Notes-Part-2-Class-12-Chemistry-Chapter-10-Halogen Derivatives-Maharashtra Board

Topics to be Learn : Part-1 Classification of halogen derivatives Nomenclature of halogen derivatives Methods of preparation of alkyl halides/ aryl halides Physical properties Optical isomerism in halogen derivatives Topics to be Learn : Part-2 Chemical properties Uses and environmental effects of some polyhalogen compounds

Chemical properties :

Laboratory test of haloalkanes :

Haloalkanes are of neutral type in aqueous medium. On warming with aqueous sodium or potassium hydroxide the covalently bonded halogen in haloalkane is converted to halide ion.

R-X + OH^—  \(\underrightarrow{Δ}\) R-OH + X^—

When this reaction mixture is acidified by adding dilute nitric acid and silver nitrate

solution is added a precipitate of silver halide is formed which confirms presence of halogen in the original organic compound.

Ag⁺(aq) + X^—(aq) → AgX↓ (s)

Nucleophilic substitution reactions of haloalkanes :

Substitution reaction : When a group bonded to a carbon in a substrate is replaced by another group to get a product with no change in state of hybridization of that carbon the reaction is called substitution reaction.

Nucleophilic substitution reactions (SN) : Alkyl halides react with a variety of nucleophiles to give nucleophilic substitution reactions. The reaction is represented in general form as shown below.

• When a substrate reacts fast it is said to be reactive.
• The reactivity of alkyl halides in SN reaction depends upon two factors, (i) the substitution state (1⁰, 2⁰ or 3⁰) of the carbon and (ii) the nature of the halogen.
• The order of reactivity : tertiary alkyl halide (3⁰) > secondary alkyl halide (2⁰) >primary alkyl halide (1⁰) and R - I > R - Br > R - Cl

Examples :

Action of aqueous KOH (or Na0H) on :

(i) Ethyl bromide : When ethyl bromide (bromoethane) is refluxed with aqueous potassium hydroxide, ethyl alcohol is formed. The reaction is called a hydrolysis reaction.

CH₃—CH₂—Br + KOH \(\underrightarrow{boil}\) CH₃—CH₂—OH + KBr

(ii) Isopropyl bromide : When isopropyl bromide (2-bromopropane) is boiled with aqueous potassium hydroxide, isopropyl alcohol is formed.

(iii) Methyl bromide : When methyl bromide (bromomethane) is heated with aq. KOH, it is hydrolysed to methyl alcohol (methanol).

CH₃—Br + KOH \(\underrightarrow{boil}\)  CH₃—OH +KBr

(iv) Tert-butyl chloride : When tert-butyl chloride is refluxed with aqueous potassium hydroxide, tert-butyl alcohol is formed.

Williamson’s synthesis : When an alkyl halide (R—X) is heated with sodium alkoxide (R—O—Na), an ether is obtained. In this reaction halide (—X) of alkyl halide is replaced by an alkoxy group (—OR). This reaction is known as Williamson’s synthesis. This method is used to prepare simple (or symmetrical) ethers and mixed

(or unsymmetrical) ethers.

Sodium alkoxide is obtained by a reaction of sodium with an alcohol.

2R-OH + 2Na → 2R-O^—Na⁺ + H₂

e. g. 2C₂H₅OH + 2Na → 2C₂H₂O^—Na⁺ + H₂

Simple (symmetrical) ether : When an alkyl halide and sodium alkoxide having similar alkyl groups are heated, symmetrical ether is obtained.

R—O—Na + RX \(\underrightarrow{Δ}\) R-O—R + NaX

e.g. When ethyl bromide is heated with sodium ethoxide, diethyl ether is formed.

C₂H₅—O—Na   +  C₂H₅—Br   \(\underrightarrow{Δ}\)  C₂H₅—O—C₂H₅ + NaBr

Mixed (unsymmetrical) ether : When an alkyl halide and sodium alkoxide having different alkyl groups are heated, unsymmetrical ether is obtained.

