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 (Effect of -NO2 on haloarene reactivity)
Step 1 (Why haloarenes are normally unreactive): In a haloarene the halogen is joined to an \( sp^2 \) carbon of the benzene ring. The lone pair of the halogen overlaps with the ring, giving the C–X bond partial double-bond character. This makes the bond short and strong, and the ring carbon becomes electron rich, so ordinary nucleophiles cannot attack easily.
Step 2 (Role of -NO2 at ortho/para): A strong electron withdrawing group such as \( -NO_2 \) pulls electron density away from the ring. When it sits at the ortho or para position relative to the halogen, it can directly stabilise the negative charge that develops on the carbon during attack. This makes nucleophilic aromatic substitution far easier.
Step 3 (Example): Chlorobenzene reacts with NaOH only under very harsh conditions (about 623 K and 300 atm). But \( p \)-nitrochlorobenzene reacts with dilute NaOH much more readily; 2,4-dinitrochlorobenzene reacts even more easily; and 2,4,6-trinitrochlorobenzene (picryl chloride) is hydrolysed just by warm water. Reactivity rises as more \( -NO_2 \) groups are added at ortho/para positions.
\[ p\text{-}O_2N\text{-}C_6H_4\text{-}Cl + NaOH \rightarrow p\text{-}O_2N\text{-}C_6H_4\text{-}ONa + NaCl \]
which on acidification gives \( p \)-nitrophenol.
Step 4 (Mechanism – addition then elimination): (a) The nucleophile \( OH^- \) adds to the ring carbon carrying the chlorine, breaking one ring double bond and forming a resonance-stabilised carbanion (the Meisenheimer intermediate). (b) The negative charge is delocalised onto the oxygen atoms of the ortho/para \( -NO_2 \) groups, which greatly stabilises this intermediate. (c) Chloride ion \( Cl^- \) is then eliminated, the ring regains its aromatic sextet, and the phenoxide (then phenol) is formed.
Step 5 (Why ortho/para only): Only when \( -NO_2 \) is ortho or para to the halogen can its resonance structures place the negative charge onto the nitro oxygen. A meta \( -NO_2 \) cannot stabilise the carbanion by resonance, so it does not increase reactivity.

Option 2 (Applications of the given compounds)
(i) DDT (p,p'-dichlorodiphenyltrichloroethane): A powerful, cheap and long-lasting insecticide. It was widely used to control malaria and typhus by killing mosquitoes and lice. Because it is very stable, it accumulates in the environment and in animal tissue (bioaccumulation) and is toxic to fish and birds, so its use is now banned or heavily restricted in many countries.
(ii) Freon (chlorofluorocarbons, e.g. \( CF_2Cl_2 \), Freon-12): Used as the refrigerant gas in refrigerators and air conditioners and as a propellant in aerosol sprays. They are chemically unreactive and non-toxic to handle, but they reach the stratosphere and destroy the ozone layer, so they are being phased out.
(iii) Carbon tetrachloride (\( CCl_4 \)): Used as an industrial solvent for oils, fats and resins, in dry cleaning, and earlier as a fire extinguisher liquid (Pyrene). Its vapours are dense and smother flames. It is toxic to the liver and, on heating, can release poisonous gases, so its use is now limited.
(iv) Chloroform (\( CHCl_3 \)): An important solvent for fats, waxes, alkaloids and iodine, and a starting material for making the refrigerant Freon (R-22). It was once used as an anaesthetic but this was stopped because it harms the liver and heart. It is stored in dark, filled bottles to stop it forming poisonous phosgene gas on exposure to light and air.
(v) Dichloromethane (\( CH_2Cl_2 \), methylene chloride): A common solvent used as a paint remover, a propellant in aerosols, a metal cleaning and degreasing agent, and in the process of decaffeinating coffee. It is harmful to the central nervous system, so it must be used with care.