Haloalkanes and Haloarenes Notes

Haloalkanes and Haloarenes

Introduction

• Replacement of hydrogen atom(s) in aliphatic or aromatic hydrocarbons by halogen atom(s) results in the formation of alkyl halide (haloalkane) and aryl halide (haloarene), respectively.
• Haloalkanes contain halogen atom(s) attached to the sp3 hybridised carbon atom of an alkyl group.
• Haloarenes contain halogen atom(s) attached to sp2 hybridised carbon atom(s) of an aryl group.
• Many halogen-containing organic compounds occur in nature and some are clinically useful.
• These compounds are used as solvents for relatively non-polar compounds and as starting materials for the synthesis of a wide range of organic compounds.
• Chloramphenicol, a chlorine-containing antibiotic, is effective for typhoid fever treatment.
• Thyroxine, an iodine-containing hormone produced by the body, deficiency causes goiter.
• Synthetic halogen compounds like chloroquine are used for malaria treatment, and halothane is used as an anaesthetic during surgery.
• Fluorinated compounds are being considered as potential blood substitutes in surgery.
• Halogenated compounds persist in the environment due to their resistance to breakdown by soil bacteria.

Classification

Based on Number of Halogen Atoms

• Haloalkanes and haloarenes can be classified as mono, di, or polyhalogen compounds depending on the number of halogen atoms in their structures.

Compounds Containing $sp^3$ C—X Bond (X= F, Cl, Br, I)

• This class includes:

  • Alkyl halides or haloalkanes (R—X):
    • The halogen atom is bonded to an alkyl group (R), represented by the formula $C<em>nH</em>{2n+1}X$.
    • They are classified as primary ($1^o$), secondary ($2^o$), or tertiary ($3^o$) based on the carbon atom to which the halogen is attached.
  • Allylic halides:
    • The halogen atom is bonded to an $sp^3$-hybridised carbon atom adjacent to a carbon-carbon double bond (C=C).
  • Benzylic halides:
    • The halogen atom is bonded to an $sp^3$-hybridised carbon atom attached to an aromatic ring.

Compounds Containing $sp^2$ C—X Bond

• This class includes:

  • Vinylic halides:
    • The halogen atom is bonded to an $sp^2$-hybridised carbon atom of a carbon-carbon double bond (C = C).
  • Aryl halides:
    • The halogen atom is directly bonded to the $sp^2$-hybridised carbon atom of an aromatic ring.

Nomenclature

• The common names of alkyl halides are derived by naming the alkyl group followed by the name of the halide.
• In the IUPAC system, alkyl halides are named as halo-substituted hydrocarbons.
• For mono-halogen-substituted derivatives of benzene, common and IUPAC names are the same.
• For dihalogen derivatives, the prefixes o-, m-, p- are used in the common system, while the numerals 1,2; 1,3; and 1,4 are used in the IUPAC system.
• Dihaloalkanes with the same type of halogen atoms are named as alkylidene or alkylene dihalides.
  • Geminal dihalides (gem-dihalides) have both halogen atoms on the same carbon atom.
  • Vicinal dihalides (vic-dihalides) have halogen atoms on adjacent carbon atoms.
• In the common name system, gem-dihalides are named as alkylidene halides, and vic-dihalides are named as alkylene dihalides.
• In the IUPAC system, they are named as dihaloalkanes.

Nature of C-X Bond

• Halogen atoms are more electronegative than carbon, so the carbon-halogen bond of alkyl halides is polarised.
• The carbon atom bears a partial positive charge ($\delta$+), and the halogen atom bears a partial negative charge ($\delta$−).
• The size of the halogen atom increases down the group in the periodic table.
• The carbon-halogen bond length also increases from C—F to C—I.
• Alkyl halides are best prepared from alcohols, which are easily accessible.

Methods of Preparation of Haloalkanes

From Alcohols

• The hydroxyl group of an alcohol is replaced by halogen on reaction with concentrated halogen acids, phosphorus halides, or thionyl chloride.
• Thionyl chloride ($SOCl<em>2$) is preferred because it forms alkyl halide along with gases $SO</em>2$ and $HCl$, which escape, giving pure alkyl halides.
• Reactions of primary and secondary alcohols with $HCl$ require a catalyst, $ZnCl_2$. Tertiary alcohols react with concentrated $HCl$ at room temperature.
• Constant boiling with $HBr$ (48%) is used for preparing alkyl bromide.
• Good yields of R—I may be obtained by heating alcohols with sodium or potassium iodide in 95% orthophosphoric acid.
• The order of reactivity of alcohols with a given haloacid is 3^o > 2^o > 1^o.
• Phosphorus tribromide and triiodide are usually generated in situ by the reaction of red phosphorus with bromine and iodine, respectively.
• The above methods are not applicable for the preparation of aryl halides because the carbon-oxygen bond in phenols has a partial double bond character and is difficult to break being stronger than a single bond.