R—O—Na + R'—X \(\underrightarrow{Δ}\) R—O—R' + NaX

When methyl bromide is heated sodium ethoxide, ethyl methyl ether is formed.

C₂H₅—O-Na   +  CH₃—Br   \(\underrightarrow{Δ}\)    C₂H₅—O—CH₃ + NaBr

When ethyl bromide is heated with sodium methoxide, ethyl methyl ether is formed.

CH₃—O—Na + C₂H₅Br \(\underrightarrow{Δ}\) C₂H₅—O—CH₃ + NaBr

Ammonolysis : When an alkyl halide is boiled under pressure with an excess of alcoholic solution of ammonia (NH₃) corresponding (primary amine) alkyl amine is formed. This reaction is known as ammonolysis of alkyl halide.

R—X + NH₃     \(\frac{413K}{pressure}\)>  R—NH₂ + H X

Action of excess of ammonia on,

(i) Ethyl bromide : When ethyl bromide is boiled under pressure with an excess of alcoholic ammonia, ethylamine (ethanamine) is formed. This is known as ‘ammonolysis’  of ethyl bromide.

CH₃—CH₂—Br   +   NH₃   \(\frac{413K}{pressure}\)> CH₃—CH₂—NH₂ + HBr

(ii) n-propyl bromide : When n-propyl bromide is boiled under pressure with an excess of ammonia, n-propyl amine (propanamine) is formed.

CH₃—CH₂—CH₂—Br + NH₃     \(\frac{413K}{pressure}\)>  CH₃—CH₂—CH₂—NH₂ + HBr

Know This : Cyanide ion is capable of attacking through more than one site (atom). Such nucleophiles are called ambident nucleophiles. KCN is predominantly ionic (K⊕C ≡ N) and provides cyanide ions. Both carbon and nitrogen are capable of donating electron pair. C-C Bond being stronger than C-N bond, attack occurs through carbon atom of cyanide group forming alkyl cyanides as major product. However AgCN (Ag-C ≡ N) is mainly covalent compound and nitrogen is free to donate pair of electron. Hence attack occurs through nitrogen resulting in formation of isocyanide. Another ambident nucleophile is nitrite ion, which can attack through ‘O’ or ‘N’.

Mechanism of S_N reaction :

Leaving group : Leaving group is the group which leaves the carbon by taking away the bond pair of electrons.

Mechanism of S_N reaction :

• The substrate undergoes two changes during a S_N
• The original C-X bond undergoes heterolysis and a new bond is formed between the carbon and the nucleophile using two electrons of the nucleophile. These changes may occur in one or more steps.
• The description regarding the sequence and the way in which these two changes take place in S_N reaction is called mechanism of S_N
• Two mechanisms are observed in various SN reactions. These are denoted as SN¹ and SN²

Nucleophilic bimolecular reaction (SN²) : The substitution reaction in which a nucleophile reacts with the substrate and the rate of the reaction depends on the concentration of the substrate and the nucleophile is called a nucleophilic bimolecular reaction.

SN² mechanism of methyl bromide using aqueous KOH. OR The mechanism of alkaline hydrolysis of methyl bromide or Bromomethane. :

Consider alkaline hydrolysis of methyl bromide (Bromomethane ), CH₃Br with aqueous NaOH or KOH.

CH₃—Br    +   OH^—   →   CH₃—OH   +   Br

Stereochemistry and Kinetics of the reaction (R.D.S.) : This hydrolysis reaction takes place only in one step which is a rate determining step i.e. R.D.S. The rate of hydrolysis reaction depends on the concentration of CH₃Br and OH^— which are present in the R.D.S. of the reaction.

Rate = R = k[CH₃Br] [OH^—], where k is rate constant of the reaction.

