In the following allylic radicals, identify the carbon where the new C–Br bond is most likely to form in the second propagation step.
(a)

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Identify the structure of the allylic radical. The image shows two resonance structures of an allylic radical, where the unpaired electron is delocalized over the allylic position.

Understand the concept of resonance. Resonance in organic chemistry refers to the delocalization of electrons across adjacent atoms, which stabilizes the molecule. In allylic radicals, the unpaired electron can be shared between the two carbon atoms adjacent to the double bond.

Determine the most stable position for the new bond formation. In the case of allylic radicals, the new bond is most likely to form at the carbon atom where the radical is most stable due to resonance stabilization.

Consider the electronic effects. The formation of the C–Br bond will occur at the carbon atom that can best stabilize the positive charge that results from the radical's electron pairing with the bromine atom.

Select the carbon atom for bond formation. Based on resonance structures, the new C–Br bond is most likely to form at the carbon atom that is part of the more stable resonance structure, which is typically the less substituted carbon in the allylic position.

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주요 개념

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Allylic Radical Stability

Allylic radicals are stabilized by resonance, where the unpaired electron is delocalized over adjacent pi bonds. This delocalization increases the stability of the radical, making allylic positions favorable sites for reactions. Understanding this stability is crucial for predicting where new bonds will form in radical reactions.

Resonance Structures

Resonance structures depict the delocalization of electrons within a molecule, showing different possible configurations. In the case of allylic radicals, resonance allows the radical to be distributed over multiple carbon atoms, influencing the site of bond formation. Recognizing resonance structures helps in identifying the most reactive sites in a molecule.

Radical Propagation Steps

Radical propagation involves the transfer of an unpaired electron to form new bonds, typically in a chain reaction. In the second propagation step, the radical reacts with another molecule, such as Br2, to form a new C–Br bond. Understanding the mechanism of propagation steps is essential for predicting the outcome of radical reactions.

In the following allylic radicals, identify the carbon where the new C–Br bond is most likely to form in the second propagation step.(a)