Question

Benzene on ozonolysis followed by reaction with Zn + H₂O gives:

• A. 3 moles of glycerol
• B. 3 moles of glyoxal
• C. 3 moles of glyoxalic acid
• D. 3 moles of acetylene

Hint:

Ozonolysis of aromatic compounds like benzene is a powerful method to cleave the entire aromatic ring. The type of workup (reductive or oxidative) determines the final product. A reductive workup (e.g., $\text{Zn}/\text{H}_2\text{O}$ or $\text{Me}_2\text{S}$) yields aldehydes, while an oxidative workup (e.g., $\text{H}_2\text{O}_2$) yields carboxylic acids.

Answer 1

The Correct Option is B

Step 1: Understanding Ozonolysis of Benzene.
Ozonolysis is an organic reaction where unsaturated compounds (alkenes, alkynes, or aromatic compounds) are cleaved by ozone (O₃). For aromatic compounds like benzene, ozonolysis involves the breaking of all three double bonds in the ring. The reaction is typically followed by a reductive workup, usually with zinc dust and water (Zn + H₂O) or dimethyl sulfide, to prevent further oxidation of the products.
Step 2: Mechanism of Benzene Ozonolysis.
Benzene has a cyclic structure with three alternating double bonds. When benzene undergoes ozonolysis, each carbon-carbon double bond is broken, and an ozonide intermediate is formed.
Benzene reacts with 3 moles of ozone (O₃) to form a triozonide.
Step 3: Reductive Workup with Zn + H₂O.
The triozonide intermediate is then treated with a reductive workup, typically Zn + H₂O. This reductive cleavage prevents the oxidation of aldehydes to carboxylic acids. Each fragment of the benzene ring that contained a C=C bond and was part of the ozonide will be converted into an aldehyde.
Specifically, each of the three carbon-carbon double bonds of the benzene ring is cleaved, and each fragment forms a dialdehyde. The product formed is glyoxal.
The overall reaction is:
C₆H₆ ⟶ _{1. 3O3} Triozonide ⟶ _{2. Zn/H2O} 3 OHC-CHO
The product, OHC-CHO, is glyoxal. Since benzene has 6 carbon atoms and 3 double bonds, and each glyoxal molecule has 2 carbon atoms, 3 moles of glyoxal are produced from 1 mole of benzene.
Step 4: Evaluate the options.

• Option (A): 3 moles of glycerol: Glycerol is a triol (CH₂OH-CHOH-CH₂OH). Ozonolysis yields aldehydes or ketones, not alcohols directly.
• Option (B): 3 moles of glyoxal: Glyoxal (OHC-CHO) is a dialdehyde formed by the cleavage of each C₂ unit of the benzene ring. This is the correct product.
• Option (C): 3 moles of glyoxalic acid: Glyoxalic acid (OHC-COOH) would be formed if the workup were oxidative (e.g., with H₂O₂), which would oxidize one aldehyde group to a carboxylic acid. With Zn + H₂O, it's a reductive workup, so aldehydes are the final products.
• Option (D): 3 moles of acetylene: Acetylene (CH≡CH) is an alkyne. Ozonolysis leads to cleavage of double or triple bonds and formation of carbonyl compounds, not other unsaturated hydrocarbons.

Step 5: Conclusion.
Benzene on ozonolysis followed by reductive workup with Zn + H₂O yields 3 moles of glyoxal.
\[ \boxed{\text{3 moles of glyoxal}} \]