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 (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

The correct answer is (a) 3-bromobutene under kinetically controlled conditions.

Step 1: Understanding Kinetic and Thermodynamic Control

In a reaction involving a conjugated diene, like 1,3-butadiene, and a halogen acid, the outcome can be determined either by kinetic or thermodynamic control. Kinetic control gives the product that forms faster, often observed at lower temperatures, while thermodynamic control gives the more stable product, typically observed at higher temperatures.

Step 2: Identifying the Products with 1,3-Butadiene

1,3-butadiene reacts with HBr to potentially form different products depending on the conditions. The two main products are 3-bromobutene and 1-bromo-2-butene. 3-bromobutene is the kinetic product formed rapidly and is usually less stable. 1-bromo-2-butene is the thermodynamic product, favored for its greater stabilization.

Step 3: Reaction Conditions Effect

At lower temperatures, such as 40°C mentioned in the question, reactions tend toward kinetic control due to the faster formation of the product. Under these conditions, the leading product should be one that forms the quickest, not necessarily the most stable.

Step 4: Identifying Correct Answer

Given the "kinetically controlled" condition and the temperature of 40°C, the primary product according to kinetic control would be 3-bromobutene. Therefore, the correct option is (a) 3-bromobutene under kinetically controlled conditions.

Key Concepts

Conjugated Diene Reactions

Conjugated diene reactions are fascinating chemical processes that involve molecules with alternating single and double bonds, like 1,3-butadiene. In such reactions, the presence of conjugated double bonds allows the dienes to participate in various types of chemical reactions, providing a rich ground for study and exploration.
The reactions of conjugated dienes with electrophiles, such as halogen acids like HBr, are particularly important. In these reactions, the electrophile attacks the diene, resulting in the addition of the electrophile across the diene's double bonds. This can produce different products under different reaction conditions.
For instance, when 1,3-butadiene reacts with HBr, the two main products that can be formed are 3-bromobutene and 1-bromo-2-butene. Understanding the conditions under which each of these products is favored helps in mastering the concept of kinetic versus thermodynamic control in chemical reactions.

Kinetic Control Products

Kinetic control refers to the conditions where the reaction product is determined by the speed at which the product forms, rather than its stability. Under kinetic control, the product that forms fastest is favored. This typically occurs under conditions of lower temperature and shorter reaction time.
In the case of the reaction between 1,3-butadiene and HBr, the kinetic control product is 3-bromobutene. This product forms more rapidly compared to its thermodynamic counterpart because the reaction follows the path with the lowest activation energy.

• Fast formation of product is key
• Lower temperature and shorter times favor kinetic products
• 3-bromobutene is less stable but forms quickly

Thus, under kinetic control conditions such as around 40°C, you can expect 3-bromobutene to be the predominant product.

Thermodynamic Control Products

Thermodynamic control is the process by which the most stable product is favored, rather than the one that forms quickest. This usually occurs when the reaction is allowed to reach equilibrium, which typically requires higher temperatures and longer reaction times.
For the reaction between 1,3-butadiene and HBr, the thermodynamic control product is 1-bromo-2-butene. This product is more stable because its formation corresponds to a lower energy state than 3-bromobutene, despite taking longer to form.

• Focus on product stability
• Higher temperatures and equilibrium conditions favor thermodynamic products
• 1-bromo-2-butene is more stable but forms more slowly

The principles of thermodynamic control are crucial for understanding why certain products are favored when reactions occur under different conditions.

Reaction Conditions Impact

The conditions under which a reaction occurs, such as temperature and time, greatly impact the outcome of the reaction. These conditions dictate whether kinetic or thermodynamic control will dominate, consequently influencing which product will be predominantly formed.
At lower temperatures, like the 40°C referenced in the exercise, reactions tend to favor kinetic control because they allow for the rapid formation of the initial product. Conversely, higher temperatures with extended time would favor thermodynamic control, allowing more time for the most stable product to form.

• Temperature and reaction time are critical factors
• Lower temperatures result in kinetic products
• Higher temperatures shift favor towards thermodynamic products

Understanding these influences is essential for predicting and manipulating the products of chemical reactions.

1,3-Butadiene Reactivity

1,3-butadiene is a simple conjugated diene with high reactivity due to its structure, which includes alternating single and double bonds. This distinct arrangement allows it to undergo diverse chemical reactions, especially when interacting with electrophiles like HBr.
When reacting with HBr, 1,3-butadiene can form different products, depending on the reaction conditions. Its reactivity is influenced by kinetic and thermodynamic principles, leading to either 3-bromobutene under kinetic control or 1-bromo-2-butene under thermodynamic control.

• High reactivity due to conjugated system
• Ability to form multiple products based on conditions
• Kinetic or thermodynamic products depend on temperature

The reactivity of 1,3-butadiene is a classic example of how molecular structure influences the course and outcome of chemical reactions.