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Chapter 15: Q54. (page 570)

Question: Draw a stepwise mechanism for the following reaction.

Short Answer

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Answer

Mechanism:

Step by step solution

01

N-bromosuccinimide

N-bromosuccinimide is utilized for “selective halogenation” at the allylic carbon, i.e., C-H bond, in the presence of light.

02

Mechanism 

Initiation:

In the presence of light, a weak Na-Br bond is NBS is homolytically cleaved, and the bromine radical is formed.

Initiation reaction

Propagation and Termination:

The bromine radical formed in the initiation step and a proton is abstracted from the C-H bond, and an allylic radical is formed. This radical can undergo resonance.

The allylic radical reacts with Br2 to form the C-Br bond.

The propagation step can occur multiple times.

When two radicals combine, a stable product is formed, which terminates the reaction.

Propagation and termination reactions

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Most popular questions from this chapter

Question: Draw a stepwise mechanism for the following addition reaction to an alkene.

Question: When 3,3-dimethylbut-1-ene is treated with HBr alone, the major product is 2-bromo-2,3- dimethylbutane. When the same alkene is treated with HBr and peroxide, the sole product is 1-bromo-3,3-dimethylbutane. Explain these results by referring to the mechanisms.

Question: When HBr adds to (CH3)2C=CH2 under radical conditions, two radicals are possible products in the first step of chain propagation. Draw the structure of both radicals and indicate which one is formed. Then draw the preferred product from HBr addition under radical conditions.

Question: Draw the structure of the four allylic halides formed when 3-methylcyclohexene undergoes allylic halogenation with NBS + hν.

Question: Rosmarinic acid is an antioxidant isolated from rosemary. Draw resonance structures for the radical that results from removal of the labeled H atom in rosmarinic acid.
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