What do NBS and CCl4 do?

NBS (N-bromosuccinimide) in CCl4 (carbon tetrachloride) solution, when combined with light, selectively brominates alkenes at the allylic position (the carbon atom adjacent to the double bond).

Detailed Explanation:

This reaction is a method for introducing a bromine atom at the allylic position of an alkene. Here's a breakdown of the roles of each component:

NBS (N-Bromosuccinimide): NBS serves as a source of low concentration of bromine (Br₂). It reacts to continuously generate bromine, ensuring a consistent and controlled supply for the reaction. This prevents a high concentration of Br₂ which would lead to unwanted addition reactions across the double bond.
CCl4 (Carbon Tetrachloride): CCl4 is an inert solvent. It's used because it's non-polar and dissolves both the NBS and the alkene. Importantly, it doesn't participate in the reaction itself but provides a suitable medium. It's worth noting that due to toxicity concerns, CCl4 is often replaced by less harmful solvents like dichloromethane (CH₂Cl₂) in modern applications.
Light (or Heat): Light, or sometimes heat, is used to initiate the reaction. It helps to homolytically cleave Br₂ into bromine radicals (Br•), which are necessary for the free-radical mechanism.

Reaction Mechanism:

The reaction proceeds through a free-radical mechanism:

Initiation: Light or heat breaks down a small amount of Br₂ into bromine radicals (Br•).
Propagation:
- A bromine radical abstracts an allylic hydrogen, forming an allylic radical.
- The allylic radical reacts with NBS to form the allylic bromide and regenerates a bromine radical, continuing the chain reaction.
Termination: Two radicals combine, terminating the chain reaction.

Summary:

In essence, NBS and CCl4, in the presence of light, provide a way to selectively substitute a hydrogen atom at the allylic position of an alkene with a bromine atom, creating an allylic bromide. The NBS acts as a source of bromine, CCl4 as an inert solvent, and light to initiate the free-radical mechanism.