Q 4P - Organic Chemistry (Mcmurry)

Question

Question: When benzene is treated with ${\mathbf{D}}_{\mathbf{2}}{\mathbf{SO}}_{\mathbf{4}}$, deuterium slowly replaces all six hydrogens in the aromatic ring. Explain.

Step-by-Step Solution

Answer

Step 1: Aromatic sulfonation

Aromatic sulphonation is done by reacting the aromatic ring with a mixture of ${\mathbf{H}}_{\mathbf{2}}{\mathbf{SO}}_{\mathbf{4}}$ and ${\mathbf{SO}}_{\mathbf{3}}$ .The reactive electrophile is ${\mathbf{HSO}}_{\mathbf{3}}^{\mathbf{+}}$or ${\mathbf{SO}}_{\mathbf{3}}$neutral based on the reaction conditions. Sulfonation happens in the presence of strong acid.

Step 2: Treatment of benzene with D 2 SO 4

Benzene can be protonated by strong acids. The resulting intermediate loses either deuterium or hydrogen. If hydrogen is lost, deuterated benzene is formed as the product. The attack by deuterium happens at all positions and leads to the replacement of all hydrogens by deuterium. The reaction can be shown as:

                                         Reaction of Benzene with ${\mathrm{D}}_2{\mathrm{SO}}_4$