Question

draw the mechanism for 1,3 butadiene = HBR for both possible products. explain which is the major product under thermodynamic control.

Answer 1

Kinetic versus thermodynamic products-

the amount of 1,2- and 1,4-addition products formed in electrophilic addition reactions of conjugated dienes depends greatly on the reaction conditions: At low temperatures, the major product is formed by 1,2-addition. The kinetic product is formed faster and predominates at low temperature:

At higher temperatures, the major product is formed by 1,4-addition. The more slowly formed thermodynamic product is more stable and predominates at higher temperatures

In fact, when a mixture containing mainly the 1,2-product is heated, the 1-4-addition product becomes the major product at equilibrium:

1,4-addition product the more stable (thermodynamic) product Because more substituted alkenes are more stable (Remember, increasing alkyl substitution stabilizes an alkene by an electron-donating inductive effect)

The 1,2-addition product is formed faster because of the proximity of Brto C2; Following H+ addition to the double bond, Br- is closer to C2 than C4 hence attack at C2 is faster.

mechenism-