When 2-methyl-1,3-butadiene reacts in the presence of HBr the major product is 1-bromo-3- methyl-2-butene. Provide a complete mechanism to show the formation of the product and explain why this product is major. (Hint: consider the reactive intermediates that are possible) 2-methyl-1,3-butadiene HBr Br 1-bromo-3-methyl-2-butene ?? ???? – ???? +Br B
The Correct Answer and Explanation is:
Of course. Here is the complete mechanism and explanation for the reaction.
The reaction of 2-methyl-1,3-butadiene with HBr is an electrophilic addition to a conjugated diene. The formation of 1-bromo-3-methyl-2-butene as the major product is governed by the formation of the most stable carbocation intermediate and the thermodynamic stability of the final product.
Complete Mechanism:
Step 1: Protonation to form a resonance-stabilized allylic carbocation.
The reaction begins when one of the pi bonds of the diene acts as a nucleophile, attacking the electrophilic proton of HBr. Protonation occurs at the C1 carbon because this generates the most stable possible carbocation: a tertiary allylic carbocation. This intermediate is highly stabilized by both the inductive effect of the methyl group and by resonance.
Step 2: Nucleophilic attack by bromide ion.
The positive charge on the allylic carbocation is delocalized across both the C2 and C4 positions, as shown in the resonance structures. The bromide ion (Br⁻), acting as a nucleophile, can attack either of these electrophilic carbons.
- 1,2-Addition (Kinetic Product): If the bromide attacks C2, it forms the 1,2-adduct, 4-bromo-3-methyl-1-butene. This product often forms faster because the C2 carbon bears more of the positive charge in the resonance hybrid (it’s a tertiary vs. a primary carbocation).
- 1,4-Addition (Thermodynamic Product): If the bromide attacks C4, it forms the 1,4-adduct, 1-bromo-3-methyl-2-butene.
Explanation for Why 1-bromo-3-methyl-2-butene is the Major Product:
The major product is determined by thermodynamic stability. We must compare the stability of the two possible products. The 1,4-adduct, 1-bromo-3-methyl-2-butene, has a double bond that is trisubstituted (it is bonded to three other carbon atoms). In contrast, the 1,2-adduct, 4-bromo-3-methyl-1-butene, has a double bond that is only monosubstituted.
According to Zaitsev’s rule, more highly substituted alkenes are more stable due to hyperconjugation. Because the 1,4-adduct contains a more substituted and therefore more stable double bond, it is the thermodynamically favored product. Under conditions that allow the reaction to reach equilibrium, the more stable product will predominate, making 1-bromo-3-methyl-2-butene the major product of this reaction.