SN² reaction : The reaction between methyl bromide and hydroxide ion to form methanol follows a second order kinetics, since the rate of the reaction depends on the concentrations of two reacting species, namely methyl bromide and hydroxide ion it is bimolecular second order (2nd) Nucleophilic Substitution reaction denoted by SN².

Salient features of SN² mechanism :

(i) It is a single step mechanism. The reaction takes place in the following steps :

Fig. : Backside attack of nucleophile in SN² mechanism

(ii) Backside attack of the nucleophile : Nucleophile, OH^— attacks carbon atom of CH₃Br from back side i.e. from opposite side to that of the leaving group i.e. Br^— to experience minimum steric repulsion and electrostatic repulsion between the incoming nucleophile (OH^—) and leaving Br^—.

(iii) Transition state : When a nucleophile, OH^— approaches carbon atom of CH₃Br, the potential energy of the system increases until a transition state (T.S.) of maximum potential energy is formed in which C—Br bond is partially broken and C—OH bond is partially formed. The negative charge is equally shared by both incoming nucleophile —OH^— and outgoing, leaving group -Br^—. (Thus, the total negative charge is diffused.)

(iv) In CH₃Br, carbon atom is sp³-hybridized and CH₃Br molecule is tetrahedral. The hybridisation of carbon atom changes to sp²-hybridisation. The transition state contains carbon having three a (sigma) bonds in one plane making bond angles of 120° with each other i.e., H₁, H₂ and H₃ atoms lie in one plane while two partial

covalent bonds containing Br and OH lie collinear and on opposite sides perpendicular to the plane.

(v) Inversion of configuration :

• The transition state decomposes fast by the complete breaking of the C—Br bond and the new C—OH bond is formed on the other side.
• The breaking of C—Br bond and the formation of C—OH bond take place simultaneously.
• The energy required to break the C—Br bond is partly obtained from the energy released when C—OH bond is formed.
• The formation of product CH₃OH is accompanied by complete or 100% inversion of configuration forming again sp³-hybridized carbon atom giving tetrahedral CH₃OH molecule. But in this structure the positions of H₂ and H₃ atoms in the reactant (CH₃Br) and in product are on the opposite side.
• This inversion of configuration is called Walden inversion.

SN¹ reaction : The substitution reaction in which a nucleophile reacts with the substrate and the rate of the reaction depends only on the concentration of the substrate is called nucleophilic unimolecular or first order reaction or SN¹ reaction.

• The reaction between tert-butyl bromide and hydroxide ion to give tert-butyl alcohol follows a first-order kinetics, that is the rate of this reaction depends on concentration of only one species, which is the substrate molecule, tert-butyl bromide. Hence it is called substitution nucleophilic unimolecular, SN₁.

Consider alkaline hydrolysis of tert-butyl bromide (2-Bromo-2-methylpropane) with aqueous NaOH or KOH

Kinetics of the reaction : Due to steric hindrance of voluminous three methyl groups around carbon, nucleophile OH^—  cannot attack carbon atom directly. Hence, the reaction takes place in two steps.

Step I : This involves heterolytic fission of C—Br covalent bond in the substrate  forming planar carbocation and Br^—. This is a slow and reversible process.

Step II : This step involves attack of nucleophile OH^— or carbocation fonning C—OH bond and product tert-butyl alcohol. Since it involves ionic charge neutralisation, it is a fast step.

Rate Determining Step (R.D.S.) : Since the first step is a slow step, it is R.D.S., and therefore the rate of the reaction depends on the concentration of only one reactant, (CH₃)₃C—Br.

Rate = R = k [(CH₃)₃C—Br] where k is a rate constant of the reaction.

Stereochemistry and mechanism of the reaction : The reaction takes place in two steps and both the steps involve formation of transition states (T.S.).

