Answer to Question #264244 in Organic Chemistry for shinal

Question

The reaction of 2,3-dimethylbuta-1,3-diene and HBr (1:1 mole) gives two products I and II.

i). Which of the two products (I or II) is the 1,4-addition product? Explain the significance of the numbers 1 and 4?

ii). Which of the two products would be more stable? Explain.

iii). The carbocation intermediate that leads to I is what? What is the structure of the carbocation that leads to II?

iv). Which product I or II is the kinetic product? Explain your choice.

v). Which product I or II is the thermodynamic product? Explain your choice.

Answer 1

i) The significance of the numbers 1 and 4 means the additions of H+ and Br- to the first and fourth carbon atoms, respectively.

ii) The 1,4-addition product is more stable, because it is less sterically hindered. Moreover, the 1,4‐addition product is the result of the formation of a stable allylic carbocation.

iv) The addition at temperatures below zero leads mainly to the 1,2‐addition product, at low temperatures, the reaction is kinetically controlled. At low temperatures, the reaction is irreversible and no equilibrium is established because the products have insufficient energy to overcome the activation energy barrier, which separates them from the initial reactant. Therefore, the 1,2-addition product is formed.

v) If addition reactions run at temperatures above 50°C produces mainly the 1,4‐addition product. At high temperatures, the reaction is thermodynamically controlled. Also if the reaction is initially run at 0°C and then warmed to 50°C or higher and held there for a period of time, the major product will be a 1,4-addition. The 1,4-addition product is more stable, it will be the major product formed.