Question

Reactivity of haloarene increases if an electron withdrawing group (\( -NO_2 \)) is present at the ortho or para position. Explain it with an example and give the reaction mechanism.
OR
Write short notes on the applications of the following: (i) DDT (ii) Freon (iii) Carbon tetrachloride (iv) Chloroform (v) Dichloromethane.

Hint:

Ortho/para \( -NO_2 \) stabilises the carbanion (Meisenheimer) intermediate by resonance onto its oxygen, driving addition–elimination. For the uses part, link each haloalkane to its solvent/refrigerant/insecticide role and its hazard.

Answer 1

Option 1 (Nucleophilic aromatic substitution activated by -NO2)
Step 1: Aryl halides resist substitution because resonance between the halogen lone pair and the ring gives the C–X bond double-bond character and makes the carbon bearing X electron rich. A nucleophile therefore finds it hard to approach.
Step 2: Introducing a strong \( -M \)/\( -I \) group like \( -NO_2 \) at a position conjugated with the halogen (ortho or para) drains electron density from that carbon and, crucially, provides a site to hold the extra electron pair created when a nucleophile adds. This turns an unreactive haloarene into a reactive one.
Step 3: The order of ease of reaction with alkali is: chlorobenzene < \( p \)-nitrochlorobenzene < 2,4-dinitrochlorobenzene < 2,4,6-trinitrochlorobenzene. Each added ortho/para \( -NO_2 \) lowers the temperature and pressure needed. For instance, 2,4-dinitrochlorobenzene gives 2,4-dinitrophenol on warming with aqueous NaOH.
Step 4 (Mechanism in words): The reaction follows an addition&ndash;elimination path. Hydroxide first adds to the carbon holding chlorine to give a carbanion (Meisenheimer complex); the ortho/para nitro groups spread this negative charge onto their oxygen atoms through resonance, lowering the energy of the intermediate. Loss of \( Cl^- \) in the second step restores aromaticity and yields the substituted product.
Step 5: A meta nitro group offers no such resonance stabilisation of the carbanion, which is why the activation works only from ortho or para positions.
\[ 2,4\text{-}(O_2N)_2C_6H_3Cl \xrightarrow{NaOH} 2,4\text{-}(O_2N)_2C_6H_3OH \]

Option 2 (Uses, compound by compound)
(i) DDT: The first widely used synthetic contact insecticide; sprayed to wipe out disease-carrying mosquitoes (malaria) and body lice (typhus). Its great stability, which made it effective, also makes it persist in soil, water and fatty tissue, causing it to be banned in most nations today.
(ii) Freon: The trade name for CFCs used as refrigerants in cooling machines and as aerosol propellants. Being inert and easily liquefied they were ideal for the job, but they damage the stratospheric ozone shield, so safer substitutes have replaced them.
(iii) Carbon tetrachloride: A non-flammable solvent for grease and rubber, used in dry cleaning and formerly filled in Pyrene fire extinguishers because its heavy vapour cuts off oxygen from a fire. Being toxic to the liver, its use has been curtailed.
(iv) Chloroform: A good solvent for fats, iodine and alkaloids and a raw material for the refrigerant R-22. Its early role as a surgical anaesthetic was abandoned due to liver and heart damage. It is kept in dark bottles filled to the brim to prevent formation of toxic phosgene.
(v) Dichloromethane: Employed as a paint stripper, aerosol propellant, degreasing solvent for metals, and to remove caffeine from coffee. Since it affects the nervous system on prolonged exposure, careful handling is required.