Animated Solution for Chemistry - Organic Chemistry: The major product of the following reaction is

Imagine you are in a laboratory, and you are handed a flask containing p-chlorotoluene. This molecule is our starting point. It consists of a central benzene ring, a methyl group ($-{\text{CH}}_3$) at the top, and a chlorine atom ($-\text{Cl}$) at the bottom.

When we look at this molecule, we must ask ourselves: where will the reaction take place? The chlorine atom attached directly to the benzene ring is exceptionally stable. Due to resonance, the carbon-chlorine bond acquires partial double-bond character, making it incredibly tough to break.

Therefore, the spotlight shifts to the benzylic methyl group. The benzylic position is highly reactive, especially under free radical conditions, because any resulting radical is stabilized by the adjacent aromatic ring.

Photochlorination

The first set of reagents given is ${\text{Cl}}_2$ in the presence of sunlight ($h u$). This is the classic recipe for free radical halogenation.

When UV light hits the chlorine molecule, it undergoes homolytic cleavage, generating highly reactive chlorine free radicals. These radicals will attack the weakest C-H bonds in our molecule, which are the benzylic C-H bonds.

But here is where we must think ahead. The reaction could theoretically replace one, two, or all three hydrogen atoms on the methyl group. To determine the correct path, we must look at the second step of the reaction sequence.

The second step involves hydrolysis with water (${\text{H}}_2\text{O}$) and heat ($\Delta$). In industrial and laboratory syntheses, when toluene derivatives are subjected to photochlorination followed by hydrolysis to yield a specific major product, the reaction is typically controlled to form a gem-dihalide intermediate.

This means two hydrogen atoms are replaced by two chlorine atoms, forming p-chlorobenzal chloride ($-{\text{CHCl}}_2$).

If only one chlorine had substituted, hydrolysis would yield an alcohol. If three chlorines had substituted, hydrolysis would yield a carboxylic acid. Given the options and standard reaction pathways, the di-chlorination route is the intended mechanism here.

Now, we introduce water and heat. This triggers a nucleophilic substitution reaction. The water molecules act as nucleophiles, attacking the benzylic carbon and displacing the two chlorine atoms.

The result is an intermediate where two hydroxyl ($-\text{OH}$) groups are attached to the exact same carbon atom. This structure is known as a gem-diol.

I know it looks like we are making progress, but there is a catch here. Gem-diols are notoriously unstable. The two bulky, electron-rich oxygen atoms repel each other strongly due to steric hindrance and electrostatic repulsion.

The Dehydration

Because of this immense instability, the gem-diol cannot exist for long. It spontaneously undergoes dehydration, meaning it loses a molecule of water ($-{\text{H}}_2\text{O}$).

As the water molecule leaves, a double bond forms between the carbon and the remaining oxygen atom, creating a stable carbonyl group ($\text{C=O}$).

This elegant transformation leaves us with our final product: p-chlorobenzaldehyde. The methyl group has been successfully oxidized to an aldehyde group, while the aryl chlorine remains completely untouched. Therefore, the correct option is (d).