Animated Solution for Chemistry - Organic Chemistry: Identify the reagent(s) 'A' and condition(s) for the reaction:

Imagine you are looking at a chemical transformation where a molecule of cyclohexene is converted into a dichlorinated product. The first thing you should notice is the position of the newly added chlorine atoms. They are not attached directly to the carbons of the double bond. Instead, they are bonded to the carbons immediately adjacent to the double bond. These specific positions are known as allylic positions.

Because the double bond remains completely intact throughout the reaction, we can immediately rule out any standard addition reactions. This is a classic example of allylic halogenation, a process that specifically targets these adjacent carbons without destroying the alkene's pi bond.

To achieve allylic halogenation, we need a reaction pathway that avoids electrophilic addition. The key to this is the free radical substitution mechanism.

Let's evaluate the given options. If we were to use $\text{HCl}$, it would act as an electrophile and add across the double bond, which is not what we want. Using ${\text{Cl}}_2$ with anhydrous ${\text{AlCl}}_3$ in the dark creates a strong electrophile (${\text{Cl}}^{+}$) intended for electrophilic aromatic substitution on benzene rings, which is also incorrect here.

What we truly need is a way to generate free radicals. The combination of ${\text{Cl}}_2$ and UV light is the perfect recipe for this. Under UV light, the chlorine molecule undergoes homolytic cleavage:

Once the highly reactive chlorine radicals are formed, the propagation phase begins. A chlorine radical abstracts a hydrogen atom from the allylic carbon of cyclohexene. This leaves behind an allylic free radical, which is exceptionally stable due to resonance delocalization with the adjacent double bond.

This stable allylic radical then reacts with another ${\text{Cl}}_2$ molecule, forming 3-chlorocyclohexene and regenerating a chlorine radical to continue the chain. However, the reaction doesn't stop there! Cyclohexene has two equivalent allylic positions. The same free radical mechanism repeats at the second allylic carbon, ultimately yielding our major product: 3,6-dichlorocyclohexene.

Thus, the correct reagent and condition is ${\text{Cl}}_2$ in the presence of UV light.