Question

When toluene $\left(C_6{H}_5{\mathrm{CH}}_3\right)$reacts with chlorine gas in the presence of an iron (III) catalyst, the product is a mixture of the ortho and para isomers of${\mathbf{C}}_{\mathbf{6}}{\mathbf{H}}_{\mathbf{4}}{\mathbf{ClCH}}_{\mathbf{3}}$ . However, when the reaction is light-catalyzed with Fe⁺³ no catalyst present, the product is ${\mathbf{C}}_{\mathbf{6}}{\mathbf{H}}_{\mathbf{5}}{\mathbf{CH}}_{\mathbf{2}}\mathbf{Cl}$. Explain.

Short answer

Iron ion increases the reactivity of hydrogen atoms that are part of the benzene ring.

Step-by-step solution

Explanation

Hydrogen atoms are attached directly to the carbon atom, which is a part of the benzene ring. These are non-reactive in the presence of light without iron ions as a catalyst. In that case, one hydrogen atom from the methyl group outside the benzene ring (more reactive hydrogen) will be replaced by the chlorine atom. With iron ion as a catalyst, a hydrogen attached to the carbon atom, that is a part of the benzene ring, will be replaced with a chlorine atom. Because the presence of iron increases the reactivity of hydrogens attached to carbon atoms that are part of the benzene ring.

Hence Iron ion increases the reactivity of hydrogen atoms that are part of the benzene ring.