Poly Halogen Compounds

POLY HALOGEN COMPOUNDS

Carbon compounds containing more than one halogen atom are usually referred to as polyhalogen compounds. Some important polyhalogen compounds are:

Dichloromethane (Methylene Chloride)

Preparation:

It is prepared industrially by direct chlorination of methane. The mixture so obtained is separated by fractional distillation.

Properties

It is a colourless, sweet smelling, volatile liquid boiling point = 313K. Because of its low boiling point & low inflammability, it is an effective extraction solvent used in pharmaceutical & food industries

Trichloro Methane (Chloroform)

Preparation

1. From Methane:

Chloroform is manufactured by chlorination of methane in presence of light or catalyst.

CHCl₃ can be separated by fractional distillation.

2. From Chloral Hydrate

3. Laboratory Method

In this method chloroform obtained from ethanol or acetone by reaction with a paste of bleaching powder and water.

4. From Carbon tetra chloride

By partial reduction of carbontetrachloride with iron filings & water.

Physical Properties

CHCl₃ is a colourless, oily liquid which has sweetish, sickly odour & taste. It is heavier than water.

Chemical Properties

1. Action of Sun light & Air

In presence of sunlight chloroform is oxidized by air to a highly poisonous compound phosgene, COCl₂.

2. Hydrolysis:

When boiled with aqueous KOH, CHCl₃ is hydrolysed to potassium formate.

3. Reduction

Chloroform can be reduced with Zn & HCl.

4. Reaction with acetone

Chloroform reacts with acetone in presence of a base such as KOH & forms the addition product, chloretone.

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

5. Reaction with Nitric acid

Chloform reacts with conc. HNO₃ on heating.

Chloropicrin is used as an insecticide & war gas.

6. Reaction with primary amines (carbyl-amine reaction)

When chloroform is warmed with a primary amine aliphatic or aromatic) in the presence of alcoholic KOH an offensive smell of isocyanide or carbyl-amine is obtained.

Illustration 1. Which one of the following compounds when heated with KOH and a primary amine gives carbylamine test?

(A) CHCl₃ (B) CH₃Cl

(C) CH₃OH (D) CH₃CN

Solution: (A)

7. Reaction with Silver Powder

On heating with silver powder, chloroform forms acetylene.

8. Chlorination

On further chlorination it gives CCl₄

9. Reimer – Tiemann reaction

CHCl₃ reacts with phenol & KOH to form salicylaldehyde.

Uses of Chloroform

1. As solvent in oils and varnishes

2. In medicine

3. As preservative

4. As laboratory reagent

Illustration 2. When chloroform reacts with acetone, the product is

(A) ethylidene dichloride (B) mesitylene

(C) Chloretone (D) chloral

Solution: (C)

Illustration 3. H atom of chloroform is acidic. Why?

Solution: The three chlorine atoms attached on carbon atom make it partially positive. Due to negative inductive effect of Cl atom carbon attracts the shared pair of electrons of C and H bond more effectively to produce partial positive charge on H to behave as acid.

Tri iodo Methane (Iodoform)

Preparation

It is prepared by heating ethanol or acetone with sodium hydroxide & iodine or sodium carbonate & iodine in water.

Physical Properties

Iodoform is a yellow coloured solid having m.p. = 382 K. It is insoluble in water but dissolves readily in organic solvents.

Chemical Properties

Uses

1. It is used as antiseptic for dressing wounds.

2. It is used to manufacture pharmaceuticals.

Tetrachloro methane (carbon tetra chloride)

Preparation

CCl₄ is manufactured by following methods:

(i) From methane: By chlorination of methane in the presence of sunlight.

(ii) From carbon disulphide: By reaction of chlorine with CS₂ in presence of AlCl­₃ as catalyst.

Physical Properties

CCl₄ is colourless oily liquid with characteristic sickly smell. It is heavier than water & is insoluble in water but is soluble in organic solvents such as ether. CCl₄ is used as solvent for organic reactions.

Chemical Properties

(i) Reduction

On reduction with moist iron filling, CCl₄ gives chloroform.

(ii) Hydrolysis

On heating with alc. KOH, it undergoes hydrolysis & gives CO₂ which dissolves in KOH to yield K₂CO₃.

(iii) Stability

CCl₄ is stable to red heat but when the vapours come in contact with water, phosgene is liberated.

Uses

1. As industrial solvent.

2. for the manufacture of chloroform.

3. As laboratory reagents.

4. In dry cleaning.

5. As a fire extinguisher.

p – Dichloro benzene

It is prepared by chlorination of benzene.

Properties

It is white volatile, solid m.p. = 325K, which readily sublimes.

Uses

1. As insecticide

2. As germicide

3. As soil fumigant

4. As deodrant & moth repellent.

Perfluoro carbon (PFC's)

Preparation

They are prepared by controlled fluorination of alkanes in vapour phase. The reaction mixture is diluted with nitrogen in order to control the reaction

Teflon is a common perfluoro carbon.

Properties

They are non – toxic, non-inflammable, non-corrosive & extremely stable compounds.

Uses

1. As lubricants

2. For surface coating

3. As electrical insulator.

Benzene haxa chloride

These are popularly known as lindane & gammaxene. It is commercially prepared by the addition of chlorine to benzene in presence of ultraviolet light.

Uses

As a pesticide in agriculture.

p - p' - Dichloro diphenyl tri chloro ethane (DDT)

It is manufactured by condensation of chloro benzene with trichloro acetaldehyde (chloral) in the presence of sulphuric acid.

Properties

It is a white powder insoluble in water but soluble in oils.

Uses

It is a powerful insecticide. However it is highly stable & is not easily decomposed in the environment. Therefore its long term effect could be potentially dangerous & its use is banned in many countries

Illustration 4. What does DDT stands for? What is its chemical name?

Solution: DDT stands for p, p' - dichlorodiphenyl trichloro ethane. Its actual name is

2, 2 – bis (4 – chlorophenyl) – 1, 1, 1 – trichloro ethane.