Problem 53 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

When isobutane is brominated in the presence of diffused sunlight then the product formed is (a) exclusively tertiary butyl bromide (b) exclusively isobutyl bromide (c) exclusively \(\mathrm{n}\) -butyl bromide (d) s-butyl bromide

Step-by-Step Solution

Answer

(a) exclusively tertiary butyl bromide

Step 1: Understanding the Problem

The problem asks us to determine the major product when isobutane undergoes bromination in the presence of diffused sunlight.

Step 2: Identify the Reactants and Conditions

Isobutane is a type of hydrocarbon with a branched chain, and bromination is a halogenation reaction. The reaction proceeds via a radical mechanism, especially in the presence of diffused sunlight, which provides energy for the homolytic cleavage of bromine molecules.

Step 3: Radical Stability Analysis

In radical reactions, the formation of more stable radicals is favored. With isobutane, the tertiary carbon is more substituted, and thus forms a more stable radical compared to a primary or secondary carbon.

Step 4: Select the Product

Since the reaction favors the formation of the most stable radical, bromine will preferentially substitute at the tertiary carbon of isobutane. This results in the formation of tertiary butyl bromide.

Key Concepts

Bromination

Bromination is a type of halogenation process where a bromine atom is introduced into an organic compound. In the context of organic chemistry, this typically involves replacing a hydrogen atom with a bromine atom. Bromination occurs commonly through a mechanism known as radical halogenation, which is initiated by sources of energy such as heat or light. This energy is needed to homolytically cleave the bond in a bromine molecule, creating bromine radicals.

• The bromine radicals are highly reactive due to their unpaired electrons.
• They seek to pair with an available electron, such as the one forming a C-H bond in hydrocarbons like isobutane.

In the presence of diffused sunlight, which provides sufficient energy, bromination proceeds quickly, favoring the formation of more stable radicals. Understanding this energy-driven initiation is crucial for predicting the products of bromination reactions.

Radical Reactions

Radical reactions are a fundamental aspect of many organic processes, especially in halogenation. Unlike other reaction types, radical reactions involve species with unpaired electrons that seek to stabilize by forming new bonds. In a radical reaction, several phases occur:

• Initiation: Light or heat splits a halogen molecule into two radicals.
• Propagation: These radicals react with organic molecules, forming new radicals and continuing the cycle.
• Termination: Radicals combine, creating stable molecules and ending the reaction.

The nature of the radical species is essential here. More substituted radicals, like tertiary radicals, are generally more stable due to the electron-donating effects of surrounding alkyl groups, which distribute electron density into the radical site. This results in the formation of the most stable product.

Isobutane

Isobutane is a branched alkane, scientifically recognized as 2-methylpropane. Its structure contains a central carbon, bonded to three other carbons and a hydrogen. This structure can be described as:

• A central tertiary carbon atom forming the core.
• Three methyl groups surrounding this central carbon.

Because of its unique branched structure, isobutane exhibits differences in chemical reactivity and physical properties compared to its straight-chain isomer, n-butane. In bromination reactions, isobutane's tertiary carbon is the main center of activity due to the stability it can provide to a forming radical, which impacts the selectivity and outcome of radical reactions, such as forming tertiary butyl bromide.

Radical Stability

Radical stability is a key aspect in predicting the outcome of radical reactions. In radical processes, intermediates with varying stabilities are formed, influencing the preference for certain products. There are several factors that affect radical stability:

• Degree of Substitution: Tertiary radicals are more stable than secondary or primary radicals because the number of alkyl groups attached to the radical center can stabilize it through hyperconjugation.
• Resonance: Radicals that can delocalize their unpaired electron through resonance structures are more stable.
• Inductive Effects: Electron-donating groups can further stabilize radical centers by providing extra electron density.

For isobutane, the tertiary radical formed is the most stable choice, enabling the preferential formation of products like tertiary butyl bromide during bromination. This concept is important for understanding why certain positions within a molecule are more likely to undergo substitution in radical reactions.