From Hydrocarbons

From alkanes by free radical halogenation

• Free radical chlorination or bromination of alkanes gives a complex mixture of isomeric mono- and polyhaloalkanes, which is difficult to separate as pure compounds.
• Consequently, the yield of any single compound is low.

From alkenes

• (i) Addition of hydrogen halides:
  • An alkene is converted to the corresponding alkyl halide by reaction with hydrogen chloride, hydrogen bromide, or hydrogen iodide.
  • Propene yields two products, but only one predominates according to Markovnikov’s rule.
• (ii) Addition of halogens:
  • Addition of bromine in $CCl_4$ to an alkene results in the discharge of the reddish-brown colour of bromine, which is an important method for detecting double bonds in a molecule.
  • The addition results in the synthesis of vic-dibromides, which are colourless.

Halogen Exchange

• Alkyl iodides are often prepared by the reaction of alkyl chlorides/bromides with $NaI$ in dry acetone (Finkelstein reaction).
  • $NaCl$ or $NaBr$ thus formed is precipitated in dry acetone, facilitating the forward reaction according to Le Chatelier’s Principle.
• The synthesis of alkyl fluorides is best accomplished by heating an alkyl chloride/bromide in the presence of a metallic fluoride such as $AgF$, $Hg<em>2F</em>2$, $CoF<em>2$, or $SbF</em>3$ (Swarts reaction).

Preparation of Haloarenes

From hydrocarbons by electrophilic substitution

• Aryl chlorides and bromides can be easily prepared by electrophilic substitution of arenes with chlorine and bromine, respectively, in the presence of Lewis acid catalysts like iron or iron(III) chloride.
• The ortho and para isomers can be easily separated due to the large difference in their melting points.
• Reactions with iodine are reversible and require the presence of an oxidising agent ($HNO<em>3$, $HIO</em>4$) to oxidise the HI formed during iodination.
• Fluoro compounds are not prepared by this method due to the high reactivity of fluorine.

From amines by Sandmeyer’s reaction

• A primary aromatic amine, dissolved or suspended in cold aqueous mineral acid, is treated with sodium nitrite to form a diazonium salt.
• Mixing the solution of freshly prepared diazonium salt with cuprous chloride or cuprous bromide results in the replacement of the diazonium group by –Cl or –Br.
• Replacement of the diazonium group by iodine does not require the presence of cuprous halide and is done simply by shaking the diazonium salt with potassium iodide.

Physical Properties

• Alkyl halides are colourless when pure, but bromides and iodides develop colour when exposed to light.
• Many volatile halogen compounds have a sweet smell.

Melting and Boiling Points

• Methyl chloride, methyl bromide, ethyl chloride, and some chlorofluoromethanes are gases at room temperature. Higher members are liquids or solids.
• Organic halogen compounds are generally polar.
• Due to greater polarity and higher molecular mass compared to the parent hydrocarbon, the intermolecular forces of attraction (dipole-dipole and van der Waals) are stronger in the halogen derivatives.
• Boiling points of chlorides, bromides, and iodides are considerably higher than hydrocarbons of comparable molecular mass.
• The attractions get stronger as the molecules get bigger in size and have more electrons.
• For the same alkyl group, the boiling points of alkyl halides decrease in the order: RI > RBr > RCl > RF.
• The boiling points of isomeric haloalkanes decrease with increased branching.
• Boiling points of isomeric dihalobenzenes are very nearly the same, but the para-isomers have higher melting points due to symmetry.

Solubility

• Haloalkanes are very slightly soluble in water.
• Energy is required to overcome the attractions between haloalkane molecules and break the hydrogen bonds between water molecules to dissolve haloalkanes in water.
• Less energy is released when new attractions are set up between the haloalkane and the water molecules, as these are not as strong as the original hydrogen bonds in water.
• Haloalkanes tend to dissolve in organic solvents because the new intermolecular attractions between haloalkanes and solvent molecules have much the same strength as the ones being broken in the separate haloalkane and solvent molecules.
• Bromo, iodo, and polychloro derivatives of hydrocarbons are heavier than water.
• Density increases with an increase in the number of carbon atoms, halogen atoms, and the atomic mass of the halogen atoms.

