Question

Provide the structure of the major product which results from 1,4-addition of br2 to the diene shown below.

Answer 1

The image provided shows the diene that is used for this question. We are told that the major product of the reaction is the 1,4-addition product. The result is the addition of a bromine atom to the first carbon and the fourth carbon of the diene. However, Br₂ can also add to just one alkene of the diene in a 1,2-addition to get the other product shown in the image.

As the first bromine atom adds to one of the alkenes, it adds to the first carbon which leads to the formation of a carbocation. The carbocation can be a stable tertiary center at the 2 carbon of the diene, or the less stable secondary center of the 4 carbon. To addition to the 4-carbon has a higher activation barrier which, but the product has a lower energy than the 1,2-product. Therefore, the 1,4-addition is the thermodynamic product and will form at higher temperatures. The 1,2-product is the kinetic product that will form at lower temperatures.