The reaction of 1,3 -butadiene with HBr is shown below. At 40^∘ C the major product is the 1 . 4

step 1 Here two product formed because two product formed because the carbocation intermediate the carb

The reaction of 1,3 -butadiene with HBr is shown below. At...

The reaction of 1,3 -butadiene with HBr is shown below. At $40^{\circ} \mathrm{C}$ the major product is the 1 . 4-addition product; however, at $-80^{\circ} \mathrm{C}$ the major product is the 1,2 -addition product. Why are two products formed? (a) The carbocation intermediate allows delocalisation of the second double bond. (b) There are two double bonds present. (c) The fact that the carbocation is planar allows attack from both sides of the plane. (d) There are 2 moles of $\mathrm{HBr}$.

The reaction of 1,3 -butadiene with HBr is shown below. At $40^{\circ} \mathrm{C}$ the major product is the 1 . 4-addition product; however, at $-80^{\circ} \mathrm{C}$ the major product is the 1,2 -addition product.
Why are two products formed?
(a) The carbocation intermediate allows delocalisation of the second double bond.
(b) There are two double bonds present.
(c) The fact that the carbocation is planar allows attack from both sides of the plane.
(d) There are 2 moles of $\mathrm{HBr}$.

Key Concepts

Electrophilic Addition Mechanism

This concept describes how electrophiles, such as H+ from HBr, add to unsaturated compounds like polyenes. In conjugated systems, the addition occurs in a stepwise manner involving the formation of a reactive intermediate, which can then react with a nucleophile. It is fundamental to understanding how different products can form from a single starting material.

Carbocation Resonance Stabilization

During the reaction, a carbocation intermediate is generated. In conjugated dienes, this intermediate is often delocalized over multiple atoms through resonance, allowing the positive charge to be distributed across different positions. This delocalization is critical as it creates different reactive sites, leading to the possibility of forming multiple products.

Kinetic versus Thermodynamic Control

Reaction conditions, especially temperature, can influence whether a reaction is under kinetic or thermodynamic control. Under kinetic control, the fastest-forming product (often the 1,2-addition product) predominates, while under thermodynamic control, the more stable product (such as the 1,4-addition product) is favored. This explains how changing temperature can shift the product distribution in the reaction.

Planar Geometry of Carbocation Intermediates

The carbocation intermediate is planar owing to sp2 hybridization, which allows nucleophilic attack from either face of the molecule. This geometric feature is essential to understanding the stereochemical outcomes and the possibility of forming different products as the nucleophile can attack at more than one site on the delocalized carbocation.

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