JEE Chemistry Haloalkanes and Haloarenes MCQs Set 10

Answer: (a) Friedel-Crafts reaction

Explanation:
1. Alkylation of the arene by an alkyl halide/AlCl3.

Teacher's Note:
1. Chloroform, DDT, BHC and chloropicrin are asked repeatedly: memorise their preparations and uses.
2. Remember the ambident nucleophiles: KCN gives nitriles, AgCN gives isocyanides, KNO2 gives nitrites, AgNO2 gives nitroalkanes.

Answer: (a) B.H.C.

Explanation:
1. Free-radical addition gives benzene hexachloride.

Teacher's Note:
1. Haloform and carbylamine reactions are tests; know the reagent, the product and the observation.
2. Separate substitution (aqueous KOH, AgCN, KCN) from elimination (alcoholic KOH) before reading the options.

Answer: (a) Ethene

Explanation:
1. Vicinal dihalide is dehalogenated by Na to give the alkene (ethene).

Teacher's Note:
1. Separate substitution (aqueous KOH, AgCN, KCN) from elimination (alcoholic KOH) before reading the options.
2. Chlorobenzene and vinyl chloride are unreactive because the C-Cl bond has partial double-bond character.

Answer: (c) Wurtz reaction

Explanation:
1. \( \ce{2CH3Br + 2Na -> C2H6 + 2NaBr} \).

Teacher's Note:
1. Chlorobenzene and vinyl chloride are unreactive because the C-Cl bond has partial double-bond character.
2. Chloroform, DDT, BHC and chloropicrin are asked repeatedly: memorise their preparations and uses.

Answer: (a) It is less reactive than alkyl halides

Explanation:
1. Benzyl chloride is more reactive than simple alkyl halides because of the stabilised benzyl cation/transition state.

Teacher's Note:
1. Remember the ambident nucleophiles: KCN gives nitriles, AgCN gives isocyanides, KNO2 gives nitrites, AgNO2 gives nitroalkanes.
2. Haloform and carbylamine reactions are tests; know the reagent, the product and the observation.

Answer: (c) Hexachloroethane

Explanation:
1. Hexachloroethane, \( \ce{C2Cl6} \), is a solid with a camphor-like odour and is used as a moth repellent.

Teacher's Note:
1. Link each compound with its use: DDT and BHC insecticides, CCl4 fire extinguisher, CHCl3 anaesthetic, freons refrigerants, iodoform antiseptic.
2. Environmental concerns about CFCs and DDT are a favourite theory point.

Answer: (a) \( \ce{(ClC6H4)2CH-CCl3} \)

Explanation:
1. DDT has two p-chlorophenyl rings on one carbon that also carries H and a CCl3 group: 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane.

Teacher's Note:
1. DDT is made from chloral and chlorobenzene with conc. sulphuric acid; it is a persistent insecticide.
2. Environmental concerns about CFCs and DDT are a favourite theory point.

Answer: (a) Freon

Explanation:
1. Chlorofluorocarbons (freons) release chlorine atoms in the stratosphere which destroy ozone.

Teacher's Note:
1. Freons are stable, volatile refrigerants; their chlorine radicals destroy stratospheric ozone.
2. Environmental concerns about CFCs and DDT are a favourite theory point.

Answer: (a) \( \ce{[-CF2-CF2-]n} \)

Explanation:
1. Teflon is poly(tetrafluoroethene); \( \ce{CF2=CF2} \) is its monomer.

Teacher's Note:
1. Link each compound with its use: DDT and BHC insecticides, CCl4 fire extinguisher, CHCl3 anaesthetic, freons refrigerants, iodoform antiseptic.
2. Environmental concerns about CFCs and DDT are a favourite theory point.

Answer: (a) True

Explanation:
1. CFCs such as \( \ce{CCl2F2} \) photolyse in the stratosphere to give Cl atoms, which catalyse ozone destruction.

Teacher's Note:
1. Freons are stable, volatile refrigerants; their chlorine radicals destroy stratospheric ozone.
2. Environmental concerns about CFCs and DDT are a favourite theory point.

Answer: (a) 5 chlorine atoms

Explanation:
1. DDT is \( \ce{C14H9Cl5} \): three Cl in CCl3 and one on each ring.

Teacher's Note:
1. DDT is made from chloral and chlorobenzene with conc. sulphuric acid; it is a persistent insecticide.
2. Environmental concerns about CFCs and DDT are a favourite theory point.

Answer: (a) Alizarin-S (with zirconium)

Explanation:
1. Fluoride decolourises the zirconium-alizarin S lake.

Teacher's Note:
1. Link each compound with its use: DDT and BHC insecticides, CCl4 fire extinguisher, CHCl3 anaesthetic, freons refrigerants, iodoform antiseptic.
2. Environmental concerns about CFCs and DDT are a favourite theory point.

Answer: (d) Tear gas

Explanation:
1. \( \ce{CCl3NO2} \) is a lachrymatory agent (tear gas) and an insecticide.

Teacher's Note:
1. Environmental concerns about CFCs and DDT are a favourite theory point.
2. Link each compound with its use: DDT and BHC insecticides, CCl4 fire extinguisher, CHCl3 anaesthetic, freons refrigerants, iodoform antiseptic.

Answer: (c) In refrigerators

Explanation:
1. \( \ce{CCl2F2} \) is volatile and non-toxic, used as a refrigerant.

Teacher's Note:
1. Freons are stable, volatile refrigerants; their chlorine radicals destroy stratospheric ozone.
2. Environmental concerns about CFCs and DDT are a favourite theory point.