T.S.-I for first step :

• In this transition state, C—Br bond is partially broken, so that carbon atom carries partial positive Charge (+ δ) and Br carries partial negative charge ( — δ) which further breaks forming carbocation and Br^—
• Tart-butyl cation (carbocation) has a planar structure and the CH₃ —C—CH₃ bond angle is 120°.
• It is the intermediate of the reaction. It is unstable. In this step, hybridisation of carbon atom changes from sp³ (tetrahedral geometry) to sp³ (planar geometry).

T.S.—II for second step :

In this transition state, C—OH bond is partially formed so that carbon atom carries partial positive charge (+δ) and OH carries partial negative charge (—δ) which further forms tert-butyl alcohol.

Fig. SN¹ mechanism

Formation of a racemic mixture : Since OH^— has equal probability of the attack on carbocation from front side and from backside, the products obtained are equal. In case of optical active alkyl halide, a racemic mixture is obtained.

Salient features of SN¹ mechanism :

• Two step
• Heterolyis of C-X bond in the slow and reversible first step to form planar
• carbocation intermediate.
• Attack of the nucleophile on the carbocation intermediate in the fast second step to form the product.
• When SN1 reaction is carried out at chiral carbon in an optically active substrate, the product formed is nearly racemic. This indicates that SN₁ reaction proceeds mainly with racemization. This means both the enantiomers of product are formed in almost equal amount. Racemization in SN¹ reaction is the result of formation of planar carbocation intermediate. Nucleophile can attack planar carbocation from either side which results in formation of both the enantiomers of the product.

Know This : A negatively charged nucleophile is more powerful than its conjugate acid. For example, R-O— is better nucleophile than R-OH. When donor atoms are from same period of periodic table, nucleophilicity decreases from left to right in a period. For example is less powerful nucleophile than NH3. When donor atoms are from same group of the periodic table, nucleophilicity increases down the group. For example,  is better nucleophile than Cl .

Question. Primary allylic and primary benzylic halides show higher reactivity by SN¹ mechanism than other primary alkyl halides. Explain.

Question. Which of the following two compounds would react faster by SN²mechanism and Why ?

Elimination reaction : Dehydrohalogenation

When alkyl halide having at least one β-hydrogen is boiled with alcoholic solution of potassium hydroxide, it undergoes elimination of hydrogen atom from β-carbon and halogen atom from α-carbon resulting in the formation of an alkene. This reaction is called β-elimination (or 1,2 - elimination) reaction as it involves elimination of halogen and a β – hydrogen atom.

Example:

Tertiary butyl bromide when heated with alcoholic solution of potassium hydroxide forms isobutylene

This reaction involves elimination of halogen and a β-hydrogen atom. As hydrogen and halogen is removed in this reaction it is also known as dehydrohalogenation reaction.

Action of alc. KOH on 2-bromobutane :

If there are two or more non-equivalent β-hydrogen atoms in a halide, then this reaction gives a mixture of products. Thus, 2-bromobutane on heating with alcoholic KOH gives mixture of but-1-ene and but-2-ene.

Saytzeff’s rule :

• In dehydrohalogenation reaction the preferred product is that alkene which has the greater number of alkyl groups attached to the doubly bonded carbon atoms.
• Hence the number of alkyl substituents on doubly bonded carbon atoms increases, the stability of the alkene giving its major products.

Hence the increasing stability of alkenes is,

There are two types of β hydrogens (β₁ and β₂) therefore two alkenes are expected

Know This : Elimination versus substitution: Alkyl halides undergo sunstitution as well as elimination reaction. Both reactions are brought about by basic reagent, hence there is always a competition between these two reactions. The reaction which actually predominates depends upon following factors. Nature of alkyl halides : Tertiary alkyl halides prefer to undergo elimination reaction where as primary alkyl halides prefer to undergo substitution reaction. Strength and size of nucleophile : Bulkier electron rich species prefers to act as base by abstracting proton, thus favours elimination. Substitution is favoured in the case of comparatively weaker bases, which prefer to act as nucleophile Reaction conditions : Less polar solvent, high temperature fovours elimination where as low tempertaure, polar solvent favours substitution reaction.

