Consider the reaction coordinate diagram for a standard electrophilic aromatic substitution. Let be — Aromatic Hydrocarbons Chemistry Question
Question
Consider the reaction coordinate diagram for a standard electrophilic aromatic substitution. Let $\Delta G^\ddagger_1$ be the free energy of activation for the formation of the arenium ion, and $\Delta G^\ddagger_2$ be the free energy of activation for the loss of a proton from the arenium ion. Which of the following relationships is generally true for non-reversible EAS reactions?
💡 Solution & Explanation
The formation of the arenium ion requires breaking the stable aromatic sextet, making this first step highly endothermic with a correspondingly large free energy of activation ($\Delta G^\ddagger_1$). The second step restores aromaticity, making it highly exothermic with a much lower activation barrier ($\Delta G^\ddagger_2$). Thus, $\Delta G^\ddagger_1 > \Delta G^\ddagger_2$.