JEE Chemistry Haloalkanes and Haloarenes MCQs Set 11

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

Explanation:
1. % Cl: CCl4 = 142/154 = 92.2; gammexane \( \ce{C6H6Cl6} \) = 73.2; chloral = 72.2; PVC = 56.8.

Teacher's Note:
1. DDT is made from chloral and chlorobenzene with conc. sulphuric acid; it is a persistent insecticide.
2. Draw the structures; most of these questions are solved in one line once the structure is on paper.

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

Explanation:
1. SN2 is fastest for the least hindered substrate: methyl > primary > secondary > tertiary.

Teacher's Note:
1. SN1 follows carbocation stability (3 degree > 2 degree > 1 degree), while SN2 is fastest for the least crowded carbon.
2. Use mechanism and stability arguments rather than memory: SN1 follows carbocation stability, SN2 follows steric hindrance.

Answer: (b) aaaeee

Explanation:
1. The gamma isomer (lindane) has three axial and three equatorial chlorine atoms.

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. Draw the structures; most of these questions are solved in one line once the structure is on paper.

Answer: (a) 2,2-Dichloropropane

Explanation:
1. A gem-dichloride on the middle carbon is hydrolysed to the gem-diol and loses water to give \( \ce{CH3COCH3} \).

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) \( \ce{CH3Br + OH- -> CH3OH + Br-} \)

Explanation:
1. Methyl bromide reacts by a one-step backside attack, SN2.
2. The others are elimination or are not SN2 in type.

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: (b) \( \ce{C2H5Cl + LiAlH4} \)

Explanation:
1. X = ethane.
2. LiAlH4 reduces ethyl chloride to ethane; the others give a Grignard reagent, diethyl ether and acetylene.

Teacher's Note:
1. Grignard reagents need dry ether and are destroyed by water, alcohols and any acidic hydrogen.
2. Use mechanism and stability arguments rather than memory: SN1 follows carbocation stability, SN2 follows steric hindrance.

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

Explanation:
1. \( \ce{CHCl3 + 3C2H5ONa -> HC(OC2H5)3 + 3NaCl} \)
2. (The question asks which compound is heated with sodium ethoxide.)

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: (b) \( \ce{H3C-O-CH3 -> CH3Cl} \)

Explanation:
1. \( \ce{PCl5} \) cleaves ethers to give alkyl chlorides: \( \ce{CH3OCH3 + PCl5 -> 2CH3Cl + POCl3} \)
2. The other conversions need HCl or base instead.

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: (c) A mixture of both (a) and (b)

Explanation:
1. The allylic carbocation formed is attacked by Br- at either end, giving a mixture of 3-bromobut-1-ene and 1-bromobut-2-ene.

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

Answer: (a) \( \ce{CH3F} \)

Explanation:
1. The C-F bond is too strong to react with magnesium under ordinary conditions.

Teacher's Note:
1. Grignard reagents need dry ether and are destroyed by water, alcohols and any acidic hydrogen.
2. Use mechanism and stability arguments rather than memory: SN1 follows carbocation stability, SN2 follows steric hindrance.

Answer: (a) tert-Butyl chloride

Explanation:
1. Wurtz coupling of \( \ce{(CH3)3CCl} \) gives 2,2,3,3-tetramethylbutane, whose 18 H atoms are all equivalent.

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. Draw the structures; most of these questions are solved in one line once the structure is on paper.

Answer: (d) p-Dichlorobenzene

Explanation:
1. The para isomer is the most symmetrical and packs best in the crystal.

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: (d) Bicyclo[1.1.0]butane

Explanation:
1. Intramolecular Wurtz coupling joins C1 and C3 across the ring, giving the strained bicyclobutane.

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. Draw the structures; most of these questions are solved in one line once the structure is on paper.

Answer: (d) A is false, but R is true.

Explanation:
1. Chloroform is stored in dark (brown) bottles because light and air oxidise it to poisonous phosgene, so the assertion is false while the reason is true.

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. Decide the truth of the assertion and of the reason separately, then ask whether the reason really explains the assertion.

Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Explanation:
1. Both are true.
2. Anti addition of Br2 to the trans alkene gives the meso product, but the reason (electrophilic addition) does not explain the stereochemical outcome, which comes from the bromonium ion and backside attack.

Teacher's Note:
1. Statements about stereochemistry and mechanism are often true separately without one explaining the other.
2. Decide the truth of the assertion and of the reason separately, then ask whether the reason really explains the assertion.

Answer: (c) A is true, but R is false.

Explanation:
1. The assertion is true: alkyl halides are non-polar to weakly polar and dissolve in organic solvents.
2. The reason is false, since p-dichlorobenzene has the highest melting point among the dichlorobenzenes.

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. Decide the truth of the assertion and of the reason separately, then ask whether the reason really explains the assertion.

Answer: (d) A is false, but R is true.

Explanation:
1. The assertion is false because CCl4 is a fire extinguisher (pyrene); the reason is true.

Teacher's Note:
1. Statements about stereochemistry and mechanism are often true separately without one explaining the other.
2. Decide the truth of the assertion and of the reason separately, then ask whether the reason really explains the assertion.

Answer: (d) A is false, but R is true.

Explanation:
1. Alkyl halides from alkenes are made with dry HX, because aqueous acid mainly gives alcohols, so the assertion is false.
2. The reason is true: HI adds easily to alkenes.

Teacher's Note:
1. Decide the truth of the assertion and of the reason separately, then ask whether the reason really explains the assertion.
2. Statements about stereochemistry and mechanism are often true separately without one explaining the other.

Answer: (c) A is true, but R is false.

Explanation:
1. Thermal dehydrohalogenation is true.
2. The reason is false: C-H bonds break faster than C-D bonds (kinetic isotope effect), so CH3CH2I reacts faster.

Teacher's Note:
1. Alcoholic KOH eliminates HX to give the more substituted alkene (Saytzeff rule); aqueous KOH substitutes.
2. Statements about stereochemistry and mechanism are often true separately without one explaining the other.

Answer: (c) A is true, but R is false.

Explanation:
1. The assertion is true.
2. The reason is false; the order is HI > HBr > HCl.

Teacher's Note:
1. Decide the truth of the assertion and of the reason separately, then ask whether the reason really explains the assertion.
2. Statements about stereochemistry and mechanism are often true separately without one explaining the other.

Answer: (d) A is false, but R is true.

Explanation:
1. The tertiary halide forms its carbocation much more readily, so it is more reactive; the assertion is false.
2. The reason is true.

Teacher's Note:
1. Statements about stereochemistry and mechanism are often true separately without one explaining the other.
2. Decide the truth of the assertion and of the reason separately, then ask whether the reason really explains the assertion.

Answer: (d) A is false, but R is true.

Explanation:
1. Nitro groups at ortho and para positions increase the reactivity, so the assertion is false; the reason is true.

Teacher's Note:
1. SN1 follows carbocation stability (3 degree > 2 degree > 1 degree), while SN2 is fastest for the least crowded carbon.
2. Decide the truth of the assertion and of the reason separately, then ask whether the reason really explains the assertion.

Answer: (d) A is false, but R is true.

Explanation:
1. Halogens deactivate the ring (-I over +M), so aryl halides are less reactive than benzene; the assertion is false.
2. The reason is 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. Statements about stereochemistry and mechanism are often true separately without one explaining the other.

Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Explanation:
1. Both are true, but the reason is about a different reaction type and does not explain why bromine addition is stereoselective.

Teacher's Note:
1. Decide the truth of the assertion and of the reason separately, then ask whether the reason really explains the assertion.
2. Statements about stereochemistry and mechanism are often true separately without one explaining the other.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. Each iodide exchange inverts the configuration (SN2); repeated exchanges equalise the amounts of both enantiomers, giving a racemic mixture.
2. The reason explains the assertion.

Teacher's Note:
1. SN1 follows carbocation stability (3 degree > 2 degree > 1 degree), while SN2 is fastest for the least crowded carbon.
2. Statements about stereochemistry and mechanism are often true separately without one explaining the other.