Final answer:
The monoiodination occurs due to the initial activation of toluene for electrophilic substitution, followed by the deactivating effect of the iodine atom that prevents further iodination. This is conceptually different from, but often discussed alongside, reversible reactions such as the formation of hydrogen iodide, which illustrates the concept of chemical equilibrium.
Step-by-step explanation:
The monoiodination of toluene (PhCH3) to produce benzyl iodide (PhCHI) can be attributed to the relative activation and deactivation properties of the substituents involved in the reaction. Toluene is an activated aromatic compound, which makes it more reactive towards electrophilic substitution reactions such as halogenation. However, after the introduction of one iodine atom, the resulting benzyl iodide becomes deactivated due to the electron-withdrawing nature of the iodine atom. This deactivation reduces the compound's reactivity towards further iodination, resulting in monoiodination as the primary outcome.
Additionally, this reaction is also subject to chemical equilibrium considerations. The reaction of hydrogen and iodine to form hydrogen iodide (H2 + I2 ⇔ 2HI) is often used as a classic example of a reversible reaction reaching chemical equilibrium. This balance of forward and reverse reactions is characterized by the reaction rates becoming equal, resulting in constant concentrations of reactants and products over time.