Chapter 22, Problem 22.58P

Interpretation Introduction

Interpretation:

Benzene can be hydroxylated by treating it with hydrogen peroxide and a strong acid such as trifluoromethanesulfonic acid ($\text{TfOH}$). A mechanism for given reaction is to be proposed.

Concept introduction:

Aromatic species tend to react with electrophiles (${\text{E}}^{+}$) in an electrophilic aromatic substitution reaction instead of an addition reaction because electrophilic aromatic substitution preserves aromaticity. Benzene consists of two steps in the overall mechanism for an electrophilic aromatic substitution reaction. First, ${\text{E}}^{+}$ undergoes electrophilic addition to the aromatic ring to produce an arenium ion intermediate, which temporarily destroys the ring’s aromaticity. The first step of electrophilic aromatic substitution that is the addition of the electrophile is slow, so it is the rate-determining step of the reaction. Thus, the reaction is first order with respect to both the aromatic ring and the electrophile. Second, a proton (${\text{H}}^{+}$) undergoes electrophile elimination, which restores aromaticity and results in a substituted aromatic ring. ${\text{H}}_{\text{2}}{\text{O}}_{\text{2}}$ and $\text{TfOH}$ reacts in a proton transfer reaction to form ${\text{HOOH}}_{\text{2}}{}^{\text{+}}$, which acts as the electrophile to add $\text{HO}$ to the benzene ring. ${\text{TfO}}^{\text{-}}$ acts as the base to regenerate the aromatic ring.