Problem 27 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The chief reaction product of reaction between \(n\)-butane and bromine at \(130^{\circ} \mathrm{C}\) is : (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Br}\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}(\mathrm{Br}) \mathrm{CH}_{3}\) (c) \(\mathrm{CH}_{3}-\mathrm{CH}_{2} \mathrm{CH}(\mathrm{Br}) \mathrm{CH}_{2} \mathrm{Br}\) (d) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{C}\left(\mathrm{Br}_{2}\right) \mathrm{CH}_{3}\)

Step-by-Step Solution

Answer

The chief reaction product is option (b): \(\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CH}( ext{Br}) \mathrm{CH}_3\).

Step 1: Identify the Type of Reaction

The reaction between an alkane (n-butane) and bromine at high temperatures often follows a free radical substitution mechanism. This is the type of reaction typically involved when an alkane reacts with bromine in the presence of heat or light.

Step 2: Determine the Likely Substitution Sites

In free radical substitution, a hydrogen atom is replaced by a bromine atom. For n-butane, \[ \mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CH}_2 \mathrm{CH}_3 \], the possible sites for substitution are the secondary carbon atoms, as they form more stable radicals compared to primary carbon atoms during the reaction.

Step 3: Determine the Major Product

In free radical halogenation, the most stable radical intermediate leads to the major product. On n-butane, the secondary carbon radical at the 2nd position (\mathrm{CH}_3\mathrm{CH}_2\mathrm{CH}(ullet)\mathrm{CH}_3) is more stable than primary radicals. Thus, bromination at the second carbon is favored, leading to the formation of \[\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CH}( ext{Br}) \mathrm{CH}_3 \] as the major product.

Step 4: Match with Given Options

From the given options, (b) \(\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CH}( ext{Br}) \mathrm{CH}_3 \) corresponds to the major product identified in Step 3 as it's formed by substitution at the secondary carbon.

Key Concepts

Free Radical Substitution

Free radical substitution is a type of organic reaction that typically occurs between an alkane and a halogen, like bromine or chlorine. This reaction involves the replacement of a hydrogen atom in the alkane molecule with a halogen atom. The process occurs under conditions of high heat or light, which provide the necessary energy to initiate the reaction.
The reaction proceeds through a three-step mechanism:

• Initiation: Free radicals are generated when the halogen molecules split, usually by the input of energy.
• Propagation: These free radicals react with the alkane, replacing a hydrogen atom, and creating a new radical that continues the reaction chain.
• Termination: The radicals eventually combine in various ways to form stable end products, ceasing the reaction.

The stability and reactivity of different radicals play a key role in determining the major products of these reactions.

Alkanes

Alkanes are hydrocarbons that consist of carbon and hydrogen atoms arranged in a linear or branched structure. They are saturated molecules, meaning they only contain single bonds between carbon atoms, which makes them relatively less reactive compared to unsaturated hydrocarbons like alkenes and alkynes.
The general formula for acyclic alkanes is \[ C_nH_{2n+2} \] where \( n \) is the number of carbon atoms. Alkanes serve as excellent substrates for free radical substitution due to their abundance and the presence of multiple hydrogen atoms available for substitution.
In substitution reactions, the type of carbon (primary, secondary, or tertiary) can significantly influence which hydrogen atoms are replaced due to differences in radical stability. For instance, secondary and tertiary radicals are generally more stable, and thus more likely to persist in the reaction mechanism.

Halogenation

Halogenation is the chemical process where one or more halogen atoms are introduced into a compound, typically replacing hydrogen atoms. In organic chemistry, halogenation is a critical step in forming many important industrial chemicals and intermediates.
For alkanes, halogenation usually involves chlorine or bromine due to their optimal reactivity. The process begins with the homolytic cleavage of the halogen molecule, forming two halogen radicals, each bearing an unpaired electron. These radicals partake in the substitution, precisely targeting hydrogen atoms in the alkane.
One key consideration in alkane halogenation is the regioselectivity—the preference of one direction of chemical bond forming or breaking over all other possible directions. Secondary and tertiary carbon atoms usually lead to more stable radical intermediates, which explains why secondary or tertiary hydrogen atoms are more frequently substituted. This principle explains the favored production of certain halogenated products over others.

Reaction Mechanisms

Reaction mechanisms provide a detailed pathway of how chemical reactions occur at the molecular level. In organic chemistry, understanding the reaction mechanism is key to predicting the behavior and outcome of chemical reactions.
In the free radical substitution mechanism observed in the halogenation of alkanes, each stage is driven by radical intermediates. The stability of these intermediates is crucial because they govern the pathway and kinetics of the reaction.

• Initiation phase involves cleaving the halogen molecules to generate reactive radicals.
• Propagation includes a series of steps where radicals react with alkanes, leading to complex radical chains that continue the reaction.
• Termination happens when radicals recombine to form stable product molecules, thus ceasing the chain reaction.

Understanding these steps allows chemists to manipulate and control the reaction conditions to favor the formation of desired products, demonstrating the practical applications of mastering reaction mechanisms.