Chemical Reactions

• The reactions of haloalkanes may be divided into the following categories:
  1. Nucleophilic substitution
  2. Elimination reactions
  3. Reaction with metals

Reactions of Haloalkanes

Nucleophilic Substitution Reactions

• Nucleophiles are electron-rich species that attack the electron-deficient part of a substrate molecule.
• The reaction in which a nucleophile replaces an already existing nucleophile in a molecule is known as a nucleophilic substitution reaction.
• Haloalkanes serve as substrates in these reactions.
• A nucleophile reacts with a haloalkane (the substrate) that has a partial positive charge on the carbon atom bonded to the halogen, leading to a substitution reaction. The halogen atom departs as a halide ion and is called leaving group.
• This is one of the most useful classes of organic reactions of alkyl halides in which a halogen is bonded to $sp^3$ hybridised carbon.
• Groups like cyanides and nitrites possess two nucleophilic centres and are called ambident nucleophiles.
• Cyanide group can act as a nucleophile in two different ways [:C≡N ↔ :C=N:], i.e., linking through carbon atom resulting in alkyl cyanides and through nitrogen atom leading to isocyanides.
• Similarly nitrite ion is also an ambident nucleophile with two different points of linkage [−O−N=O].
  • The linkage through oxygen results in alkyl nitrites while through nitrogen atom, it leads to nitroalkanes.

Mechanism

• This reaction proceeds by two different mechanisms:

  • Substitution Nucleophilic Bimolecular ($S_N2$):
    • The reaction between $CH_3Cl$ and hydroxide ion to yield methanol and chloride ion follows second-order kinetics.
    • The rate depends upon the concentration of both reactants.
    • The incoming nucleophile interacts with the alkyl halide, causing the carbon-halide bond to break as a new bond forms between the carbon and the attacking nucleophile.
    • These two processes take place simultaneously in a single step, and no intermediate is formed.
    • As the reaction progresses, the bond between the incoming nucleophile and the carbon atom starts forming, and the bond between the carbon atom and leaving group weakens.
    • The three carbon-hydrogen bonds of the substrate start moving away from the attacking nucleophile.
    • In the transition state, all three C-H bonds are in the same plane, and the attacking and leaving nucleophiles are partially attached to the carbon.
    • As the attacking nucleophile approaches closer to the carbon, the C-H bonds keep moving in the same direction until the attacking nucleophile attaches to the carbon and the leaving group leaves the carbon.
    • As a result, the configuration inverts, and the carbon atom under attack inverts much the same way as an umbrella is turned inside out when caught in a strong wind.
    • This process is called inversion of configuration.
    • In the transition state, carbon is simultaneously bonded to five atoms.
    • The presence of bulky substituents on or near the carbon atom have a dramatic inhibiting effect.
    • Of the simple alkyl halides, methyl halides react most rapidly in $S_N2$ reactions because there are only three small hydrogen atoms. Tertiary halides are the least reactive because bulky groups hinder the approaching nucleophiles.
    • The order of reactivity followed is: Primary halide > Secondary halide > Tertiary halide.
  • Substitution Nucleophilic Unimolecular ($S_N1$):
    • $S_N1$ reactions are generally carried out in polar protic solvents (like water, alcohol, acetic acid, etc.).
    • The reaction between tert-butyl bromide and hydroxide ion yields tert-butyl alcohol and follows first-order kinetics.
    • The rate of reaction depends upon the concentration of only one reactant, which is tert-butyl bromide.
    • It occurs in two steps:
      • Step I: The polarised C—Br bond undergoes slow cleavage to produce a carbocation and a bromide ion.
      • Step II: The carbocation thus formed is then attacked by nucleophile to complete the substitution reaction.
    • Step I is the slowest and reversible.
    • It involves the C–Br bond breaking for which the energy is obtained through solvation of halide ion with the proton of protic solvent.
    • Since the rate of reaction depends upon the slowest step, the rate of reaction depends only on the concentration of alkyl halide and not on the concentration of hydroxide ion.
    • Further, the greater the stability of carbocation, the greater will be its ease of formation from alkyl halide and the faster will be the rate of reaction.
    • Primary halides undergo $S_N1$ reaction very fast because of the high stability of primary carbocations.
    • The order of reactivity of alkyl halides towards $S<em>N1$ and $S</em>N2$ reactions is as follows:
    • R–I> R–Br>R–Cl>>R–F.
    • Allylic and benzylic halides show high reactivity towards the $S_N1$ reaction because the carbocation thus formed gets stabilised through resonance.

