CHAPTER 6: Aryl Halides and Aralkyl Halides

B.sc 1st year Book
Organic Chemistry
(Page 3)

# DDT(Dichlorodiphenyltrichloroethane)

Molecular Formula : C14H9Cl5
Structural Formula : (ClC6H4)2CHCCl3
IUPAC Name : 1,1'-(2,2,2-trichloroethane-1,1-diyl)bis(4-chlorobenzene)

Introduction: DDT is one of the most famous pesticides in the world, and it has been with us for quite a long me. It is an organochlorine compound that was first synthesized in 1874 but it was not until 1939 that Paul Muller, at Geigy pharmaceutical in Switzerland, discovered the effectiveness of DDT as an insecticide. Paul Muller was awarded the Nobel Prize in medicine and physiology in 1948 for this pioneering work.

## Method of Preparation:

DDT is prepared by heating chlorobenzene with chloral(trichloroacetaldehyde; Cl3CCHO ) in presence of conc. H2SO4.

#### How does DDT work?

DDT affects the nervous system by interfering with normal nerve impulses.

## How toxic is DDT?

DDT is slight to moderately acutely toxic to mammals. including people, if eaten. It is poorly absorbed through mammalian skin, but it is easily absorbed through an insect’s outer covering known as an exoskeleton.

#### Does DDT cause cancer?

The EPA has categorized DDT as a B2 carcinogen. This means that DDT has been shown to cause cancer in laboratory animals, mammals exposed to DDT develop liver tumors and have an increased risk of liver tumors. But there is no evidence that shows that it may cause cancer in humans.

#### Use:

DDT is a cheap but powerful insecticide. It is widely used for sugarcane and fodder crops and to kill mosquitoes. It is particularly very effective against Anopheles mosquitoes which spread malaria and lice that carry typhus. DDT can kill organisms by interfering with the plasma membrane.

The chemical stability of DDT and its fat solubility increased the problems. It is not completely biodegradable(i.e it is not very rapidly metabolized by animals). Instead, it gets deposited and stored in fatty tissues. If this ingestion continues at a steady rate, DDT builds up within the animal over time. stored in fatty tissues. If this ingestion continues at a steady rate, DDT builds up in the animal over time.
This affects the reproductive system of animals. Rat became sterile after being fed DDT. One of the
the breakdown product of DDT is DDE which causes thinning of eggs shell in birds. People exposed to Dichlorodiphenyltrichloroethane while working with the chemical or by accidental exposure report a prickling sense Because of this side effect, the use of Elizario, headache, lethargy, incoordination, vomiting, fatigue, and tremors Protection Agency(EPA). It was banned because it persists in the environmental tissues and can cause adverse health effects on wildlife in the environment, accumulates in fa insects(like the house fly) that develop the ability to quickly metabolize the resistance that occurs in so many other Asian countries, DDT is still being to quickly metabolize the DDT. However, in India a cheaper insecticide.
DDT is responsible for genocide and menticide which has already condemned one entire genus

## Points to remember :

(1) Halogen atom attached to the benzene nucleus is relatively inert to nucleophiles(different from al halides); while the halogen atom present on the side chain is very reactive and can easily undergo
(2) In C6H5−Cl, due to resonance C−Cl bond acquires a partial double bond character, hence it becomes shorter and stronger. Thus, the Cl atom cannot be easily replaced by nucleophiles.
(3) Aryl halides undergo nucleophilic substitution reactions either
•  Under drastic conditions e.g. at high temperatures, pressures, or using strong nucleophiles like −OH,−OR et:
•  By the presence of electron-withdrawing groups like −NO2,−COOH,−CHO, etc in the ortho and
para position to the nuclear halogen para position to the nuclear halogen.
(4) Polar solvents favour SN1 mechanisms while strong nucleophiles favour SN2 mechanisms.
(5) SN2 reactions of optically active alkyl halides lead to Walden inversion, and SN1 reactions of optically active alkyl halides lead to Racemisation.
(6) Heating of an aryl halide with copper powder is called the UIImann reaction.
(7) Condensation of aryl halides with Na in presence of dry ether is called a Fitting reaction.
(8) Condensation of alkyl halides with Na in presence of dry ether is called the Wurtz reaction.
(9) Condensation of aryl halides and alkyl halides with Na in presence of dry ether is called Wurtz-Fitting
(10) The replacement of the diazonium group (−N2X) by halogen is known as the Sandmeyers reaction. In this reaction, a chloro or bromo arene is formed when an aqueous solution of aryl diazonium chloride is heated with Cu2Cl2 in HCl or Cu2Br2 in HBr respectively at 60∘C.
(11) The formation of chloro and bromo arenes by heating of an aryl diazonium salt with copper powder in presence of halogen acid( HCl or HBr) is called Gattermann Reaction.
(12) When an aryl diazonium salt is heated with fluoroboric acid, fluorobenzene is formed. This reaction is known as the “Schiemann Reaction“.