Answer: (a) \( \ce{CCl2F2} \)

Explanation:
1. Freon-12 is \( \ce{CCl2F2} \).

Teacher's Note:
1. Freons are stable, volatile refrigerants; their chlorine radicals destroy stratospheric ozone.
2. Link each compound with its use: DDT and BHC insecticides, CCl4 fire extinguisher, CHCl3 anaesthetic, freons refrigerants, iodoform antiseptic.

Answer: (d) Insecticide

Explanation:
1. The gamma isomer (gammexane/lindane) is a powerful insecticide.

Teacher's Note:
1. BHC is formed by free-radical addition of chlorine to benzene in sunlight; only the gamma isomer is a strong insecticide.
2. Environmental concerns about CFCs and DDT are a favourite theory point.

Answer: (c) DDT

Explanation:
1. This is the structure of DDT.

Teacher's Note:
1. DDT is made from chloral and chlorobenzene with conc. sulphuric acid; it is a persistent insecticide.
2. Environmental concerns about CFCs and DDT are a favourite theory point.

Answer: (b) Both are insecticides

Explanation:
1. Both are chlorinated insecticides.

Teacher's Note:
1. DDT is made from chloral and chlorobenzene with conc. sulphuric acid; it is a persistent insecticide.
2. Link each compound with its use: DDT and BHC insecticides, CCl4 fire extinguisher, CHCl3 anaesthetic, freons refrigerants, iodoform antiseptic.

Answer: (d) \( \ce{CCl4} \)

Explanation:
1. Carbon tetrachloride vapour is non-combustible and heavier than air, so it smothers flames (pyrene).

Teacher's Note:
1. Environmental concerns about CFCs and DDT are a favourite theory point.
2. Link each compound with its use: DDT and BHC insecticides, CCl4 fire extinguisher, CHCl3 anaesthetic, freons refrigerants, iodoform antiseptic.

Answer: (b) \( \ce{CHCl3} \)

Explanation:
1. Chloroform was widely used as a general anaesthetic.

Teacher's Note:
1. Chloroform is oxidised by air and light to poisonous phosgene, so it is stored in dark, full bottles with a little ethanol.
2. Environmental concerns about CFCs and DDT are a favourite theory point.

Answer: (c) DDT

Explanation:
1. Chloral + 2 chlorobenzene (conc.
2. \( \ce{H2SO4} \)) gives DDT.

Teacher's Note:
1. DDT is made from chloral and chlorobenzene with conc. sulphuric acid; it is a persistent insecticide.
2. Link each compound with its use: DDT and BHC insecticides, CCl4 fire extinguisher, CHCl3 anaesthetic, freons refrigerants, iodoform antiseptic.

Answer: (b) \( \ce{CCl4} \)

Explanation:
1. Pyrene is the trade name of carbon tetrachloride.

Teacher's Note:
1. Environmental concerns about CFCs and DDT are a favourite theory point.
2. Link each compound with its use: DDT and BHC insecticides, CCl4 fire extinguisher, CHCl3 anaesthetic, freons refrigerants, iodoform antiseptic.

Answer: (a) Hypnotic

Explanation:
1. The product chloretone is a hypnotic.

Teacher's Note:
1. Chloroform is oxidised by air and light to poisonous phosgene, so it is stored in dark, full bottles with a little ethanol.
2. Environmental concerns about CFCs and DDT are a favourite theory point.

Answer: (c) They eat away the ozone in the atmosphere

Explanation:
1. CFCs release chlorine radicals that destroy stratospheric ozone.

Teacher's Note:
1. Freons are stable, volatile refrigerants; their chlorine radicals destroy stratospheric ozone.
2. Link each compound with its use: DDT and BHC insecticides, CCl4 fire extinguisher, CHCl3 anaesthetic, freons refrigerants, iodoform antiseptic.

Answer: (a) \( \ce{CH3Cl} \)

Explanation:
1. CH3Cl (about 1.9 D) > CH2Cl2 (1.6 D) > CHCl3 (1.0 D) > CCl4 (zero).

Teacher's Note:
1. Chloroform is oxidised by air and light to poisonous phosgene, so it is stored in dark, full bottles with a little ethanol.
2. Draw the structures; most of these questions are solved in one line once the structure is on paper.

Answer: (c) sp3

Explanation:
1. The product is ethanol, \( \ce{CH3CH2OH} \), in which both carbons are sp3.

Teacher's Note:
1. Use mechanism and stability arguments rather than memory: SN1 follows carbocation stability, SN2 follows steric hindrance.
2. Draw the structures; most of these questions are solved in one line once the structure is on paper.

Answer: (a) Vinyl chloride

Explanation:
1. The C-Cl bond in vinyl chloride is shortened and strengthened by resonance, so it resists nucleophilic substitution.

Teacher's Note:
1. SN1 follows carbocation stability (3 degree > 2 degree > 1 degree), while SN2 is fastest for the least crowded carbon.
2. Draw the structures; most of these questions are solved in one line once the structure is on paper.

Answer: (d) \( \ce{NO2} \) withdraws electrons from ortho/para positions

Explanation:
1. The nitro groups at the ortho and para positions withdraw electrons and stabilise the Meisenheimer intermediate.

Teacher's Note:
1. Aryl and vinyl halides are inert in ordinary nucleophilic substitution because the C-X bond has partial double-bond character.
2. Use mechanism and stability arguments rather than memory: SN1 follows carbocation stability, SN2 follows steric hindrance.