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Chapter: 11th Chemistry : Haloalkanes and Haloarenes

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Poly halogen compounds

Carbon compounds containing more than one halogen atoms are called poly halogen compounds.

Poly halogen compounds

 

Carbon compounds containing more than one halogen atoms are called poly halogen compounds. Some of the important poly halogen compounds are described below.

They are classified as

 

a) gem – dihalides


 

b) vic – dihalides

For Example


 

Preparation


a) gem- Dihalides

Ethylidene dichloride (1, 1 - Dichloro ethane) is prepared by

 

(i) Treating acetaldehyde with PCl5


 

(ii) Adding hydrogen chloride to acetylene


 

b) vic- Dihalides

Ethylene dichloride (1, 2 - Dichloro ethane) is prepared by the following methods.

 

i) Addition of chlorine to ethylene


 

ii) Action of PCl5 (or HCl) on ethylene glycol


 

Properties

 

Physical Properties

i) They are sweet smelling, colourless liquids having relatively high boiling points.

ii) The boiling point of ethylidene chloride is less than that of ethylene dichloride.

 

Chemical properties

 

1) Hydrolysis with aqueous NaOH or KOH

gem-Dihalides, on hydrolysis with aqueous KOH give an aldehyde or a ketone vic-Dihalides, on hydrolysis with aqueous KOH gives glycols.


This reaction can be used to distinguish the gem- Dihalides and vic- Dihalides.

 

2) Reaction with Zinc (Dehalogenation)

gem- Dihalides and vic- Dihalides on treatment with zinc dust in methanol give alkenes.


 

3) Reaction with Alcoholic KOH (Dehydrohalogenation)

gem- Dihalides and vic- Dihalides on treatment with alcoholic KOH give alkynes.


 

Methylene chloride (Di chloromethane)

 

Preparation

Methylene chloride is prepared by the following methods

 

1) Reduction of chloroform

a) Reduction of chloroform in the presence of Zn + HCl gives methylene chloride.


b) Reduction of chloroform using H2/Ni


 

2) Chlorination of methane

Chlorination of methane gives methylene chloride


 

Uses of methylene chloride

Methylene chloride is used as

i. aerosol spray propellant

ii. solvent in paint remover

iii. process solvent in the manufacture of drugs

iv. a metal cleaning solvent

 

Trihaloalkane

 

Trihaloalkanes are compounds obtained by replacing three hydrogen atoms of a hydrocarbon by three halogen atoms.

Example


 

1) Chloroform

Chloroform is an important trihaloalkane. Dumas named CHCl3 as chloroform as it gives formic acid on hydrolysis.

 

Preparation:

Chloroform is prepared in the laboratory by the reaction between ethyl alcohol with bleaching powderfollowed by the distillation of the product chloroform. Bleaching powder act as a source of chlorine and calcium hydroxide. This reaction is called haloform reaction. The reaction proceeds in three steps as shown below.

 

Step – 1: Oxidation

CH3CH2OH + Cl2 → CH3CHO + 2HCl

Ethyl alcohol        Acetaldehyde

Step – 2: Chlorination

CH3CHO + 3Cl2 → CCl3CHO + 3HCl

Acetaldehyde Trichloro acetaldehyde

Step – 3: Hydrolysis

2CCl3CHO + Ca(OH)2 → 2CHCl3 + (HCOO)2 Ca

Chloral                  chloroform


Properties

 

Physical properties

i. Chloroform is a colourless liquid with peculiar sickly smell and a burning taste

ii. The vapours of chloroform when inhaled cause unconsciousness (depress the central nervous system) and hence it is used as an anaesthetic.

 

Chemical properties

 

1) Oxidation

Chloroform undergoes oxidation in the presence of light and air to form phosgene (carbonyl chloride)


Since phosgene is very poisonous, its presence makes chloroform unfit for use as anaesthetic.

 

2) Reduction

Chloroform undergoes reduction with zinc and HCl in the presence of ethyl alcohol to form methylene chloride.


 

3) Nitration

Chloroform reacts with nitric acid to form chloropicrin.(Trichloro nitro methane)


It used as an insecticide and soil sterilising agent.

 

4) Carbylamine reaction

Chloroform reacts with aliphatic or aromatic primary amine and alcoholic caustic potash, to give foul smelling alkyl isocyanide (carbylamines)


This reaction is used to test primary amine.

 

Tetra haloalkane

 

Carbon tetrachloride is a good example for tetra haloalkane

Carbon tetrachloride

 

Preparation

 

1. Chlorination of methane

The reaction of methane with excess of chlorine in the presence of sunlight will give carbon tetrachloride as the major product.


 

2. Action of carbondisulphide with chlorine gas

Carbon disulphide reacts with chlorine gas in the presence of anhydrous AlCl3 as catalyst giving carbon tetrachloride


 

Physical properties

i. Carbon tetrachloride is a colourless liquid with its specific smell

ii. It is insoluble in water and soluble in organic solvents

 

Chemical properties

 

(i) Oxidation

Carbon tetrachloride reacts with hot water or with hot water vapour producing the poisonous gas, phosgene.


 

(ii) Reduction

Carbon tetrachloride is reduced by iron powder in dilute HCl medium to form chloroform


 

Freons (CFC)

 

The chloro fluoro derivatives of methane and ethane are called freons.

 

Nomenclature

Freon is represented as Freon-cba

Where c = number of carbon atoms – 1

b = number of hydrogen atoms + 1

a = total number of fluorine atoms

Example


Freon – 12 is prepared by the action of hydrogen fluoride on carbon tetrachloride in the presence of catalylic amount of antimony patachloride. This is called swartz reaction


 

Physical properties

Freons are highly stable, unreactive, non corrosive, non toxic, easily liquefiable gases.


Uses:

i. Freons are a used as refrigerants in refrigerators and air conditioners.

ii. It is used as a propellant for aerosols and foams

iii. It is used as propellant for foams to spray out deodorants, shaving creams, and insecticides.

 

DDT (p,p’-dichloro diphenyl tri-chloro ethane)

 

DDT, the first chlorinated organic pesticide was prepared in 1873, and in 1939 Paul Muller discovered the effectiveness of DDT as an insecticide. He was awarded Noble prize in medicine and physiology in 1948 for this discovery.

DDT can be prepared by heating a mixture of chlorobenzene with chloral (Trichloro acetaldehyde) in the presence of con.H2SO4.


 

Uses:

i. DDT is used to control certain insects which carries diseases like malaria and yellow fever

ii. It is used in farms to control some agricultural pests

iii. It is used in building construction as pest control

iv. It is used to kill various insects like housefly and mosquitoes due to its high and specific toxicity.


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Study Material, Lecturing Notes, Assignment, Reference, Wiki description explanation, brief detail


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