Alkylbenzenes such as toluene (methylbenzene) react with NBS...

Name: Problem 29, Chapter 10: Organic Chemistry
Uploaded: 2020-07-01T06:54:08-08:00
Duration: 2 min 23 s
Channel: Tom Rutherford
Description: Alkylbenzenes such as toluene (methylbenzene) react with NBS to give products in which bromine substitution has occurred at the position next to the aromatic ring (the benzylic position). Explain, based on the bond dissociation energies in Table 6.3 on page 204.(FIGURE CANNOT COPY)

Alkylbenzenes such as toluene (methylbenzene) react with NBS to give products in which bromine substitution has occurred at the position next to the aromatic ring (the benzylic position). Explain, based on the bond dissociation energies in Table 6.3 on page 204.(FIGURE CANNOT COPY)

Key Concepts

Role of N-Bromosuccinimide (NBS) in Bromination

NBS is a brominating agent frequently used in radical bromination reactions, particularly for allylic and benzylic positions. Under the reaction conditions, NBS serves as a source of bromine radicals in a controlled manner, ensuring that bromine substitution occurs selectively at positions where the C-H bond dissociation energy is lower, such as the benzylic position in alkylbenzenes.

Free Radical Halogenation Mechanism

The free radical halogenation mechanism involves initiation, propagation, and termination steps. In the initiation step, radicals are generated (often through exposure to light or heat), which then abstract a hydrogen atom to form a more stable radical at a reactive site (like the benzylic position). The propagation step continues as the formed radical reacts with a halogen source to yield the halogenated product while regenerating another radical, allowing the chain reaction to proceed.

Bond Dissociation Energy

Bond dissociation energy refers to the amount of energy required to break a bond homolytically. In the context of alkylbenzenes, the benzylic C-H bonds have lower bond dissociation energies compared to typical alkane C-H bonds, meaning they are more easily broken. This concept is crucial to understand why the benzylic position is selectively reactive under radical conditions, as less energy is needed to generate the intermediate radicals.

Benzylic Radical Stabilization

When a hydrogen atom is abstracted from the benzylic position, a benzyl radical is formed. This radical is especially stabilized by resonance delocalization, where the unpaired electron is spread over the aromatic ring. The enhanced stability of the benzylic radical lowers the activation energy for its formation and makes the benzylic hydrogen more susceptible to abstraction compared to other C-H bonds.

Please give Ace some feedback