Question. Convert Ethanol to propane nitrile.

Question. Convert But-1-ene to n-butyl iodide.

Reaction with active metals :

Organometallic compounds : Active metals like sodium, magnesium cadmium readily combine with alkyl chlorides, bromides and iodides to form compounds containing carbon-metal bonds. These are known as organometallic compounds.

Grignard reagent : An organometallic compound in which the divalent magnesium is directly linked to an alkyl group (R—) and a halogen atom (X), and has general formula R—Mg—X is called Grignard reagent. OR

When alkyl halide is treated with magnesium in dry ether as solvent, it gives alkyl magnesium halide. It is known as Grignard reagent.

R—X + Mg \(\underrightarrow{dry\,,ether}\) R—Mg—X

• The carbon-magnesium bond is highly polar and magnesium-halogen bond is in ionic in nature.
• Grignard reagent is highly reactive.
• It is an important reagent and used in the preparation of a large number of organic compounds.

Preparation of Grignard reagent :

(i) When an alkyl halide like CH₃—I is added from a dropping funnel to a flask containing pieces of pure Mg in pure and dry ether (dielhyl ether) and a trace of iodine, Grignard reagent, CH₃—Mg—I is formed.

CH₃—I    +  Mg  \(\underrightarrow{dry\,,ether}\)   CH₃—Mg—I

(ii) Ethyl iodide when treated with magnesium in presence of dry ether forms ethyl magnesium iodide

C₂H₅—I  +  Mg  \(\underrightarrow{dry\,,ether}\)   C₂H₅—Mg—I

Grignard reagents’ reaction with water or compounds containing hydrogen attached to electronegative element :

Examples :

Action of water on :

(1) Methyl magnesium iodide : When methyl magnesium iodide is treated with water, methane is obtained

(2) Ethyl magnesium iodide : When ethyl magnesium iodide is treated with water, ethane is obtained.

Know This : Carbon-magnesium bond in Grignard reagent is a polar covalent bond. The carbon pulls electrons from the electropositive magnesium. Hence carbon in Grignard reagent has negative polarity and acts as a nucleophite Victor Grignard received Nobel Prize in 1912 for synthesis and study of organomagnesium compounds. Grignard reagent is a very versatile reagent used by organic chemist. Vinyl and aryl halides also form Grignard reagent.

Wurtz reaction : Alkyl halides react with metallic sodium in dry ether as solvent, and form higher alkanes containing double the number of carbon atoms present in alkyl halide. This reaction is called Wurtz reaction.

2R—X + 2Na    R—R + 2NaX

• In this reaction the alkyl radicals from two molecules of the reacting alkyl halide combine or couple to form the higher alkane.

Thus, methyl bromide reacts with sodium in ether to form ethane (C₂H₆), whlle ethyl bromide under the same conditions forms n-butane (C₄H₁₀)

2CH₃Br   +   2Na   \(\underrightarrow{dry\,,ether}\)     CH₃-CH₃ + 2NaBr

2C₂H₅Br   +   2Na   \(\underrightarrow{dry\,,ether}\)     CH₃-CH₂-CH₂-CH₃ + 2NaBr

When a mixture of two different alkyl halides is used, all the three possible alkanes are formed.

Example: a mixture of CH₃Br and C₂H₅Br gives propane along with C₂H₆ and C₄H₁₀

Reaction of haloarenes :

Wurtz- Fittig reaction : The reaction of aryl halide with alkyl halide and sodium metal in dry ether to give substituted aromatic compounds is known as Wurtz- Fittig reaction.

• This reaction is an extension of Wurtz reaction and was carried out by Fittig.
• This reaction allows alkylation of aryl halides.

Action of aryl halide on sodium metal : In this case only aryl halide takes part in the reaction, the product is biphenyl and the reaction is known as Fittig reaction.

