Propose mechanisms to account for the observed products in the following reactions.
(b)

Verified step by step guidance

1. Step 1: Recognize that the reaction involves allylic bromination using NBS (N-Bromosuccinimide) under light (hv). NBS is commonly used to selectively brominate allylic or benzylic positions in the presence of a double bond.
2. Step 2: Initiation phase: Light (hv) causes homolytic cleavage of the N-Br bond in NBS, generating a bromine radical (Br•). This radical is highly reactive and initiates the reaction.
3. Step 3: Propagation phase: The bromine radical abstracts a hydrogen atom from the allylic position of the cyclohexene molecule, forming an allylic radical. The allylic radical is stabilized by resonance, creating two possible resonance structures.
4. Step 4: Bromine from NBS reacts with the allylic radical to form the brominated products. The resonance structures of the allylic radical lead to two different products: (1) 3-bromocyclohexene and (2) 1-bromocyclohexene.
5. Step 5: Termination phase: Any remaining radicals combine to form stable molecules, completing the reaction. The observed products are the result of the selective bromination at the allylic positions due to the resonance stabilization of the intermediate radical.

Key Concepts

Allylic Bromination

Allylic bromination is a reaction where bromine is added to the allylic position of an alkene or alkane. This process typically involves the formation of a radical intermediate, allowing for the substitution of a hydrogen atom with a bromine atom. The reaction is often facilitated by reagents like N-bromosuccinimide (NBS) in the presence of light, which generates bromine radicals that can abstract hydrogen atoms from the allylic position.

Radical Mechanism

The radical mechanism involves the formation and reaction of free radicals, which are highly reactive species with unpaired electrons. In the context of allylic bromination, the mechanism proceeds through three main steps: initiation (formation of bromine radicals), propagation (reaction of radicals with the substrate), and termination (recombination of radicals). Understanding this mechanism is crucial for predicting the products of the reaction and the regioselectivity of bromination.

Regioselectivity

Regioselectivity refers to the preference of a chemical reaction to yield one structural isomer over others when multiple possibilities exist. In allylic bromination, the regioselectivity is influenced by the stability of the radical intermediates formed during the reaction. More stable radicals, such as those formed at more substituted carbon centers, are favored, leading to specific products. This concept is essential for predicting the distribution of products in the reaction.