Online Chemistry notes

Chloroform – Lab preparation, Properties, Uses and Question/Answer

Chemist

Laboratory preparation of chloroform (trichloromethane) (CHCl3) 

Principle:

Chloroform is prepared in the laboratory by heating ethanol or acetone with aqueous bleaching powder paste. Bleaching powder paste acts as oxidizing, chlorinating and hydrolyzing agent.

From ethanol :

Step I : Oxidation :

 

 

Step II : Chlorination :

Step III : Hydrolysis :

From acetone (propanone) :

Step I : Chlorination :

Step II : Hydrolysis :

Procedure : First of all, bleaching powder paste is prepared by mixing 100 gm of bleaching powder with 200 ml of water in one liter round bottomed flask and 25 ml of ethanol or acetone is added to it. The flask is heated gently on a water bath until a mixture of chloroform and water distills over, as shown in figure . The mixture from receiver is transferred into a separating funnel and the lower layer of chloroform is separated.

Purification : The impure chloroform is washed with dilute caustic soda (NaOH) solution and then with water successively in the separating funnel. It is then dried over anhydrous calcium chloride and re-distilled between 60 – 650C. In this way, pure and dry chloroform is obtained.

Physical properties of chloroform

Chemical properties/reactions of chloroform

1. Reaction with air : Oxidation of chloroform 

In the presence of sunlight, chloroform is oxidized by air to produce highly poisonous gaseous compound called phosgene (carbonyl chloride).

Thus, chloroform is stored in a dark/coloured bottle to prevent the oxidation of chloroform into phosgene.

A small amount of ethanol is also added into the chloroform at the time of packing because ethanol converts highly poisonous phosgene ( if formed) to a non poisonous diethyl carbonate.

2. Reaction with aq. KOH solution :

When boiling with aqueous KOH solution, chloroform is hydrolysed to form potassium formate which on acidification gives formic acid.

3. Reaction with silver powder :

Chloroform when heated with silver powder gives acetylene (ethyne).

4. Reaction with primary amines ( carbylamine reaction) :

When chloroform is warmed with a primary amine in the presence of alcoholic KOH, an offensive(unpleasant) smell of carbylamines ( i.e. isocyanide) is obtained. This reaction is known as carbylamine reaction.

Note : Secondary and tertiary amines do not respond to this reaction and therefore, this reaction is used as test reaction for primary amines.

5. Reaction with phenol (Riemer- Tiemann reaction) :

When chloroform is heated with phenol and sodium hydroxide followed by hydrolysis, o – hydroxy benzaldehyde ( salicylaldehyde) is formed. This reaction is called Riemer – Tiemann reaction.

6. Reaction with acetone (propanone) :

Chloroform reacts with acetone in presence of a base such as KOH to give chloretone.

Chloretone is used as a sleep – inducing (hypnotic) drug.

7. Reaction with HNO3 :

On heating with conc. HNO3, chloroform gives chloropicrin.

Chloropicrin is used as an insecticide and tear gas.

8. Reduction : 

Chloroform undergoes reduction with Zn/HCl to give methylene chloride but when treated with Zn/H2O (neutral medium) methane is obtained.

Uses of chloroform 

  1. It is used as an anesthetic. It is now being replaced by other safe anesthetics because chloroform in some cases causes cardiac and respiratory failure.
  2. It is used as a laboratory reagent for testing primary amines.
  3. It is used for the preparation of chloropicrin, chloretone, salicylaldehyde, etc.
  4. It is used in medicines such as a cough syrups.
  5. It is used as preservative for biological specimens.

Questions and Answers from Chloroform

Q) Why does chloroform does not give white precipitation with aqueous silver nitrate solution?

→ Chloroform does not give white precipitation with aqueous silver nitrate because C – Cl bond in chloroform is covalent and non – polar and does not ionize in aq. Solution to produce chloride ion (Cl –).

If chloroform is impure, precipitation will occur with aqueous AgNO3 because it may contains HCl as impurities.

Q) Write the action of monohydroxy benzene with trichloromethane in presence of alcoholic caustic soda.

Q) Identify the products of given reactions.

Answer :

References