Trihalogen Derivatives Preparation and Properties
→ Two important compounds of trihalogen derivatives are chloroform (CHCl3) and iodoform (CHI3) which are also known as haloform and they aïe prepared by haloforrn reaction.
Haloform Reaction : Compound which have
→ where R = alkyl group or hydrogen atom (H) give haloform reaction. Compounds having (A) structure, on oxidation with halogen changes into structure (B) when compounds with structure A or B are heated with halogen and base, then through haloform reaction formation of haloform occurs, which involve the following steps:
Trichioro methane (Chloroform) CHCl3
Methods of Preparation
→ Laboratory Method : When ethyl alcohol or acetone heated with bleaching powder and water, then chloroform is obtained. This reaction is known as “Haloform reaction” which completes in following steps:
→ Bleaching powder (CaOCl2) reacts with water and form nascent chlorine and calcium hydroxide.
→ Nascent chlorine oxidises ethyl alcohol to acetyldehyde.
CH3—CH2 – OH + 2Cl → CHŠCHO + 2HCl
→ Chlorination of acetaldehyde occurs on reaction with nascent chlorine and trichloro acetaldehyde (chlorai) forms.
→ Chlorai on hydrolysis with calcium hydroxide form chloroform.
→ If acetone is taken in place of ethyl alcohol then haloform reaction occurs in following way:
→ Experiment : Take loo g bleaching powder, 200 mL water and 25 mL ethyl alcohol or acetone in a 500 mL round bottom flask. Heat the flask on sand bath, Chloroform from condenser is collected in water, which is kept in a receiver.
→ The chloroform thus obtained is washed with dilute solution of NaOH, dried with anhydrous CaCl2 and then on again distillation, pure chloroform is obtained.
→ From Sodium Hypochiorite (NaOCl) : Formation of chloroform from sodium hypochiorite solution through chloroform reaction, occurs as follows:
Here R = Alkyl group or H atom.
→ From Chlorai Hydrate : On reaction of chlorai hydrate with aqueous NaOH, it is hydrolysed and pure chloroform is obtained.
→ From Carbon Tetrachloride : Chloroform can be prepared on partial reduction of carbon tetrachioride with Fe + H2O.
→ By Chlorination of Methane : By this method, chloroform can be prepared on an industrial scale.
Chloroform is separated from the mixture
(CH3Cl + CH2Cl3 + CHCl3 + CCl4) by fractional distillation.
Physical Properties of Trihalogen Derivatives
→ It is a colourless, sweet smelling liquid. It is sparingly soluble in water ad is heavier than water. It is readily soluble in ether and alcohol. The vapours when inhaled cause unconsciousness.
→ Due to this reason, chloroform is used as an anaesthetic. Chloroform is slowly oxidised into a poisonous compound phosgene (carhonyl chloride) in the presence of light and air
→ Since phosgene is very poisonous, its presence makes chloroform unfit for use as anaesthetic. So that before use of chloroform as anesthetic íts purity should be checked.
→ Test for Purity of Chloroform : Pure Chloroform does not give white precipitate with AgNO3 solution whereas impure chloroform gives white precipitate because due to presence of air and light, HCl gas forms which gives white precipitate of AgCl.
Chemical Properties of Trihalogen Derivatives
→ Oxidation : Chloroform is oxidised by atmospheric oyxgen in the presence of light, in to a poisonous gas carbonyl chloride which is aho known as phosgene.
→ Therefore impure chloroform is unfit for use as anaesthetic. Therefore to prevent it from oxidation, the following precautions are taken :
It is stored in dark, coloured bottles which are completely filled upto brim to protect from air and sunlight.
→ A small amount of 1% ethyl alcohol is added to the bottles of chloroform. This reacts with phaogene, that may he formed during storage, to non-toxic harmles diethyl carbonate.
→ Reduction : Reduction with different reducing agent yield different products.
→ Chloroform reduction with Zn + HCl gives methylene chloride and if the reduction is carried with Zn and water, methane is obtained.
→ Reaction with Acetone: Chloroform gives addition reaction with acetone in the presence of KOH to give chioretone which is hypnotic.
→ Hydrolysis : Chloroform on reaction with aqueous KOH gives formic acid.
→ Nitration : Chloroform reacts with conc. HNO3 to form chioropicrin. Chioropicrin is used as an insecticide and as war gas.
→ Chlorination : Chlorination of chloroform with chlorine in the presence of sunlight give carbon tetrachioride.
→ Reaction with Silver Powder : On warming with silver powder, chloroform is converted into acetylene.
→ Hoffmann Carbylamine Reaction or Isocyanide Reaction : When chloroform is heated with primary amine and alcoholic KOH then isocyanide is formed with an extremely unpleasent smell which is also known as carbylamme.
→ This reaction is used as a test for primary amine and chloroform therefore it is also known as isocyanide test.
→ Reimer Tiemann Reaction : Chloroform and phenol are heated with alcoholic KOH to give salicylaldehyde.
→ Addition with Alkenes : Addition of ch’oroform to alkenes in presence of peroxide, takes place against the Markownikoffs rule.
Uses of Chloroform
- It is used as an anaesthetic.
- It is used as solvent.
- It is used as a laboratory reagent.
- It is used in the preparation of chioropicrin, chioretone etc.