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

When toluene reacts with chlorine gas in the presence of iron(III) catalyst, the products are a mixture of ortho and para isomers of \(C_6H_4ClCH_3\). The reaction is: \[C_6H_5CH_3 + Cl_2 \xrightarrow{FeCl_3} C_6H_4ClCH_3 + HCl\] However, when the reaction is light-catalyzed with no iron(III) catalyst present, the product is \(C_6H_5CH_2Cl\). The reaction is: \[C_6H_5CH_3 + Cl_2 \xrightarrow{h\nu} C_6H_5CH_2Cl + HCl\]

When there is an iron(III) catalyst present, the reaction involves an electrophilic aromatic substitution mechanism. This is a reaction where an electrophile attacks the pi system of the aromatic ring, and the resulting intermediate is stabilized by delocalization of the positive charge. Finally, the aromaticity is restored when one of the hydrogen atoms of the ring is removed along with its incoming negative charge. On the other hand, when light-catalysis is used without iron(III) catalyst, the reaction mechanism is different. In this case, it is a radical substitution called a free radical halogenation. The light induces the formation of radicals, and these radicals react with the toluene in a less-selective way.

Electrophilic aromatic substitution with iron(III) catalyst yields a mixture of ortho and para isomers due to the directing effect of the methyl group, as it donates electron density into the benzene ring, making it more reactive. By introducing iron(III) catalyst, the regioselectivity is affected, leading to predominantly ortho and para substitution. In the case of light-catalysis, free radical halogenation takes place. Light generates chlorine radicals, which then abstract one hydrogen atom from the methyl group of toluene, forming a stable benzyl radical. Afterward, the benzyl radical reacts with the chlorine molecule to form \(C_6H_5CH_2Cl\) and the regeneration of a chlorine radical. This reaction does not involve the benzene ring and thus has different selectivity compared to the reaction involving the iron(III) catalyst. In conclusion, the two different reaction conditions, presence of iron(III) catalyst and light-catalysis without iron(III) catalyst result in different mechanisms and products due to the directing effects and different selectivities associated with electrophilic aromatic substitution and free radical halogenation mechanisms, respectively.