Nucleophilic substitution SN of haloarenes :

Haloarenes (Aryl halides) show low reactivity towards nucleophilic substitution reactions. The low reactivity of aryl halides is due to :

• Resonance effect and
• sp² hybrid state of C .

Resonance effect : In haloarenes, the electron pairs on halogen atom are in conjugation with π-electrons of the benzene ring. The delocalization of these electrons C-Cl bond acquires partial double bond character.

For example, different resonance structures of chlorobenzene.

Resonance structures II, III and IV show double bond character to carbon-chlorine bond.

• Due to partial double bond character of C-Cl bond in aryl halides, the bond cleavage in haloarene is difficult and are less reactive. On the other hand, in alkyl halides, carbon is attached to chlorine by a single bond and it can be easily broken.
• Aryl halides are stabilized by resonance but alkyl halides are not. Hence, the energy of activation for the displacement of halogen from aryl halides is much greater than that of alkyl halides.

sp² hybrid state of C :

Different hybridization state of carbon atom in C-X bond :

• In alkyl halides. the carbon of C-X bond is sp³-hybridized with less s-character and greater bond length of 178 pm. which requires less energy to break the C-X bond.
• In aryl halides the carbon of C-X bond is sp²-hybridized with more s-character and shorter bond length which requires more energy to break C-X bond. Therefore. aryl halides are less reactive than alkyl halides.
• Polarity of the C-X bond : In aryl halide C-X bond is less polar than in alkyl halides. Because sp²-hybrid carbon of C-X bond has less tendency to release electrons to the halogen than a sp³-hybrid carbon in alkyl halides. Thus halogen atom in aryl halides cannot be easily displaced by nucleophile.

Instability of Phenyl cation :

• Phenyl cation produced due to selfionization of haloarene will not be stabilized by resonance, which rules out possibility of SN¹
• Back side attack of nucleophile is blocked by the aromatic ring, which rules out SN²
• Thus cations are not formed and hence aryl halides do not undergo nucleophilic substitution reaction easily.

π-bond :

• As any halides are electron rich molecules due to the presence of π-bond, they repel electron rich nucleophilic attack. Hence, aryl halides are less reactive towards nucleophilic substitution reactions.
• However, the presence of electron withdrawing groups at o/p position activates the halogen of aryl halides towards substitution.

Electrophilic substitution (SE) in aryl halides :

Aryl halides undergo electrophilic substitution reactions slowly and it can be explained as follows :

Inductive effect : The strongly electronegative halogen atom withdraws the electrons from carbon, atom of the ring (—I effect). This deactivates the ring hence aryl halides show reactivity towards electrophilic attack.

Resonance effect : The resonating structures of aryl halides show increase in electron density at ortho and para position, hence it is o, p directing.

The inductive effect and resonance effect compete with each other. The inductive effect is stronger than resonance effect. The reactivity of aryl halides is controlled by stronger inductive effect and o, p orientation is controlled by weaker resonating effect.

The attack of electrophile (Y) on haloarenes at ortho and para positions are more stable due to formation of chloronium ion. The chloronium ion formed is comparatively more stable than other hybrid structures of carbonium.

Uses and Environmental effects of some polyhalogen compounds :

(i) Dichloromethane/ methylene chloride (CH₂Cl₂) :

It is a colourless volatile liquid with moderately sweet aroma.

Uses :

• Dichloromethane dissolves wide range of organic compounds, hence it is used as solvent for many chemical reactions.
• It is used as a solvent as a paint remover and degreaser.
• It is used as propellant in aerosols and as a fumigant pesticide for grains and strawberries.
• It is used to decaffinate tea or coffee.

Environmental effects :

• Higher levels of dicliloromethane in air causes nausea, numbness in fingers and toes, dizziness
• Lower levels of dichloromethane causes impaired vision and hearing.
• Direct contact with eyes can damage cornea.

(ii) Chloroform / trichloromethane (CHCl₃) :

It is a colourless liquid with peculiar sweet smell.

