Question

Diphenyl acetylene $\xrightarrow{\begin{smallmatrix} 1. O_3 \\ 2. Zn \end{smallmatrix}} P \xrightarrow{\begin{smallmatrix} 1. Conc. KOH \\ 2. H^+ \end{smallmatrix}} Q \xrightarrow{H_3O^+} R$

Answer 1

• P: Benzil ($\text{PhCO–COPh}$)
• Q: Benzilate ion ($\text{Ph}_2\text{C(OH)COO}^-$)
• R: Benzilic acid ($\text{Ph}_2\text{C(OH)COOH}$)

Answer 2

Reaction Sequence:

1. Ozonolysis Reaction:
   Diphenyl acetylene, $\text{Ph–C}\equiv\text{C–Ph}$, when treated with ozone followed by reductive workup (Zn), cleaves the triple bond to produce a 1,2-diketone. This product is benzil:

   $$\textbf{P}: \; \text{PhCO–COPh}$$

2. Benzilic Acid Rearrangement:
   On treatment of benzil with concentrated KOH (followed by acidification), a rearrangement occurs (the benzilic acid rearrangement). In the process, the benzil is converted to the benzilate ion which, on further protonation, gives benzilic acid.
   Thus,

   $$\textbf{Q}: \; \text{Ph}_2\text{C(OH)COO}^- \quad \text{(benzilate ion)}$$ $$\textbf{R}: \; \text{Ph}_2\text{C(OH)COOH} \quad \text{(benzilic acid)}$$

Summary:

• Ozonolysis of diphenyl acetylene gives benzil.
• Benzil undergoes benzilic acid rearrangement with conc. KOH and acid to yield benzilic acid.