Preparation
It is prepared by the following methods:
Laboratory method of chloroform preparation
Ethanol (ethyl alcohol) or propanone (acetone) when treated with bleaching powder gives chloroform. This reaction is known as haloform reaction.
- From ethanol
The reaction occurs as the bleaching powder reacts with water giving chlorine and calcium hydroxide as:
The reaction proceeds in three steps:
First step is the oxidation of ethyl alcohol to give acetaldehyde.
From propanone (acetone)
The reaction occurs when chlorine is obtained from bleaching powder.
Then the chlorination of acetone to give trichloroacetone takes place
Lastly, combination of trichloroacetone with lime takes place to form chloroform.
By chlorination of methane
On an industrial scale, chloroform can be prepared by chlorination of methane at 675 K.
Chloroform is separated from this mixture of products by fractional distillation.
By reduction of carbon tetrachloride
On an industrial scale chloroform can be prepared by reducing carbon tetrachloride with iron filings and steam.
Pure chloroform
It is prepared by heating chloral hydrate with concentrated solution of caustic soda.
Physical properties of chloroform
- Chloroform is a colorless oily liquid with a peculiar sickly smell and a burning taste.
- It is heavier than water. It is sparingly soluble in water but readily soluble in organic solvents such as ethanol and ether.
- The boiling point is 334 K and freezing point is 210 K.
- It is a good solvent for oils, fats and waxes.
- It is a poisonous compound, the vapors of which, when inhaled, cause unconsciousness. Hence, chloroform is used as an anaesthetic.
Chemical properties of chloroform
The important chemical properties of chloroform are:
Oxidation
In the presence of air or light chloroform is slowly oxidized into a poisonous compound phosgene (carbonyl chloride).
Since phosgene is very poisonous, its presence makes chloroform unfit for use as an anaesthetic.
Preservation of chloroform
If chloroform is to be used for anaesthetic purposes, its slow oxidation to phosgene must be prevented. The following precautions are taken to keep chloroform pure:
- It should be stored in dark colored bottles to protect it from sunlight.
- The bottles containing chloroform are completely filled up to the brim and are properly stoppered to keep out air.
- A small amount of 0.6 to 1% alcohol is added to the bottles of chloroform. This reacts with phosgene that may be formed during storage to form non-toxic, harmless diethyl carbonate.
Reduction
Chloroform gets reduced with zinc and hydrochloric acid to form methylene chloride. If the reduction is carried with zinc dust and water, methane is obtained.
Nitration
Chloroform reacts with concentrated nitric acid to form nitro chloroform or chloropicrin.
Chloropicrin is used as an insecticide and war gas.
Chlorination
Chlorination of chloroform with chlorine in the presence of diffused sunlight gives carbon tetrachloride.
Hydrolysis
Chloroform gives potassium formate on warming with concentrated aqueous or alcoholic potassium hydroxide.
Carbylamine reaction
Chloroform reacts with primary amines (both aliphatic and aromatic) and alcoholic caustic potash, to give isocyanide compound, which is commonly known as carbylamine. It has an extremely unpleasant smell and is used as a test for primary amines.
Reaction with silver powder
Chloroform is converted into acetylene on warming with silver powder.
Condensation with acetone
Chloroform combines with acetone in the presence of potassium hydroxide to give chloretone, which is used as a sleep producing (hypnotic) drug.
Reaction with phenol
Chloroform reacts with phenol in sodium hydroxide at 340 K to give salicylaldehyde.
This reaction is called 'Reimer Tiemann reaction'.
Uses
- In industry, chloroform is used as an important solvent particularly for fats, waxes, rubber, etc.
- As an anaesthetic. But these days, it has been replaced by other anaesthetics as it is very toxic in nature.
- As a laboratory reagent.
- In the preparation of chloropicrin, chloretone, etc.
- In medicines.
