Trihalogen Derivatives Preparation and Properties

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.

Mechanism

→ 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.

Chemistry Notes