Q.Which of the following compounds is formed when - C6H6 + Cl2 (excess) --sunlight--> ?

Chloroform, CHCl3, has three chlorine atoms on one carbon, so it is a trihalogen derivative of methane.

Chloroform is trichloromethane, CHCl3. It is formed by replacing three of the four hydrogen atoms of methane (CH4) by chlorine atoms. Because it contains three halogen atoms, it is a trihalogen (trihalo) derivative.

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CHCl3 + 6[H] -> CH4 + 3HCl (reduction gives methane).

Zinc and water (or Zn/HCl) supply nascent hydrogen [H]. This reduces chloroform by replacing each chlorine atom with hydrogen:

CHCl3 + 6[H] -> CH4 + 3HCl

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Reduction of chloroform by nascent hydrogen (Zn + H2O) removes all Cl atoms and gives methane.

Chloroform is trichloromethane, CHCl3. When treated with zinc and water, nascent hydrogen [H] is generated, which reduces the C-Cl bonds one after another. All three chlorine atoms are replaced by hydrogen:

CHCl3 + 6[H] --(Zn/H2O)--> CH4 + 3HCl

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In sunlight benzene undergoes ADDITION (not substitution) with excess Cl2 to give benzene hexachloride, C6H6Cl6.

Benzene reacts with chlorine in two very different ways depending on conditions:

• With a Lewis-acid catalyst (e.g. anhydrous AlCl3/FeCl3) in the dark → electrophilic SUBSTITUTION → chlorobenzene.
• In the presence of sunlight/UV light and no catalyst → free-radical ADDITION of three Cl2 molecules across the ring:

C6H6 + 3Cl2 --(sunlight)--> C6H6Cl6

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Chloroform + O2 in light -> phosgene (COCl2) + HCl.

On exposure to air and light, chloroform (CHCl3) is slowly oxidised to a highly poisonous gas, phosgene (carbonyl chloride):

2 CHCl3 + O2 --light--> 2 COCl2 + 2 HCl

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