Stereochemical Aspects of Nucleophilic Substitution Reactions

• (i) Optical activity:

  • Plane of plane polarised light produced by passing ordinary light through Nicol prism is rotated when it is passed through the solutions of certain compounds.
  • Such compounds are called optically active compounds.
  • The angle by which the plane polarised light is rotated is measured by an instrument called polarimeter.
  • If the compound rotates the plane of plane polarised light to the right, i.e., clockwise direction, it is called dextrorotatory (Greek for right rotating) or the d-form and is indicated by placing a positive (+) sign before the degree of rotation.
  • If the light is rotated towards left (anticlockwise direction), the compound is said to be laevo-rotatory or the l-form and a negative (–) sign is placed before the degree of rotation.
  • Such (+) and (–) isomers of a compound are called optical isomers, and the phenomenon is termed as optical isomerism.

• (ii) Molecular asymmetry, chirality, and enantiomers:

  • The observation of Louis Pasteur (1848) that crystals of certain compounds exist in the form of mirror images laid the foundation of modern stereochemistry.
  • He demonstrated that aqueous solutions of both types of crystals showed optical rotation, equal in magnitude (for solution of equal concentration) but opposite in direction.
  • He believed that this difference in optical activity was associated with the three-dimensional arrangements of atoms in the molecules (configurations) of two types of crystals.
  • Dutch scientist, J. Van’t Hoff and French scientist, C. Le Bel in the same year (1874), independently argued that the spatial arrangement of four groups (valencies) around a central carbon is tetrahedral and if all the substituents attached to that carbon are different, the mirror image of the molecule is not superimposed (overlapped) on the molecule; such a carbon is called asymmetric carbon or stereocentre.
  • The resulting molecule would lack symmetry and is referred to as an asymmetric molecule.
  • The asymmetry of the molecule along with the non-superimposability of mirror images is responsible for the optical activity in such organic compounds.
  • The symmetry and asymmetry are also observed in many everyday objects: a sphere, a cube, a cone, are all identical to their mirror images and can be superimposed.
  • The objects which are non- superimposable on their mirror image (like a pair of hands) are said to be chiral, and this property is known as chirality.
  • Chiral molecules are optically active, while the objects which are superimposable on their mirror images are called achiral. These molecules are optically inactive.
  • The presence of a single asymmetric carbon atom can assist in recognising chiral molecules.
  • Stereoisomers related to each other as non-superimposable mirror images are called enantiomers.
  • Enantiomers possess identical physical properties namely, melting point, boiling point, refractive index, etc.
  • They only differ with respect to the rotation of plane polarised light.
  • If one of the enantiomers is dextro rotatory, the other will be laevo rotatory.
  • A mixture containing two enantiomers in equal proportions will have zero optical rotation, as the rotation due to one isomer will be cancelled by the rotation due to the other isomer.
  • Such a mixture is known as a racemic mixture or racemic modification.
  • A racemic mixture is represented by prefixing dl or (±) before the name.
  • The process of conversion of an enantiomer into a racemic mixture is known as racemisation.

• (iii) Retention:

  • Retention of configuration is defined as the preservation of the spatial arrangement of bonds to an asymmetric centre during a chemical reaction or transformation.
  • In general, if during a reaction, no bond to the stereocentre is broken, the product will have the same general configuration of groups around the stereocentre as that of reactant.
  • Such a reaction is said to proceed with retention of the configuration.

• (iv) Inversion, Retention, and Racemisation:

  • There are three outcomes for a reaction at an asymmetric carbon atom when a bond directly linked to an asymmetric carbon atom is broken.
    • If (A) is the only compound obtained, the process is called retention of configuration.
    • If (B) is the only compound obtained, the process is called inversion of configuration.
    • If a 50:50 mixture of A and B is obtained then the process is called racemisation, and the product is optically inactive, as one isomer will rotate the plane-polarised light in the direction opposite to another.
  • $S_N2$ reactions of optically active halides are accompanied by inversion of configuration because the nucleophile attaches itself on the side opposite to the one where the halogen atom is present.
  • SN1 reactions are accompanied by racemisation because the carbocation formed in the slow step, being $sp^2$ hybridised, is planar (achiral).
    • The attack of the nucleophile may be accomplished from either side of the plane of carbocation resulting in a mixture of products, one with the same configuration and the other having the opposite configuration.

