Problem 194 - A Complete Resource Book in Chemistry for JEE Main

Question

Reaction of one molecule of HBr with one molecule of 1,3 -butadiene at \(40^{\circ} \mathrm{C}\) gives predominantly [2005] (a) 3-bromobutene under kinetically controlled conditions (b) 1-bromo-2-butene under thermodynamically controlled conditions(c) 3-bromobutene under thermodynamically controlled conditions (d) 1-bromo-2-butene under kinetically controlled conditions.

Step-by-Step Solution

Answer

At 40°C and kinetically controlled conditions, 3-bromobutene is predominantly formed.

Step 1: Understand the Situation

The reaction is between HBr and 1,3-butadiene, a conjugated diene, at 40°C. Depending on the conditions, the reaction can yield different products. The products depend on kinetic versus thermodynamic control.

Step 2: Analyze Under Kinetically Controlled Conditions

Under kinetically controlled conditions, reactions proceed via the fastest pathway and product formation is determined by the rate of reaction. For 1,3-butadiene with HBr, at lower temperatures, the 1,2-addition is faster. This results in 3-bromobutene, as the H+ initially adds to the 1 position leading to the most stable carbocation for rapid bromide ion addition.

Step 3: Analyze Under Thermodynamically Controlled Conditions

Under thermodynamically controlled conditions, the reaction reaches an equilibrium favoring the more stable product, despite potentially slower formation. At higher temperatures, the 1,4-addition becomes favored. For the HBr and 1,3-butadiene reaction, this forms 1-bromo-2-butene, a more stable (thermodynamically favored) product due to a more substituted alkene.

Step 4: Evaluate the Reaction Temperature

At 40°C, temperature is relatively low but could influence the reaction pathway. Generally, lower temperatures favor kinetic control. Hence the outcome would likely be different if thermodynamic conditions were distinctly at higher temperatures like 100°C.

Key Concepts

kinetically controlled conditions

Kinetically controlled conditions focus on the speed of reaction. Under these conditions, chemical reactions tend to choose the path that leads to products most quickly. In the reaction of HBr with 1,3-butadiene at lower temperatures, this means the reaction takes the route with the fastest rate. This often results in the 1,2-addition product, which in this case is 3-bromobutene.
The priority here is on getting to the product fast, regardless of whether it is the most stable form. Rapid formation is important, as it is driven by the speed at which intermediates form and convert to products.
The key implications of kinetically controlled reactions are:

• The product forms at a fast pace.
• Reactions tend to take this path at lower temperatures.
• 1,2-addition is favored because it provides a quick, stable intermediate.

This means that the environment and temperature can heavily influence the resulting product structure.

thermodynamically controlled conditions

Thermodynamically controlled conditions prioritize the stability of the product. Here, reactions have the flexibility to reach a more stable state, even if it takes longer to get there. For the reaction between HBr and 1,3-butadiene, this typically means the 1,4-addition is favored as it leads to a more stable, substituted alkene product. In the context of this reaction, extended time or higher temperatures allow the molecules to rearrange into more stable configurations, resulting in 1-bromo-2-butene.
The important insights about thermodynamically controlled reactions include:

• Equilibrium is central, as it allows formation of the most stable product possible.
• Reactions under these conditions require sufficient energy to overcome barriers to rearrangement.
• Typically favored when the reaction temperature is higher.

Choosing the pathway determined by the most stable product can mean sacrificing speed for equilibrium and final product stability.

1,2-addition vs. 1,4-addition

Understanding 1,2-addition and 1,4-addition is crucial in reactions involving conjugated dienes like 1,3-butadiene. When HBr reacts with 1,3-butadiene, two major pathways can occur. In the 1,2-addition, the hydrogen and bromine are added to adjacent carbon atoms, more or less immediately following each other. This pathway is often kinetically controlled because it forms quickly and usually results in 3-bromobutene. It's rapid but doesn't always produce the most stable final product.
In contrast, the 1,4-addition involves the attachment of atoms to non-adjacent carbons. Although slower, this pathway frequently results in a more stable product, like 1-bromo-2-butene, due to a more stable arrangement of the double bonds. Key takeaways about these additions include:

• 1,2-addition is faster but may not be the most stable.
• 1,4-addition tends to occur when the reaction is allowed to reach equilibrium, focusing on final stability.
• Reactivity can depend greatly on reaction temperature and conditions.

Both types of addition illustrate how reaction conditions affect product formation, with kinetics favoring speed and thermodynamics favoring stability.