uses and environmental effects of dichloromethane, trichloromethane, tetrachloromethane, iodoform, freons & DDT.

Dichloromethane (methylene chloride), CH₂Cl₂ :-

It is prepared industrially by the direct chlorination of methane. The mixture so obtained is separated by fractional distillation. It can also be prepared by the reduction of chloroform.

CHC1₃ + 2H    —(Zn, HCl/Alcohol) —–>CH₂Cl₂     +  HC1                                                                   Chloroform                                              Methylene chloride

Properties:-

(i) Methylene chloride is a colourless, sweet smelling liquid.
(ii) It is a volatile liquid having low boiling point of 313 K.
(iii) Its specific gravity is 1.37.

Uses:-

(i) It is used as a solvent in paint remover and also as a metal cleaning and finishing solvent.
(ii) Because of its low boiling point and low inflammability, it is an effective solvent for extraction in pharmaceutical in the manufacturing of drugs and food industries.
(iii) It is also used as a propellent in aerosols.
(iv) It is used as refrigerant and dewaxing agent.

Methylene chloride is known to have harmful effects on the human central nervous system. Exposure to low levels of methylene chloride in air can lead to slightly impaired hearing and vision. However, exposure to high levels of methylene chloride can cause dizziness, nausea, tingling and numbness in the fingers and the toes. Moreover, in humans, direct contact of methylene chloride with skin causes intense burning and mild redness of the skin. Direct contact with the eyes can even burn the cornea. It was observed that the cornea of animals was damaged when exposed vapours of methylene chloride.

Freons, CCl₂F₂ (Dichlorofluoromethane):

The chlorofluoro compounds of methane and ethane are collectively known as freons. These are extremely stable, low boiling, non-inflammable, non-toxic, non-corrosive easily liquefiable and highly unreactive compounds. These are stable even under high pressures and temperatures and therefore, find use as refrigerants. Among these dichlorofluoromethane (CC1₂F₂) known as Freon- 12 is the most important compound of this class because of its industrial uses. It is manufactured by the action of antimony fluoride on carbon tetrachloride in the presence of antimony pentachloride.

3CCl₄  + + 2SbF₅  —– SbCl₅ —–>2SbC1₃  +     3CC1₂F₂                                                                                                                                                Freon—12

Alternatively, it may be prepared by the action of hydrogen fluoride on carbon tetrachloride in the presence of carbon and ferric chloride.

CCl₄    + + 2HF  —-C, FeCl₃ —–> 2HC1    +   CC1₂F₂                                                                                                                                                   Freon—12

Uses:-

(i) Freons are used as refrigerants in refrigerators and air conditioners and hence is the name freons.
(ii) These have also been used extensively as propellants for aerosols and foams to spray out deodorants, cleansers, shaving creams, hair sprays and insecticides.

However, freons are persistent because micro organisms and other agents that cause biodegradation of natural organic materials are unable to break these stable molecules or can do so only extremely slowly. Therefore, these molecules move to upper part of air atmosphere and undergo photochemical decomposition and deplete the protective ozone layer surrounding our earth. Therefore, the use of freons as propellants and refrigerants has been drastically discouraged. It has also been banned in many countries.

DDT -2, 2-bis (p-Chlorophenyl) -1, 1, 1- Trichloroethane:

It is synthesised by heating a mixture of chloral (1 mol) with chlorobenzene (2-mol) in the presence of concentrated H₂S0₄.

Properties and Uses of D.D.T.:-

(i) D.D.T. is almost insoluble in water but it is moderately soluble in polar solvents.
(ii) D.D.T. is a powerful insecticide. It is widely used as an insecticide for killing mosquitoes and other insects.

Side Effects of D.D.T:-

D.D. T. is one of the most powerful and effective insecticides. The use of D.D. T. increased enormously after World War II because of its effectiveness against the mosquito that spreads malaria and lice that carry typhus. However, it was realised that excessive use of D.D. T. leads to many problems. Many species of insects developed resistance to D.D.T. and also D.D.T. was formed to have high toxicity towards fish. D.D.T. is not biodegradable. Its residues accumulate in environment and its long term effects could be highly dangerous. D.D.T. is not metabolised very repidly by animals rather it gets deposited and stored in fatty tissues. This raised alarming dangers.             Therefore, its use has been abandoned in many western countries. However, inspite of its dangerous side effects, DDT is still being widely used in India due to non-availability of other cheaper insecticides.

Tetra-chloromethane (Carbon Tetrachloride), CCl₄:

Carbon tetrachloride is prepared industrially by the following methods

(1) From methane:

Carbon tetrachloride can be prepared by the chlorination of methane in the presence sunlight.                                     CH₄ + 4C1₂ —-(uv-light)—-> CC1₄   +  4HC1        

(2) From carbon disulphide:

Carbon tetrachloride is prepared by the action of chlorine carbon disulphide in the presence of A1C1₃ (catalyst).

CS₂ + 3C1₂ ———–>  CC1₄     +   S₂C1₂

Sulphur monochloride

Carbon tetrachloride is separated from S₂C1₂ by fractional distillation.

