Question

Which of the following compounds gives trichloromethane on distilling with bleaching powder?
A. Methanal
B. Phenol
C. Ethanol
D. Methanol

Answer 1

Hint: Aldehydes and ketones which have at least one methyl group linked to the carbonyl carbon atom are oxidised by bleaching powder to calcium salts of corresponding carboxylic acids having one carbon atom less than that of carbonyl compound. The methyl group is converted to haloform.

Complete step-by-step answer: The bleaching powder is a chloronating agent as well as oxidising agent. It has many industrial uses due to this property. The bleaching powder breaks into calcium hydroxide and chlorine gas which are the main reagents of haloform test.
$CaOC{l_2} + {H_2}O \to Ca{(OH)_2} + C{l_2} \\$

The equation of the ethanol, and chlorine in the presence of calcium hydroxide is as follows:
1. In the first step ethanol gets converted into acetaldehyde.
$C{l_2} + C{H_3}C{H_2}OH \to C{H_3}CHO + 2HCl $

2. In the second step, $CC{l_3}COH$ forms by the reaction of acetaldehyde and chlorine.
$C{H_3}CHO + 3C{l_2} \to CC{l_3}COH + 3HCl \\$

3. In the third step, $CC{l_3}COH$ hydrolysed to give chloroform and calcium salts of
carboxylic acids.
$2CC{l_3}COH+ Ca{(OH)_2} \to 2CHCl_3 + 2HCOO^-+Ca^{2+}$

Therefore, ethanol gives trichloromethane when distilled with bleaching powder.

Option ‘C’ is correct

Note: Ethanol is the only primary alcohol that undergoes haloform reaction, other primary alcohols don’t give haloform test. The reason for this exception is that ethanol can oxidize to acetaldehyde. Acetaldehyde is a methyl aldehyde, it has a methyl group attached to carbonyl carbon. All the methyl aldehyde and ketone show haloform tests.