Question

The compound added to prevent chloroform to form phosgene gas is:
A. \[{{\rm{C}}_{\rm{2}}}{{\rm{H}}_{\rm{5}}}{\rm{OH}}\]
B. \[{\rm{C}}{{\rm{H}}_3}{\rm{COOH}}\]
C. \[{\rm{C}}{{\rm{H}}_3}{\rm{COC}}{{\rm{H}}_3}\]
D. \[{\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{OH}}\]

Answer 1

Hint: Chloroform reacts with oxygen initiated by light to form a highly toxic gas, carbonyl chloride. To prevent the chloroform from reacting with oxygen it is kept in dark bottles.

Complete Step by Step Solution:
Carbonyl chloride is a poisonous gas which is more commonly known as phosgene. The reaction of chloroform and oxygen is initiated by light to form carbonyl chloride along with another product, hydrochloric acid. Phosgene exists as a colourless gas at room temperature. Phosgene has a suffocating odour at high concentrations and at lower concentrations, it has a smell of green corn or like new mown hay.

The balanced chemical equation for the reaction is as shown below:
\[{\rm{2CHC}}{{\rm{l}}_{\rm{3}}} + {{\rm{O}}_{\rm{2}}} \xrightarrow[air]{light} {\rm{2COC}}{{\rm{l}}_{\rm{2}}} + {\rm{2HCl}}\]
Here, \[{\rm{CHC}}{{\rm{l}}_{\rm{3}}}\] = chloroform
\[{{\rm{O}}_{\rm{2}}}\] = oxygen
\[{\rm{COC}}{{\rm{l}}_{\rm{2}}}\] = carbonyl chloride or phosgene
\[{\rm{HCl}}\] = hydrochloric acid or hydrogen chloride

The above reaction will not occur if either light or oxygen (air) is not present. In absence of light and sufficient temperature (slightly high), the reaction of chloroform with oxygen will produce the products as carbon monoxide and hydrochloric acid. Hence, it can be said that in the presence of light and air, chloroform reacts with oxygen to form carbonyl chloride (phosgene) and hydrochloric acid. In order to prevent this oxidation process, ethanol (\[{{\rm{C}}_{\rm{2}}}{{\rm{H}}_{\rm{5}}}{\rm{OH}}\]) is used since it is known to float over chloroform and thus oxidation does not proceed as a result of it.
Therefore, option A is correct.

Note: The number of carbon atoms in the products must be equal to the number of carbon atoms present in chloroform, \[{\rm{CHC}}{{\rm{l}}_{\rm{3}}}\]. Chloroform is generally stored in dark coloured bottles in order to avoid the formation of highly toxic carbonyl chloride, \[{\rm{COC}}{{\rm{l}}_{\rm{2}}}\,\]. Dark coloured bottles are used because it does not allow light to come in contact with chloroform. Chloroform is filled right up to the top (brim) to avoid contact with atmospheric oxygen.