Physical Properties:-

(i) Carbon tetrachloride is a colourless oily liquid with sickly smell.

(ii) It is insoluble in water but is soluble in organic solvents such as ether, alcohol.

(iii) Its boiling point is 350 K.

(iv) The vapour of carbon tetrachloride are inflammable.

Chemical Properties:-

(i) Reduction:- On reduction with moist iron filings, CC1₄ gives chloroform.

CCl₄ + 2[H]—– Fe/H₂O—-> CHC1₃   +   HC1

Chloroform

(ii) Hydrolysis:- Upon passing steam, the vapour of CC1₄ decomposes to give phosgene (poisonous gas).

CC1₄ + H₂0 (steam) —–> COC1₂    +   2HC1

Phosgene

(iii) Action with KOH: On heating with alcoholic KOH, it decomposes to give CO₂. The CO₂ dissolves in KOH to give potassium carbonate.

CC1₄ + 4KOH —-Heat,-4KCl —-> [C(OH)₄] ——> CO₂ + 2H₂0

Unstable

2KOH + CO₂ ———–> K₂CO₃    +     H₂O

Potassium carbonate

iv) Action with antimony trifluoride: Carbon tetrachloride reacts with SbF₃ in the presence of antimony pentachloride as catalyst to give dichlorodifluoromethane (Freon).

3CCl₄  + + 2SbF₅  —–SbCl₅ —-> 2SbC1₃ +  CC1₂F₂                                                                                                                                                   Freon—12

Freon is easily liquefied and is used as and propellant in aerosols.

Uses: – The important uses of CC1₄ are-

1. It is used as a solvent for oils, fats, waxes, etc.
2. It is used as a fire extinguisher under the name pyrene.
3. It is used in dry cleaning.
4. It is used for the manufacture of Freon.

Trichloromethane, CHCl₃ (Chloroform):-

Preparation of trichloromethane: Trichloromethane is prepared in the laboratory from ethanol (ethyl alcohol) or propanone (acetone) with bleaching powder. This reaction is known as haloform reaction.

(a) From ethanol: – The reaction occurs as:

The bleaching powder reacts with water giving chlorine and calcium hydroxide as:

CaOCl₂    +    H₂0 —–Oxidation—–> Ca(OH)₂    +      Cl₂

Bleaching powder

The reaction proceeds in three steps as-
(i) Oxidation of ethyl alcohol to give acetaldehyde.

CH₃CH₂OH     + Cl₂ ——> CH₃CHO     +   2HC1
Ethyl alcohol                        Acetaldehyde
(ii) Chlorination of acetaldehyde to form chloral (trichloroethanal)
CH₃CHO + 3Cl₂    ——–>  CC1₃CHO   +   3HC1
Trichloroacetaldehyde (Chloral)
(iii) Reaction of chloral with lime to give chloroform.

2CC1₃CHO  + Ca(OH)₂ ——>CHCl₃     +    Ca(HCOO)₂

Calcium formate
(b) From propanone (acetone):- The reaction occurs as:

Chlorine is obtained from bleaching powder.

CaOCl₂    +    H₂0 —-Oxidation—–> Ca(OH)₂    +      Cl₂

Bleaching powder

(i) Chlorination of acetone to give trichloroacetone

CH₃COCH₃ + 3C1₂ —–> CC1₃COCH₃    +  3HC1
Acetone Trichloroacetone

(ii) Combination of trichloroacetone with lime to form chloroform.

2CC1₃CH₃  + Ca(OH)_{2   ——->} 2CHCl₃   +    Ca(CH₃OO)₂

Calcium acetate
Physical properties of chloroform:-

The important physical properties of chloroform are:
(i) It is a colourless oily liquid with a peculiar sickly smell and a burning taste.

(ii) It is heavier than water. It is sparingly soluble in water but readily soluble in organic solvents such as ethanol and ether.

(iii) Its boiling point is 334 K and freezing point is 210 K.

(iv) It is poisonous compound. The vapour when inhaled cause unconsciousness. Due to this reason, chloroform is used as an anaesthetic.

(v) It is a good solvent for oils, fats and waxes.

Chemical properties of chloroform:

The important chemical properties of chloroform are:

i) Oxidation: – Chloroform is slowly oxidised into a poisonous compound phosgene (carbonyl chloride) in the presence of air or light.

CHCl₃ +    ½ O₂ —–^Air/light —-> COCl₂         +    HCl

Carbonyl chloride (Phosgene)

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

ii) Reduction: – Chloroform is reduced with zinc and hydrochloric acid to give methylene chloride. If the reduction is carried with zinc dust and water, methane is obtained as product.

CHC1₃    +    2H —–Zn/HCI—->  CH₂C1₂  + HC1

Chloroform                              Methylene chloride                                                                                              
CHC1₃    +    6H—–Zn/H2O—–>   CH₄     +  3HC1

Chloroform                                      Methane

iii) Carbylamine reaction: – Chloroform reacts with primary amine (both aliphatic and aromatic) and alcoholic caustic potash, to give isocyanides which is commonly known as carbylamine.