Elimination Reactions

• When a haloalkane with a β-hydrogen atom is heated with an alcoholic solution of potassium hydroxide, there is elimination of a hydrogen atom from the β-carbon and a halogen atom from the α-carbon atom.
• As result, an alkene is formed as a product.
• Since β-hydrogen atom is involved in elimination, it is often called β-elimination.
• If there is the possibility of formation of more than one alkene due to the availability of more than one β-hydrogen atoms, usually one alkene is formed as the major product.
• In dehydrohalogenation reactions, the preferred product is that alkene which has the greater number of alkyl groups attached to the doubly bonded carbon atoms (Zaitsev’s rule).

Reaction with Metals

• Most organic chlorides, bromides, and iodides react with certain metals to give compounds containing carbon-metal bonds.
• Such compounds are known as organometallic compounds.
• An important class of organometallic compounds discovered by Victor Grignard in 1900 is alkyl magnesium halide, RMgX, referred to as Grignard Reagents.
• Grignard reagents are obtained by the reaction of haloalkanes with magnesium metal in dry ether.
• In the Grignard reagent, the carbon-magnesium bond is covalent but highly polar, with carbon pulling electrons from electropositive magnesium; the magnesium halogen bond is essentially ionic.
• Grignard reagents are highly reactive and react with any source of proton to give hydrocarbons.
• Even water, alcohols, amines are sufficiently acidic to convert them to corresponding hydrocarbons.
• It is therefore necessary to avoid even traces of moisture from a Grignard reagent; that is why the reaction is carried out in dry ether.
• Alkyl halides react with sodium in dry ether to give hydrocarbons containing double the number of carbon atoms present in the halide (Wurtz reaction).

Reactions of Haloarenes

Nucleophilic Substitution

• Aryl halides are extremely less reactive towards nucleophilic substitution reactions due to the following reasons:
  • (i) Resonance effect:
    • In haloarenes, the electron pairs on halogen atom are in conjugation with π-electrons of the ring.
    • C—Cl bond acquires a partial double bond character due to resonance.
    • As a result, the bond cleavage in haloarene is difficult than haloalkane, and therefore, they are less reactive towards nucleophilic substitution reaction.
  • (ii) Difference in hybridisation of carbon atom in C—X bond:
    • In haloalkane, the carbon atom attached to halogen is $sp^3$ hybridised, while in the case of haloarene, the carbon atom attached to halogen is $sp^2$-hybridised.
    • The $sp^2$ hybridised carbon with a greater s-character is more electronegative and can hold the electron pair of C—X bond more tightly than $sp^3$-hybridised carbon in haloalkane with less s-chararcter.
    • Thus, C—Cl bond length in haloalkane is 177pm, while in haloarene is 169 pm.
    • Since it is difficult to break a shorter bond than a longer bond, therefore, haloarenes are less reactive than haloalkanes towards nucleophilic substitution reaction.
  • (iii) Instability of phenyl cation:
    • In the case of haloarenes, the phenyl cation formed as a result of self-ionisation will not be stabilised by resonance, and therefore, the $S_N1$ mechanism is ruled out.
  • (iv) Because of the possible repulsion, it is less likely for the electron-rich nucleophile to approach electron-rich arenes.

Replacement by Hydroxyl Group

• Chlorobenzene can be converted into phenol by heating in aqueous sodium hydroxide solution at a temperature of 623K and a pressure of 300 atmospheres.
• The presence of an electron-withdrawing group (-$NO_2$) at ortho- and para-positions increases the reactivity of haloarenes.
• The effect is pronounced when (-$NO_2$) group is introduced at ortho- and para-positions.
• The presence of a nitro group at ortho- and para-positions withdraws the electron density from the benzene ring and thus facilitates the attack of the nucleophile on haloarene.

