JEE Chemistry Haloalkanes and Haloarenes MCQs Set 09

Answer: (b) p,p-Dichlorodiphenyltrichloroethane

Explanation:
1. This is the preparation of DDT.

Teacher's Note:
1. DDT is made from chloral and chlorobenzene with conc. sulphuric acid; it is a persistent insecticide.
2. Separate substitution (aqueous KOH, AgCN, KCN) from elimination (alcoholic KOH) before reading the options.

Answer: (c) Vinyl chloride is more reactive than allyl chloride

Explanation:
1. Vinyl chloride is very unreactive (resonance); allyl chloride is highly reactive.
2. The other statements are true.

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. Chlorobenzene and vinyl chloride are unreactive because the C-Cl bond has partial double-bond character.

Answer: (c) Dehydrohalogenation

Explanation:
1. Alcoholic KOH removes HX from alkyl halides.

Teacher's Note:
1. Alcoholic KOH eliminates HX to give the more substituted alkene (Saytzeff rule); aqueous KOH substitutes.
2. Remember the ambident nucleophiles: KCN gives nitriles, AgCN gives isocyanides, KNO2 gives nitrites, AgNO2 gives nitroalkanes.

Answer: (a) 1,1-Dichloroethane

Explanation:
1. Markovnikov addition: H goes to CH2, Cl to the carbon already bearing Cl, giving \( \ce{CH3CHCl2} \).

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. Haloform and carbylamine reactions are tests; know the reagent, the product and the observation.

Answer: (a) Nucleophilic substitution

Explanation:
1. OH- is the nucleophile replacing X.

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: (b) \( \ce{C2H6} \)

Explanation:
1. Wurtz-type coupling gives ethane.

Teacher's Note:
1. Wurtz coupling joins two alkyl groups with sodium in dry ether; fluorides do not react and mixed halides give a mixture of alkanes.
2. Chloroform, DDT, BHC and chloropicrin are asked repeatedly: memorise their preparations and uses.

Answer: (b) Salicylaldehyde

Explanation:
1. Reimer-Tiemann reaction.

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. Remember the ambident nucleophiles: KCN gives nitriles, AgCN gives isocyanides, KNO2 gives nitrites, AgNO2 gives nitroalkanes.

Answer: (c) \( \ce{HCOOK} \)

Explanation:
1. \( \ce{CHI3 + 4KOH -> HCOOK + 3KI + 2H2O} \).

Teacher's Note:
1. The iodoform test is given by compounds with a CH3CO- or CH3CH(OH)- group; methanol, formaldehyde and diethyl ketone do not respond.
2. Haloform and carbylamine reactions are tests; know the reagent, the product and the observation.

Answer: (d) CO

Explanation:
1. CO binds haemoglobin irreversibly.
2. CHCl3 is a liquid, \( \ce{CO2} \) is non-toxic, \( \ce{CH4} \) is non-toxic.

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. Chlorobenzene and vinyl chloride are unreactive because the C-Cl bond has partial double-bond character.

Answer: (a) \( \ce{CH3I} \)

Explanation:
1. Methyl iodide transfers CH3 readily because iodide is an excellent leaving group.

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: (b) \( \ce{C6H3Cl3} \)

Explanation:
1. Three molecules of HCl are eliminated to give trichlorobenzene.

Teacher's Note:
1. Alcoholic KOH eliminates HX to give the more substituted alkene (Saytzeff rule); aqueous KOH substitutes.
2. Remember the ambident nucleophiles: KCN gives nitriles, AgCN gives isocyanides, KNO2 gives nitrites, AgNO2 gives nitroalkanes.

Answer: (a) Fluoroform

Explanation:
1. \( \ce{CHF3} \): 12 + 1 + 57 = 70.

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. Separate substitution (aqueous KOH, AgCN, KCN) from elimination (alcoholic KOH) before reading the options.

Answer: (a) \( \ce{CH4} \)

Explanation:
1. Reduction of CHCl3 with Zn/water replaces all chlorine by hydrogen to give methane.

