Question

Arrange the following free radicals in order of stability: (1) Benzyl (2) Allyl (3) Methyl (4) Vinyl (a) \(4>3>2>1\) (b) \(1>2>3>4\) (c) \(1>3>4>2\) (d) \(4>3>1>2\)

Short answer

The correct order is (b): Benzyl > Allyl > Methyl > Vinyl.

Step-by-step solution

Identify the Factors

First, we need to identify which factors affect the stability of free radicals. The stability is primarily influenced by resonance stabilization, the nature of the carbon (sp, sp2, sp3), and hyperconjugation.

Analyze Benzyl Radical

The benzyl radical is stabilized by resonance. The unpaired electron can delocalize into the aromatic ring, spreading over several atoms, which increases its stability significantly.

Analyze Allyl Radical

The allyl radical is also resonance-stabilized due to conjugation with a double bond. This type of stabilization makes it more stable than a simple alkyl radical but less stable than benzyl due to fewer resonance structures.

Analyze Methyl Radical

The methyl radical has no resonance stabilization and is only slightly stabilized through hyperconjugation with adjacent C-H bonds. It is a basic alkyl radical with moderate stability.

Analyze Vinyl Radical

The vinyl radical is on an sp2 carbon involved in a double bond, with no resonance stabilization. Vinyl radicals are highly unstable due to the rigidity of the double bond limiting hyperconjugation and lack of resonance.

Arrange in Order of Stability

Taking into account the observations: Benzyl > Allyl > Methyl > Vinyl. Thus, option (b) corresponds to this order: (1) > (2) > (3) > (4).

Key concepts

Resonance Stabilization

Resonance stabilization is a key factor that enhances the stability of free radicals. It occurs when the unpaired electron of a radical can be delocalized over two or more adjacent atoms, spreading the radical character through alternate structures called resonance forms.
This delocalization reduces the energy and increases stability. Some common examples include benzyl and allyl radicals.

• Benzyl radicals benefit significantly from resonance as their unpaired electron can disperse into the aromatic ring.
• Allyl radicals also enjoy stabilization, but with fewer resonance forms than benzyl radicals, making them moderately stable.

Resonance is not present in methyl or vinyl radicals, which makes them more reactive and less stable.

Hyperconjugation

Hyperconjugation is another mechanism that affects the stability of free radicals. This effect involves the overlap of sigma bonds (specifically C-H or C-C bonds) with the p-orbital containing the unpaired electron.
In radicals like methyl, hyperconjugation provides some stability by allowing electron density from adjacent C-H bonds to stabilize the unpaired electron.

• Methyl radicals rely primarily on hyperconjugation for stability, though this effect is relatively weak compared to resonance.

Vinyl radicals, having double bonds and an sp2 configuration, do not benefit from hyperconjugation, contributing to their higher instability.

Vinyl Radical

Vinyl radicals are highly unstable due to their specific structural features. Positioned on an sp2 hybridized carbon next to a double bond, these radicals lack both resonance stabilization and significant hyperconjugation opportunities.

• The double bond's rigidity diminishes the flexibility needed for significant hyperconjugation.
• The absence of resonance forms means there is no way to distribute the radical electron across multiple atoms, leaving it highly reactive.

Benzyl Radical

The benzyl radical is an exceptionally stable radical, thanks to resonance. The unpaired electron can delocalize over the aromatic ring, spreading through multiple resonance structures.

• This delocalization significantly lowers energy, leading to increased stability.
• With a large number of resonance forms, benzyl radicals can efficiently stabilize the radical electron, making it the most stable among the discussed radicals.

Allyl Radical

Allyl radicals are an interesting case where resonance plays a stabilizing role, though not as efficiently as in benzyl radicals. These radicals involve an electron delocalized over a region involving a double bond and an adjacent p orbital, forming three resonance structures.

• While more stable than simple alkyl radicals, they do not achieve the extensive delocalization possible in benzyl radicals.
• This makes allyl radicals moderately stable.

Methyl Radical

Methyl radicals are simple alkyl radicals with a minimal degree of stability. Without resonance opportunities, they rely solely on hyperconjugation for stabilization.

• The stability gained from adjacent C-H bonds is modest, preventing the methyl radical from achieving the stability of its more conjugated counterparts.

Overall, methyl radicals are more reactive and less stable than radicals with additional stabilization mechanisms like resonance.