Question

The stability of the free radicals allyl, benzyl, \(3^{\circ}, 2^{\circ}\), \(1^{\circ}\) and \(\mathrm{CH}_{3}\) is of the order (1) allyl=benzyl \(>3^{\circ}>2^{\circ}>1^{\circ}>\mathrm{CH}_{3}\) (2) allyl \(>\) benzyl \(>3^{\circ}>2^{\circ}>1^{\circ}>\mathrm{CH}_{3}\) (3) \(3^{\circ}>2^{\circ}>1^{\circ}>\mathrm{CH}_{3}>\) allyl \(>\) benzyl (4) \(3^{\circ}>2^{\circ}>1^{\circ}>\mathrm{CH}_{3}>\) allyl=benzyl

Short answer

The stability order is allyl = benzyl > 3° > 2° > 1° > CH₃ (Option 1).

Step-by-step solution

- Understand Free Radical Stability

Free radicals are atoms or molecules with an unpaired electron. Their stability can be impacted by resonance stabilization and the type of carbon they are attached to (primary, secondary, tertiary).

- Resonance Stabilization

Resonance effects are significant in stabilizing free radicals. Allyl and benzyl radicals benefit greatly from resonance, making them very stable.

- Comparison of Radicals

Compare the stability: Allyl and benzyl radicals are highly stabilized by resonance. Tertiary radicals are more stable than secondary, which are more stable than primary. The methyl radical (CH₃) is the least stable due to the lack of alkyl group's stabilizing effect.

- Identify the Correct Order

Based on the above steps, the correct stability order is: allyl = benzyl > 3° (tertiary) > 2° (secondary) > 1° (primary) > CH₃.

Key concepts

Resonance Stabilization

Resonance stabilization is a critical concept in organic chemistry. It explains how certain molecules or ions distribute electrons more evenly across different atoms. This distribution lowers the potential energy of the molecule, making it more stable.
In the case of free radicals, resonance stabilization is particularly significant. For example, both allyl and benzyl radicals are highly stable because their electrons can be delocalized through resonance.
Here's how it works:

• Allyl radical: The unpaired electron in an allyl radical can be shared between three carbon atoms through the formation of resonance structures.
• Benzyl radical: Similarly, the unpaired electron in a benzyl radical can be delocalized through the aromatic ring, creating multiple resonance structures.

The ability to spread out the unpaired electron through resonance makes these radicals significantly more stable than those without such stabilization. This makes resonance a key player in explaining why certain radicals are more stable than others.

Primary, Secondary, Tertiary Radicals

In organic chemistry, the stability of radicals is also influenced by the type of carbon atom they are attached to. Radicals can be classified into three types based on this: primary, secondary, and tertiary.
Let's break it down:

• Primary (1°) radicals: These are attached to a carbon that is bonded to only one other carbon.
• Secondary (2°) radicals: These are bonded to a carbon that is connected to two other carbons.
• Tertiary (3°) radicals: These are attached to a carbon that is connected to three other carbons.

Tertiary radicals are the most stable among these because they benefit from the electron-donating effect of three alkyl groups. This effect helps to stabilize the unpaired electron in the radical. Secondary radicals have moderate stability, while primary radicals are the least stable in this group.
Understanding this ranking is important for predicting the behavior of different organic molecules in radical reactions.

Organic Chemistry and Free Radicals

Free radicals play a crucial role in many organic chemistry reactions. A free radical is an atom or a group of atoms that has an unpaired electron, making it highly reactive.
The stability of free radicals affects how they participate in chemical reactions. More stable free radicals are less reactive, while less stable ones react more readily.
In organic reactions such as polymerization, halogenation, and combustion, free radicals are often intermediates. Their stability is influenced by a combination of factors:

• Resonance stabilization, which spreads out the electron density.
• The type of carbon they are bonded to (primary, secondary, tertiary).
• Other stabilizing effects, such as hyperconjugation.

Hyperconjugation is an effect where electron density from a bond adjacent to the radical can help to stabilize the unpaired electron.
Mastering these concepts can help you predict the outcomes of radical reactions and understand why certain radicals are more stable than others. This grasp over free radical stability is foundational for deeper studies in organic chemistry.