Uses :

• Chloroform in the production of chlorofluoromethane, freon refrigerant R-22.
• It is used as solvent in pharmaceuticals, pesticides, gums, fats, resins and dye industry.
• It is a good source of dichlorocarbene species.

Environmental effects :

• When chloroform is exposed to air in the presence of sunlight, it slowly oxidised to phosgene, a poisonous compound, therefore it is stored in dark, amber coloured bottles.
• Chloroform vapour when inhaled for a short time causes dizziness, headache and fatigue and if inhaled for a long time affects central nervous system.

(iii) Tetrachloromethane or carbon tetrachloride (CCl₄) :

It is a colourless liquid with sweet smell.

Uses :

• Carbon tetrachloride is used in the manufacture of refrigerants.
• It is used as a dry cleaning agent and as a pesticide for stored grains.
• It is very useful solvent for oils, fats and resins. It senses as a source of chlorine.

Environmental effects :

• Exposure to carbon tetrachloride causes eye irritation, damages nerve cells, vomiting sensation, dizziness, unconciousness or death. Long exposure to chloroform may affect liver.
• When mixed with air it causes depletion of the ozone layer, which affects human skin leading to cancer.

(iv) Iodoform or triiodomethane (CHI₃) :

It is a yellow crystalline substance with disagreeable smell.

Uses :

• Iodoform is used as antiseptic, dressing of wounds and sores.
• On small scale it is used as disinfectant.

Environmental effects :

• Iodoform has a strong smell. It causes irritation to skin and eyes. It may cause respiratory irritation or breathing difficulty, dizziness, nausea, depression of central nervous system, visual disturbance.

(v) Freons :

These are organic compounds of chlorine and fluorine, chlorofluorocarbons,

The most common representative is dichlorodifluromethane (Freon-12) others

include chlorodifluromethane (R-22), trichlorofluromethane (R-11)

Uses :

• Freons are widely used as propellants in aerosol, products of food, cosmetics and pharmaceutical industries.
• Freons containing bromine in their molecules are used as fire extinguishers.
• They are used in aerosol insecticides, solvent for cleaning clothes and metallic surfaces. It is used as foaming agents in the preparation of foamed plastics and in production of certain fluorocarbons.
• It is used as refrigerants and air conditioning purposes.

Environmental effects :

• Freon as refrigerant causes ozone depletion.
• Freons have low toxicity and low biological activity.
• Freons from propane group are more toxic in nature.
• Regular large inhalation of freon results in breathing problems, organ damage, loss of consciousness.

(vi) Dichlorodiphenyltrichloroethane (DDT) :

It is colourless, tasteless and odorless crystalline compound having insecticidal property.

Uses :

• DDT is used as insecticide against malaria and typhus.
• It is used to kill various insects like housefly and mosquitoes

Environmental effects :

• DDT is not readily metabolised by animals.
• It is deposited and stored in fatty tissues.
• Exposure to high doses of DDT may cause vomiting, tremors or shakiness.
• Laboratory animal studies showed adverse effect of DDT on liver and reproduction.
• DDT is a pressistent organic pollutant, readily absorbed in soils and tends to accumulate in the ecosystem.
• When dissolved in oil or other lipid, it is readily absorbed by the skin. It is resistant to metabolism. There is a ban on use of DDT due to all these adverse effects.

Know This : Q. How do CFC distroy the ozone layer in the atmosphere ? Answer : When ultraviolet radiation (UV) strikes CFC (CFCl3) molecules in the upper atmosphere, the carbon-chlorine bond breaks and produces highly reactive chlorine atom (Cl). CFCl3 → CFCl2 + Cl This reactive chlorine atom decomposes ozone (O3) molecule into oxygen molecule (O2). O3 + Cl → O2 + ClO ClO + O → O2 + Cl One atom of chlorine can destroy upto 100,000 ozone molecules.