Question

The most stable radical is (A) ${\mathrm{CH}}_3^{\ast }$ (B) ${\mathrm{CH}}_3-{\mathrm{CH}}_2^{\ast }$ (C) ${\mathrm{CH}}_3-{\mathrm{CH}}_2-{\mathrm{CH}}_2^{\ast }$ (D)

Short answer

The most stable radical is (D) ${\mathrm{CH}}_3-{\mathrm{CH}}_2-{\mathrm{CH}}_2-{\mathrm{CH}}_2^{\ast }$ because it has the most electron delocalization, allowing the unpaired electron to be dispersed among the most carbon atoms, reducing electron repulsion and increasing stability.

Step-by-step solution

Observe the Radicals

Each radical in the exercise has a distinct structure. The presence of the "*" sign marks the point of the unpaired electron. The major point of focus should be on its position.

Evaluate the Electron Stability in each Radical

Now, assess each radical based on the location of the unpaired electron and the potential for electron delocalization. Remember that a radical is more stable if the unpaired electron is more dispersed.

Compare Electron Delocalization in all Radicals

In (A), the unpaired electron is localized in one place as there are no other atoms for delocalization. Radicals in both (B) and (C) have their electron at the secondary and tertiary positions respectively which offers some electron delocalization. But in (D), the unpaired electron is on a quaternary carbon, with more surrounding carbon atoms providing better delocalization and hence more stability.

Determine the Most Stable Radical

After evaluating all the Radicals, we can conclude that Radical (D) ${\mathrm{CH}}_3-{\mathrm{CH}}_2-{\mathrm{CH}}_2-{\mathrm{CH}}_2^{\ast }$ is the most stable because it has the most electron delocalization. It can disperse the unpaired electron among the most carbon atoms, therefore distributing the charge even more than the other options. This leads to a reduction in electron repulsion and hence increased stability.

Key concepts

Electron Delocalization

Electron delocalization is a fundamental concept in organic chemistry that explains the distribution of electrons across several adjacent atoms within a molecule. It's a stabilizing force, especially in organic radicals, which are reactive species with an unpaired electron.

Imagine you are at a dance holding a balloon (the unpaired electron) in a crowd; it's much better if people (the surrounding atoms) can help to spread out and share the balloon's push and pull. That is precisely what delocalization does in a radical: It allows the unpaired electron to be spread out or 'delocalized' over several atoms, thereby reducing the electron density at any one point and increasing the stability of the molecule.

For students preparing for competitive exams like the JEE Main and Advanced in Organic Chemistry, understanding this principle is crucial. It's often a determining factor in the reactivity patterns of organic molecules and is frequently tested through questions on radical stability and reaction mechanisms.

Radical Stability Factors

Radical stability in organic chemistry can be influenced by a number of factors. A key to mastering questions about radical stability, be it in homework or competitive exams like JEE, is to understand these factors thoroughly.

Hybridization and Orbital Overlap

The type of hybridized orbitals involved can affect radical stability. Sp3 hybridized carbons are less stable due to poorer orbital overlap compared to sp2 hybridized carbons.

Inductive Effect

Electronegative atoms adjacent to the radical center can help stabilize it via the inductive effect. The electronegative atoms pull electron density away from the radical center, helping to disperse the charge.

Resonance

Perhaps the most influential factor for radical stability. When a radical can be delocalized through resonance, the unpaired electron's density is spread over multiple atoms or even multiple pi bonds, significantly increasing stability.

Remember, a stable radical is less reactive, so understanding these factors can greatly assist in predicting the behavior of organic molecules in various reactions.

JEE Main and Advanced Organic Chemistry

When diving into Organic Chemistry for competitive exams like JEE Main and Advanced, students must have a strong grasp of concepts such as radical stability. These concepts underpin many organic reactions and mechanisms that are tested in these exams.

For students, it is essential not just to memorize the factors affecting radical stability but to comprehend their practical implications. Some of the critical questions might involve predicting the products of radical reactions, understanding reaction pathways, and explaining the stability of intermediates.

Developing the ability to visualize electron movement and understanding how electron delocalization affects molecular structure will set a strong foundation for solving complex problems. Furthermore, application-based questions that require a deep understanding of radical stability can be a deciding factor in achieving high ranks in JEE examinations.