Question

Which of the following halogen derivative forms a poisonous phosgene, when come in contact with air and light?

• A. \(\text{CHCl}_3\)
• B. \(\text{PCl}_3\)
• C. \(\text{C}_6\text{H}_5\text{Cl}\)
• D. \(\text{CH}_2\text{Cl}_2\)

Hint:

Chloroform is stored in dark bottles because it forms phosgene.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Chloroform is slowly oxidised by air in the presence of light. It forms carbonyl chloride, which is called phosgene and is very poisonous.

Step 2: Reaction:
\[ 2CHCl_3 + O_2 \xrightarrow{\text{light}} 2COCl_2 + 2HCl \]

Step 3: Why chloroform is stored carefully:
To stop this, chloroform is kept in dark brown bottles filled to the brim, and about 1 percent ethanol is added. The ethanol turns phosgene into harmless ethyl carbonate.

Step 4: Check the others:
\(PCl_3\) is not a halogen derivative of a hydrocarbon. Chlorobenzene and dichloromethane do not form phosgene this way. So the answer is \(CHCl_3\), option (A).

Final Answer:
Chloroform forms phosgene in air and light. \[ \boxed{A:\ \text{CHCl}_3} \]