C₂H₅NH₂   +   CHCl₃ + Alco. KOH ——-> C₂H₅NC    +   KCl  +  H₂O

Ethyl amine                                                 Ethyl Isocyanide

C₆H₅NH₂   +   CHCl₃ + Alco. KOH  ——–> C₆H₅NC    +   KCl      +     H₂O

Aniline                                                        Phenyl Isocyanide

iv) Reaction with Phenol: – Chloroform reacts with phenol in sodium hydroxide at 340K to give salicyladehyde (2-Hydroxy benzaldehyde).

C₆H₅OH   +   CHCl₃ +  NaOH  ——> C₆H₄(OH)CHO   +   NaCl +     H₂O

Phenol

Structure: Chloroform has tetrahedral structure as shown below

Uses of chloroform:

Chloroform has the following uses-

• In industry, chloroform is used as an important solvent particularly for fats, alkaloids, iodine, waxes, rubber, etc.
• It is used as an anaesthetic. But these days, it has been replaced by other anaesthetics such as ether, as it is very toxic in nature.

In the earlier days, chloroform was used as a general anaesthetic in surgery. This is because inhaling chloroform vapours depresses the central nervous system. It has been observed that breathing about 900 parts of chloroform per million parts of air (900 ppm) for a short time causes dizziness, fatigue and headache. Chronic chloroform exposure may damage lever and kidneys because chloroform is metabolised to poisonous phosgene. Some people develop sores when the skin is immersed in chloroform. Therefore, the use of chloroform as an anaesthetic has been replaced by less toxic anesthetic such as ether.

(iii) It is used as a laboratory reagent.

(iv) It is used in the preparation of chloropicrin, chloretone, etc.

(v) It is used in medicines.

(vi) It is used in the production of Freon refrigerant, R-22.

Iodoform (Tri-iodomethane) CHI₃:-

Iodoform is prepared in the laboratory by the action of iodine on ethyl alcohol or acetone in the presence of alkali. This is called haloform reaction.

(a) From ethyl alcohol (ethanol):-

With ethyl alcohol, the reactions taking place are-
2NaOH   +       I₂  ———>  NaOI   +     NaI    +  H₂0

Sod. Hypoiodite

(i) Oxidation: CH₃CH₂OH   +   NaOI ——–> CH₃CHO   +   NaI +   H₂0

(ii) Iodination: CH₃CHO + 3NaOI   ————> CI₃CHO + 3NaOH
Tri-Iodoacetaldehyde (lodal)

(iii) Cleavage: CI₃CHO + NaOH  ———>   CHI₃ +  HCOONa
lodoform
(b) From propanone (acetone):

The reactions taking place are-

2NaOH +I₂  ———->  NaOI  +  NaI   +  H₂0

Sod. Hypoiodite

(i) Iodination: CH₃COCH₃ + 3NaOI ——–—> CI₃COCH₃    +    3NaOH

Propanone                        1, 1, 1-Triiodopropanone

(ii) Cleavage: CI₃COCH₃ + NaOH ——->  CHI₃          +     CH₃COONa

Iodoform             Sod. acetate

Physical properties of Iodoform:

The important physical properties of Iodoform are:

• It is a yellow coloured crystalline solid with m.p. 392 K.
• It has characteristic unpleasant odour.
• It is insoluble in water but readily dissolves in ethyl alcohol and ether.
• It has an antiseptic action due to liberation of free iodine.

Chemical properties:

The important chemical properties are:

i) Stability: On heating, Iodoform decomposes to give iodine vapour. This can also be accelerated by moisture, air or light.

ii) Reduction: lodoform can be reduced with P and HI to give methylene iodide.

CHI₃ + 2H ——–> CH₂I₂    +    2HI

Methylene iodide

iii) Hydrolysis: On boiling with aqueous or alcoholic KOH, Iodoform gives potassium formate.

Heat
CHI₃ + 3KOH ——-> HCOOH         +      3KI   +        H₂0

HCOOH + KOH  ——-> HCOOK    +     H₂0

Pot. Formate

iv) Carbylamine reaction: – Chloroform reacts with primary amine (both aliphatic and aromatic) and alcoholic caustic potash, to give isocyanides which is commonly known as carbylamine.

C₂H₅NH₂   +   CHI₃ + Alco. KOH ——–> C₂H₅NC    +   KI     +     H₂O

Ethyl amine                                              Ethyl Isocyanide

C₆H₅NH₂   +   CHI₃ + Alco. KOH ——–> C₆H₅NC    +   KI     +     H₂O

Aniline                                                     Phenyl Isocyanide

5. With silver powder: – Iodoform, when heated with silver powder gives acetylene.

CHI₃    +   6Ag     +     CHI_{3 ——–>} HC ≡ CH    +    6AgI

Iodoform                                           Acetylene

 Uses of Iodoform:-

The important uses of Iodoform are:

• It is used as an antiseptic and this nature is due to iodine that it liberates. However, because of its very unpleasant smell, it has now been replaced by better antiseptics.
• It is used in the manufacture of pharmaceuticals.