Question

The following model is that of an allylic carbocation intermediateformed by protonation of a conjugated diene with HBr. Show the structureof the diene and the structures of the final reaction products.

Short answer

A conjugated diene is a compound with the alkene functional group of two “double bonds or pie bonds” (=), also known as alkadienes.

The conjugated diene is separated by one single bond, “alternative position single and doublebond,” and is more stable than isolated and cumulated diene due to its resonating structure.

Step-by-step solution

Conjugated Diene

A conjugated diene is a compound with the alkene functional group of two “double bonds or pie bonds” (=), also known as alkadienes.

The conjugated diene is separated by one single bond, “alternative position single and doublebond,” and is more stable than isolated and cumulated diene due to its resonating structure.

Structure of conjugated diene and product

The given allylic carbocation can be formed by three possible conjugated dienes, which are as follows:

• 3-methyl-2,4-hexadiene
• 4-methyl-1,3-hexadiene
• 2-Ethyl-1,3-pentadiene

The three conjugated dienes reacting with hydrogen halide (HBr) formed the given allylic carbocation, resonance stabilized, with the two resonating structures.

The two resonating structures of the given allylic carbocation reacting with bromine ion $\left({\text{Br}}^{\text{-}}\right)$formed the desired products 3-bromo-4-methyl-3-hexene and 4-bromo-4-methyl-2-hexene.

The structure of conjugated diene and the product formed is shown as follows:

Structure of diene and product