Question

Write the product of the following reactions along with their mechanism:
i) p-nitrochlorobenzene + NaOH (623 K, 300 atm), then H⁺ → [A]
ii) Chlorobenzene + CH₃COCl (anhydrous AlCl₃) → [B] + [C]
iii) CH₃–CH=C(CH₃)₂ + HBr → [D]
OR
Write short notes on the following: i) D.D.T. ii) Freon iii) Carbon tetrachloride

Hint:

For Option 1: the para NO2 group activates chlorobenzene toward nucleophilic substitution (gives p-nitrophenol), CH3COCl with AlCl3 is Friedel-Crafts acylation giving o- and p-chloroacetophenone, and HBr without peroxide follows Markovnikov rule. For Option 2: DDT is an insecticide, Freon is a CFC refrigerant, and CCl4 is a solvent and fire-extinguisher liquid.

Answer 1

Option 1:
Step 1: Reaction (i) product. Plain chlorobenzene needs very drastic conditions (NaOH, 623 K, 300 atm) for the chlorine to be replaced by an OH group. When a strong electron-withdrawing group such as NO₂ is present at the ortho or para position, this replacement becomes much easier. Here the para nitro group activates the ring, so the product [A] after acidification is p-nitrophenol (4-nitrophenol).
Mechanism (nucleophilic aromatic substitution, addition-elimination): The hydroxide ion OH^- attacks the carbon that carries the Cl atom, giving a negatively charged intermediate (a carbanion / Meisenheimer complex). This negative charge is spread out by resonance and is stabilised mainly by the para NO₂ group. In the next step the chloride ion Cl^- leaves and the ring becomes aromatic again, giving sodium p-nitrophenoxide. On treatment with H⁺ this gives p-nitrophenol.
Step 2: Reaction (ii) products. This is a Friedel-Crafts acylation. Anhydrous AlCl₃ reacts with acetyl chloride to form the acylium ion CH₃CO⁺. In chlorobenzene the Cl atom is an ortho/para directing group, so the two products [B] and [C] are p-chloroacetophenone (major) and o-chloroacetophenone (minor).
Mechanism (electrophilic aromatic substitution): (1) CH₃COCl + AlCl₃ → CH₃CO⁺ + AlCl₄^-, forming the electrophile. (2) The acylium ion attacks the benzene ring at the position ortho or para to Cl, giving a resonance-stabilised arenium ion (carbocation). (3) Loss of H⁺ restores aromaticity and regenerates AlCl₃, giving chloroacetophenone.
Step 3: Reaction (iii) product. The alkene CH₃CH=C(CH₃)₂ is 2-methylbut-2-ene. Since no peroxide is present, HBr adds by Markovnikov rule. Product [D] = 2-bromo-2-methylbutane, that is (CH₃)₂CBr–CH₂–CH₃.
Mechanism (electrophilic addition): The H⁺ of HBr adds to the =CH carbon (the one already bearing more hydrogens), so the positive charge appears on the C(CH₃)₂ carbon, which is a tertiary (3 degree) carbocation and hence the most stable. The bromide ion Br^- then attacks this tertiary carbon to give the Markovnikov product.

Option 2:
Step 1: D.D.T. D.D.T. is p,p'-dichlorodiphenyltrichloroethane. It was the first chlorinated organic insecticide, prepared by reacting chlorobenzene with chloral (trichloroacetaldehyde) in the presence of concentrated H₂SO₄. It is very effective against mosquitoes and lice, but it is stable and non-biodegradable and gets stored in fatty tissues, so its use is now banned or restricted in many countries.
Step 2: Freon. Freons are the common name for chlorofluorocarbons (CFCs), gaseous compounds of carbon with chlorine and fluorine. The most common one is Freon-12 (CCl₂F₂), prepared from CCl₄ and HF in the presence of SbF₃/SbCl₅. Freons are used as refrigerants in fridges and air conditioners and as propellants in aerosol sprays. In the upper atmosphere they release Cl atoms which destroy the ozone layer.
Step 3: Carbon tetrachloride. CCl₄ (tetrachloromethane) is a colourless, heavy, non-inflammable liquid used as an industrial solvent for oils and fats, in fire extinguishers (Pyrene) and formerly in dry cleaning. Its vapour is toxic and can cause liver damage, on heating in air it forms poisonous phosgene gas (COCl₂), and it also contributes to ozone depletion.