Q35E - Organic Chemistry (Mcmurry)

Question

Benzene and alkyl-substituted benzenes can be hydroxylated by reaction with ${\mathbf{H}}_{\mathbf{2}}{\mathbf{O}}_{\mathbf{2}}$ in the presence of an acidic catalyst. What is the structure of the reactive electrophile? Propose a mechanism for the reaction.

Step-by-Step Solution

Answer

The proposed mechanism is shown below

Step1: Reaction of hydrogen peroxide in presence of an acid catalyst

When the reaction of hydrogen peroxide in the presence of an acid catalyst leads to the formation of $\mathrm{HO}-\stackrel{+}{\mathrm{O}}{\mathrm{H}}_2$ , carbocation acts as an electrophile in reaction with the benzene.

Step 2: Reaction of benzene with an electrophile

When the dichloro carbocation reacts with benzene, one of the double bonds of the aromatic ring breaks and the addition of $\mathrm{HO}-\stackrel{+}{\mathrm{O}}{\mathrm{H}}_2$ occurs at the ring, followed by hydrolysis which protonates the hydrogen and the ring regains its aromaticity and the formation of phenol occurs.

Step 3: Mechanism

Mechanism for the formation of electrophiles:

1. Addition of electrophile to a benzene ring.
2. Hydrolysis for deprotonation of hydrogen.

      3.Formation of phenol occurs as shown below

Formation of electrophile

Mechanism