Question

Chloroform oxidizes in the presence of light to

• A. Phosgene
• B. Carbon tetrachloride
• C. Freons
• D. Methylene chloride

Hint:

Chloroform + \(\text{O}_2\) + sunlight \(\rightarrow\) Phosgene (\(\text{COCl}_2\)) + \(\text{HCl}\).
Amber bottles keep light out, and adding \(1\%\) ethanol converts any phosgene into harmless diethyl carbonate.

Answer 1

The Correct Option is A

Concept:
Trihalomethanes are susceptible to radical-induced photo-oxidation upon prolonged exposure to atmospheric air and light.
For chloroform, this photo-oxidation generates a toxic, suffocating gas that poses significant safety and handling concerns.

Step 1: Mechanism of Photo-Oxidation:
When chloroform (\(\text{CHCl}_3\)) is exposed to atmospheric oxygen in the presence of light, it undergoes homolytic cleavage and auto-oxidation via a free-radical chain reaction.
The balanced chemical equation representing this transformation is:
\[ 2\text{CHCl}_3 + \text{O}_2 \xrightarrow{h\nu} 2\text{COCl}_2 + 2\text{HCl} \] The principal oxidation product obtained is carbonyl chloride, commonly known as phosgene (\(\text{COCl}_2\)).

Step 2: Chemical Properties of Phosgene and Precautions:
Phosgene is a toxic gas that can cause severe pulmonary edema and respiratory failure.
Because of this hazard, chloroform used for medicinal or laboratory applications is stored in completely filled, dark amber-colored bottles to exclude air and prevent light penetration.
Additionally, approximately \(1\%\) ethanol is added to react with any phosgene formed, converting it into harmless diethyl carbonate:
\[ \text{COCl}_2 + 2\text{C}_2\text{H}_5\text{OH} \rightarrow (\text{C}_2\text{H}_5\text{O})_2\text{C}=\text{O} + 2\text{HCl} \]

Step 3: Verification of Options:
The direct oxidation product is carbonyl chloride (phosgene).
Compounds such as carbon tetrachloride (\(\text{CCl}_4\)), freons (\(\text{CF}_2\text{Cl}_2\)), and methylene chloride (\(\text{CH}_2\text{Cl}_2\)) are produced through alternative synthetic pathways and not by the oxidation of chloroform.
Final Answer:
Thus, chloroform oxidizes in the presence of light to form phosgene, which corresponds to option (A).