When 1,3 -butadiene reacts with one mole of HBr, two isolable products result. Propose a mechanism to explain this.
The Correct Answer and Explanation is:
When 1,3-butadiene reacts with one mole of HBr, the reaction proceeds through an electrophilic addition mechanism, leading to two different products due to the possibility of two carbocation intermediates forming at different positions on the diene. Here’s a detailed breakdown of the mechanism:
Step 1: Protonation of 1,3-butadiene
The first step in the mechanism involves the protonation of 1,3-butadiene by HBr. Since 1,3-butadiene has conjugated double bonds, the proton can add to either of the two carbon atoms of the double bonds (C1 or C2), leading to two possible carbocation intermediates:
- Initial protonation at C1: This results in a secondary carbocation at C2 (C1-H).
- Initial protonation at C2: This results in a secondary carbocation at C3 (C2-H).
Step 2: Carbocation rearrangement (if applicable)
The formed carbocations are not always stable in their initial form. In this case, the carbocation at C1 can rearrange to form a more stable carbocation at C3 through a 1,2-hydride shift. Similarly, the carbocation formed at C2 can rearrange to form a more stable carbocation at C4 through a 1,4-hydride shift.
Step 3: Nucleophilic attack by Br⁻
Once the carbocation is formed, the bromide ion (Br⁻) attacks the carbocation, resulting in the formation of two possible products:
- Product 1: When protonation occurs at C1, the Br⁻ attacks C2, resulting in the formation of 1-bromo-2-butene (a mono-substituted alkene).
- Product 2: When protonation occurs at C2, the Br⁻ attacks C3, resulting in the formation of 3-bromo-1-butene (another mono-substituted alkene).
Final Products
Thus, two isolable products result from this reaction:
- 1-bromo-2-butene (from protonation at C1)
- 3-bromo-1-butene (from protonation at C2)
Both products are formed due to the regioselectivity of protonation and the resulting carbocation intermediate’s stability. The reaction is governed by the Markovnikov rule, where the proton adds to the carbon with the most hydrogen atoms, leading to the more stable carbocation intermediate.
In summary, the mechanism involves:
- Electrophilic attack by HBr
- Formation of two possible carbocations
- Rearrangement to more stable carbocations
- Nucleophilic attack by Br⁻ to yield two products.
