Stability of Conjugated Intermediates: Videos & Practice Problems

Conjugation stabilizes carbocations by allowing the positive charge to be delocalized over multiple atoms through resonance. This delocalization reduces the electron deficiency at the carbocation center, making it more stable. For example, a primary allylic carbocation is more stable than a typical primary carbocation because the positive charge can resonate with the adjacent double bond. Similarly, a benzylic carbocation is stabilized by resonance with the aromatic ring. The general trend is that tertiary carbocations are the most stable due to hyperconjugation, but conjugated carbocations like allylic and benzylic are also significantly stabilized by resonance.

Resonance has a more pronounced effect on the stability of radicals compared to carbocations. While carbocations are stabilized by both hyperconjugation and resonance, radicals are primarily stabilized by resonance. This is because the unpaired electron in a radical can be delocalized over multiple atoms, reducing its reactivity. Allylic and benzylic radicals are particularly stable due to their ability to resonate. In contrast, the stability of carbocations is more influenced by the number of alkyl groups attached, with tertiary carbocations being the most stable due to hyperconjugation. Therefore, resonance plays a more critical role in stabilizing radicals than carbocations.

Allylic and benzylic positions are particularly stable for radicals due to the extensive resonance stabilization they provide. In an allylic radical, the unpaired electron can be delocalized over the π-system of the adjacent double bond, spreading the electron density over multiple atoms. Similarly, in a benzylic radical, the unpaired electron can resonate with the aromatic ring, distributing the electron density across the ring. This delocalization significantly lowers the energy of the radical, making it more stable. As a result, radicals at allylic and benzylic positions are among the most stable types of radicals.

The stability of conjugated intermediates significantly influences reaction pathways by determining the most favorable sites for reactions to occur. For example, in reactions involving carbocations or radicals, the formation of intermediates at allylic or benzylic positions is preferred due to their enhanced stability from resonance. This stability can lead to specific reaction mechanisms, such as nucleophilic attacks on carbocations or radical termination steps at these positions. Understanding the stability trends of conjugated intermediates helps predict the most likely reaction pathways and products in organic synthesis, making it a crucial concept in organic chemistry.

Hyperconjugation and resonance are two different mechanisms that stabilize carbocations. Hyperconjugation involves the delocalization of electrons from adjacent σ-bonds (typically C-H or C-C bonds) to the empty p-orbital of the carbocation, which helps to disperse the positive charge. This effect is most pronounced in tertiary carbocations, where multiple alkyl groups provide extensive hyperconjugation. In contrast, resonance stabilization occurs when the positive charge is delocalized over a π-system, such as in allylic or benzylic carbocations. Resonance allows the positive charge to be shared among multiple atoms, reducing its intensity. Both mechanisms increase carbocation stability, but they operate through different electron delocalization processes.