transition state leading to the 1,2-addition, kinetic product, but not over the higher energy transition state leading to the 1,4-addition thermodynamic product. If this reaction is performed under kinetic conditions at -78°C, the 1,2-addition product is made in 80% yield.
Kinetic conditions for HBr addition to 1,3-butadiene
If we want to make more of the thermodynamic product, we do the opposite. We heat up the reaction. If this reaction is performed at 40°C, we get the 1,4-addition product in 85% yield. At warmer temperatures, there is plenty of energy in the reaction to go over either transition state. We can make 1,2-addition product, and it can go back to the intermediate and then go on to make 1,4-addition product. In easily reversible reactions like this, the relative stability of the two products determines how much of each we get. Since the 1,4-addition product is more stable and lower in energy (Saytzeff’s rule), we get more of the thermodynamic 1,4-addition product.
Thermodynamic conditions of HBr addition to 1,3-butadiene
7. Draw the product of the following reactions. Attempt to answer this by thinking through the mechanism and NOT looking at the information on the previous pages.
a)
b)
Answers
7.
a)
b)
