Electrophilic Attack on Conjugated Dienes-Kinetic and Thermodynamic Control III
The reactivity of conjugated dienes (hydrocarbons that contain two double bonds) varies depending on the location of double bonds and temperature of the reaction.These reactions can produce both thermodynamic and kinetic products.
Isolated double bonds provide dienes with less stability thermodynamically than conjugated dienes.
However, they are more reactive kinetically in the presence of electrophiles and other reagents.
This is a result of Markovnikov addition to one of the double bonds. A carbocation is formed after a double bond is opened. This carbocation has two resonance structures and addition can occur at either of the positive carbons.
Kinetic and Thermodynamic Products:
Kinetic products form the fastest. They usually occur at or below 0°C. This is also known as the 1,2-adduct because the substituents are added to the first and second carbons. Kinetic products contain a terminal double bond and the reaction is irreversible.
Thermodynamic products form at higher temperatures, generally greater than 40°C. These are known are the 1,4-adducts because they add to the first and fourth carbons. Thermodynamic products contain an internal double bond and the reaction is reversible. Also, when reactions are carried out, thermodynamic products are more stable than kinetic products because they are more substituted.
Kinetic and Thermodynamic Product Ratios:
To ensure the greatest possible yield of thermodynamic products, the reaction should be carried out at a temperature of 40°C or greater. This is known as thermodynamic control. At higher temperatures and longer reaction times, thermodynamic products are favored.
On the contrary, at lower temperatures, one would tend to see a greater yield of kinetic products. These products are generally formed at or around 0°C. Carrying out reactions around these temperatures is known as kinetic control and kinetic products form before thermodynamic products.
Since the thermodynamic product contains an internal double bond, it is more stable than the kinetic product, and this is due to hyperconjugation with neighboring atoms. Additionally, a higher activation energy results in the thermodynamic product forming slower than the kinetic product. Therefore, a thermodynamically controlled reaction gives a more stable product and kinetically controlled reaction gives a less stable product.
1) Determine both kinetic and thermodynamic products of the reaction of 1,3-butadiene and HBr.
2) Show the carbocations that lead to the thermodynamic and kinetic products.
3) Determine which of the products from the first example listed under Basic Reactions is thermodynamic and which is kinetic. Also label which is the 1,2-adduct and which is the 1,4-adduct.
4) React 1,3-butadiene with HCl at 40°C, what would the major organic product be?
5) React 1,3 -butadiene with Br2 and list the major organic products after a long period of time.