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. Chlorobenzene and vinyl chloride are unreactive because the C-Cl bond has partial double-bond character.

Answer: (b) Iodine powder

Explanation:
1. A crystal of iodine activates the magnesium surface.

Teacher's Note:
1. Grignard reagents need dry ether and are destroyed by water, alcohols and any acidic hydrogen.
2. Chloroform, DDT, BHC and chloropicrin are asked repeatedly: memorise their preparations and uses.

Answer: (a) Vinyl chloride

Explanation:
1. The C-Cl bond in vinyl chloride has partial double bond character, 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. Haloform and carbylamine reactions are tests; know the reagent, the product and the observation.

Answer: (b) Bases

Explanation:
1. A nucleophile is an electron-pair donor, i.e. a Lewis base (such as OH-, CN-, NH3).

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: (d) Chloroform

Explanation:
1. Chloroform is saturated and has no activating group; amines, phenol and alkenes all react.

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. Chlorobenzene and vinyl chloride are unreactive because the C-Cl bond has partial double-bond character.

Answer: (b) Sodium benzoate

Explanation:
1. \( \ce{C6H5CH3 -> C6H5CCl3} \); hydrolysis gives benzoic acid, which NaOH converts to sodium benzoate.

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) 1-Bromo-2-methylbutane

Explanation:
1. 1-Bromo-2-methylbutane has beta-hydrogen on only one carbon, so only 2-methylbut-1-ene forms; hydrogenation gives 2-methylbutane. 2-Bromo-2-methylbutane gives two alkenes; neopentyl bromide has no beta-H.

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) 2-Chloropropane and chloromethane

Explanation:
1. Cross-coupling of isopropyl and methyl gives \( \ce{(CH3)2CH-CH3} \)
2. (The others would give 2-methylbutane, butane or pentane mixtures.)

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-Fittig reaction

Explanation:
1. Aryl halide + alkyl halide + Na gives an alkylbenzene (Wurtz-Fittig).

Teacher's Note:
1. Wurtz coupling joins two alkyl groups with sodium in dry ether; fluorides do not react and mixed halides give a mixture of alkanes.
2. Chloroform, DDT, BHC and chloropicrin are asked repeatedly: memorise their preparations and uses.

Answer: (a) \( \ce{C2H5OH} \)

Explanation:
1. About 1 percent ethanol converts any phosgene formed into harmless ethyl carbonate.

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. Remember the ambident nucleophiles: KCN gives nitriles, AgCN gives isocyanides, KNO2 gives nitrites, AgNO2 gives nitroalkanes.

Answer: (c) p-Methoxybenzyl bromide

Explanation:
1. Solvolysis proceeds through the benzylic cation; the strongly electron-donating methoxy group stabilises it most.

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: (b) Sodium in dry ether

Explanation:
1. Sodium in dry ether is the reagent; water would react with sodium.

Teacher's Note:
1. Wurtz coupling joins two alkyl groups with sodium in dry ether; fluorides do not react and mixed halides give a mixture of alkanes.
2. Chlorobenzene and vinyl chloride are unreactive because the C-Cl bond has partial double-bond character.

Answer: (b) \( \ce{COCl2} \)

Explanation:
1. Air oxidation gives phosgene.

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. Chloroform, DDT, BHC and chloropicrin are asked repeatedly: memorise their preparations and uses.

Answer: (b) Tear gas

Explanation:
1. Chloropicrin, a tear gas.

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. Remember the ambident nucleophiles: KCN gives nitriles, AgCN gives isocyanides, KNO2 gives nitrites, AgNO2 gives nitroalkanes.

Answer: (d) \( \ce{C2H2} \)

Explanation:
1. Acetylene is formed.

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. Separate substitution (aqueous KOH, AgCN, KCN) from elimination (alcoholic KOH) before reading the options.

Answer: (b) Ether

Explanation:
1. \( \ce{2RX + Ag2O -> R-O-R + 2AgX} \).

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.