Electrophilic addition of Br2 to isoprene (2-methyl-1, 3-hutadiene) yields the following product mixture: Of the 1, 2-addition products, explain why 3, 4-dibromo-3-methyl-1-butene (21 %) predominates over 3, 4-dibromo-2-mcthyl-1-butene(3%).
The Correct Answer and Explanation is :
The electrophilic addition of Br₂ to isoprene (2-methyl-1,3-butadiene) leads to a mixture of products due to the formation of carbocations in the reaction mechanism. The two major possible 1,2-addition products are 3,4-dibromo-3-methyl-1-butene and 3,4-dibromo-2-methyl-1-butene. The reason why 3,4-dibromo-3-methyl-1-butene predominates over 3,4-dibromo-2-methyl-1-butene (21% vs. 3%) is due to the stability of the carbocation intermediate formed during the reaction.
Here’s the breakdown of the process:
- Electrophilic addition mechanism:
- The Br₂ molecule undergoes homolytic cleavage, creating a Br⁺ ion (the electrophile) and a Br⁻ ion.
- The Br⁺ ion attacks the electron-rich double bond of isoprene (2-methyl-1,3-butadiene), forming a carbocation intermediate.
- Carbocation formation:
- In the case of 3,4-dibromo-3-methyl-1-butene, the intermediate carbocation is formed at the C-2 position (the carbon adjacent to the double bond), leading to a 3-methyl-1-butyl cation.
- For 3,4-dibromo-2-methyl-1-butene, the carbocation intermediate forms at C-4, resulting in a secondary carbocation.
- Stability of the carbocations:
- The carbocation formed at C-2 in the 3,4-dibromo-3-methyl-1-butene product is tertiary (adjacent to the methyl group on C-3), which is significantly more stable than the secondary carbocation formed at C-4 in the 3,4-dibromo-2-methyl-1-butene product.
- More stable carbocations are more likely to form, leading to a higher concentration of 3,4-dibromo-3-methyl-1-butene in the product mixture.
Thus, the 3,4-dibromo-3-methyl-1-butene product predominates because the carbocation intermediate involved in its formation is more stable than the one for 3,4-dibromo-2-methyl-1-butene, making the reaction pathway leading to the former more favorable.
Now, let me generate an image for you that visualizes this concept.
Here is the diagram illustrating the electrophilic addition of Br₂ to isoprene, showing the two possible products and the formation of carbocations at C-2 and C-4. As discussed, the more stable tertiary carbocation formed at C-2 leads to the predominance of 3,4-dibromo-3-methyl-1-butene over the other product. Let me know if you need further clarification!