Electrophilic Substitution Reactions

• Haloarenes undergo the usual electrophilic reactions of the benzene ring such as halogenation, nitration, sulphonation, and Friedel-Crafts reactions.
• Halogen atom besides being slightly deactivating is o, p-directing; therefore, further substitution occurs at ortho- and para-positions with respect to the halogen atom.
• Due to resonance, the electron density increases more at ortho- and para-positions than at meta-positions.
• Further, the halogen atom because of its –I effect has some tendency to withdraw electrons from the benzene ring.
• As a result, the ring gets somewhat deactivated as compared to benzene, and hence the electrophilic substitution reactions in haloarenes occur slowly and require more drastic conditions as compared to those in benzene.
• Chlorine withdraws electrons through inductive effect and releases electrons through resonance.
• Through inductive effect, chlorine destabilises the intermediate carbocation formed during the electrophilic substitution.
• Through resonance, halogen tends to stabilise the carbocation, and the effect is more pronounced at ortho- and para- positions.
• The inductive effect is stronger than resonance and causes net electron withdrawal and thus causes net deactivation.
• The resonance effect tends to oppose the inductive effect for the attack at ortho- and para- positions and hence makes the deactivation less for ortho- and para- attack.
• Reactivity is thus controlled by the stronger inductive effect and orientation is controlled by the resonance effect.

Reaction with Metals

• Wurtz-Fittig reaction:
  • A mixture of an alkyl halide and an aryl halide gives an alkylarene when treated with sodium in dry ether.
• Fittig reaction:
  • Aryl halides also give analogous compounds when treated with sodium in dry ether, in which two aryl groups are joined together.

Polyhalogen Compounds

• Carbon compounds containing more than one halogen atom are usually referred to as polyhalogen compounds.
• Many of these compounds are useful in industry and agriculture.

Dichloromethane (Methylene Chloride)

• Widely used as a solvent, paint remover, propellant in aerosols, and process solvent in the manufacture of drugs.
• Also used as a metal cleaning and finishing solvent.
• It harms the human central nervous system.
• Exposure to lower levels can lead to impaired hearing and vision.
• Higher levels can cause dizziness, nausea, tingling, and numbness.
• Direct skin contact causes intense burning and mild redness.
• Direct contact with the eyes can burn the cornea.

Trichloromethane (Chloroform)

• Employed as a solvent for fats, alkaloids, iodine, and other substances.
• Major use today is in the production of the freon refrigerant R-22.
• Was once used as a general anaesthetic in surgery but has been replaced by less toxic, safer anaesthetics, such as ether.
• Inhaling chloroform vapours depresses the central nervous system.
• Chronic chloroform exposure may cause damage to the liver and kidneys.
• Chloroform is slowly oxidised by air in the presence of light to carbonyl chloride (phosgene).

Triiodomethane (Iodoform)

• Was used earlier as an antiseptic, but the antiseptic properties are due to the liberation of free iodine and not due to iodoform itself.
• Due to its objectionable smell, it has been replaced by other formulations containing iodine.

Tetrachloromethane (Carbon Tetrachloride)

• Produced in large quantities for use in the manufacture of refrigerants and propellants for aerosol cans.
• Also used as feedstock in the synthesis of chlorofluorocarbons and other chemicals, pharmaceutical manufacturing, and general solvent use.
• There is some evidence that exposure to carbon tetrachloride causes liver cancer in humans.
• Common effects include dizziness, light-headedness, nausea, and vomiting, which can cause permanent damage to nerve cells.
• Exposure to Carbon tetrachloride can make the heart beat irregularly or stop.
• The chemical may irritate the eyes on contact.
• When carbon tetrachloride is released into the air, it rises to the atmosphere and depletes the ozone layer.

Freons

• The chlorofluorocarbon compounds of methane and ethane are collectively known as freons.
• They are extremely stable, unreactive, non-toxic, non-corrosive, and easily liquefiable gases.
• Freon 12 ($CCl<em>2F</em>2$) is one of the most common freons in industrial use.
• These are usually produced for aerosol propellants, refrigeration, and air conditioning purposes.
• In the stratosphere, freon is able to initiate radical chain reactions that can upset the natural ozone balance.

p,p’-Dichlorodiphenyltrichloroethane (DDT)

• The first chlorinated organic insecticides.
• The use of DDT increased enormously on a worldwide basis after World War II, primarily because of its effectiveness against the mosquito that spreads malaria and lice that carry typhus.
• Many species of insects developed resistance to DDT, and it was also discovered to have high toxicity towards fish.
• The chemical stability of DDT and its fat solubility compounded the problem.
• DDT is not metabolised very rapidly by animals; instead, it is deposited and stored in the fatty tissues.
• The use of DDT was banned in the United States in 1973, although it is still in use in some other parts of the world.