Question

Two percent of ethanol is added during the oxidation of chloroform to stop the formation of carbonyl chloride. In this reaction ethanol acts as:
A. Auto catalyst
B. Negative catalyst
C. Positive catalyst
D. None of these

Answer 1

Hint: The chemical formula of chloroform is \[{\rm{CHC}}{{\rm{l}}_{\rm{3}}}\]. Chloroform is a colourless gas that evaporates quickly into gas. Oxidation is the process which involves the loss of electrons by atoms, ions or molecules.

Complete Step by Step Solution:
Chloroform reacts with oxygen initiated by light to form a highly toxic gas, carbonyl chloride. 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 molecular 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

From the reaction, it can be seen that two moles of chloroform react with one mole of oxygen to give two moles of carbonyl chloride (phosgene) and two moles of 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 presence of light and air, chloroform reacts with oxygen to form carbonyl chloride (phosgene), \[{\rm{COC}}{{\rm{l}}_{\rm{2}}}\,\] and hydrochloric acid, \[{\rm{HCl}}\] . 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.
A negative catalyst is one which decreases the rate of a chemical reaction. Hence, ethanol acts as a negative catalyst here.
Therefore, option B is correct.

Note: Friedel-Crafts acylation reaction can be used in preference to Friedel-Crafts alkylation as it is free from the two chief drawbacks of the later; skeletal rearrangement and polysubstituted.