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Haloalkanes and Haloarenes Set-4 MCQ Online Practice Test | Class 12 | Quiz Tweek

Haloalkanes and Haloarenes Set-4

Practice Important MCQs for Haloalkanes and Haloarenes — Chemistry, Class 12.

The Haloalkanes and Haloarenes chapter is crucial because it links structure to reaction mechanism (SN1/SN2, E1/E2, SNAr, benzyne) and teaches how substitution/elimination selectivity depends on substrate type, solvent, and nucleophile strength. This makes it a high-yield area for board exams and competitive tests (JEE/NEET), where numerical half-life/kinetics, order of reactivity, and mechanism-based reasoning are frequently asked.

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Q1. A tert‑butyl chloride ( $(\mathrm{CH_3})_3\mathrm{CCl}$ ) undergoes solvolysis in ethanol by an $S_N1$ mechanism with half‑life $t_{1/2}=10\ \mathrm{min}$ at a given temperature. How long will it take for $75\%$ of the tert‑butyl chloride to be consumed?

(A)

$10\ \mathrm{min}$

(B)

$20\ \mathrm{min}$

(C)

$30\ \mathrm{min}$

(D)

$40\ \mathrm{min}$

Q2. Under conditions favouring an $S_N1$ mechanism (polar protic solvent, weak nucleophile), which of the following halides will undergo nucleophilic substitution most rapidly?

(A)

1‑Bromobutane, $\mathrm{CH_3CH_2CH_2CH_2Br}$

(B)

Bromobenzene, $\mathrm{C_6H_5Br}$

(C)

2‑Bromo‑2‑methylpropane (tert‑butyl bromide), $(\mathrm{CH_3})_3\mathrm{CBr}$

(D)

Benzyl bromide, $\mathrm{C_6H_5CH_2Br}$

Q3. Which of the following will undergo nucleophilic aromatic substitution (addition–elimination, $S_{N}Ar$ ) most readily with methoxide ion ( $\mathrm{CH_3O^-}$ ) at 25°C?

(A)

1‑Chloro‑2,4‑dinitrobenzene, $\mathrm{C_6H_3Cl(NO_2)_2}$

(B)

4‑Nitrochlorobenzene, $\mathrm{C_6H_4ClNO_2}$

(C)

3‑Nitrochlorobenzene, $\mathrm{C_6H_4ClNO_2}$

(D)

Chlorobenzene, $\mathrm{C_6H_5Cl}$

Q4. Chlorobenzene is virtually inert to nucleophilic attack by hydroxide ion ( $\mathrm{OH^-}$ ) under mild conditions, whereas benzyl chloride reacts rapidly with $\mathrm{OH^-}$ to give benzyl alcohol. Which of the following best explains this difference?

(A)

Benzyl chloride is a primary benzylic halide that undergoes rapid $S_N2$ because it is less hindered, whereas the $sp^2$ carbon of chlorobenzene cannot undergo $S_N2$ .

(B)

Chlorobenzene reacts only by the benzyne (elimination–addition) route which requires a very strong base and high temperature; benzyl chloride reacts by a radical chain with $\mathrm{OH^-}$ under mild conditions.

(C)

In chlorobenzene the C–Cl bond has significant partial double‑bond character due to resonance (delocalisation of lone pair of Cl into the ring) and the carbon is $sp^2$ , making nucleophilic attack energetically unfavourable; benzyl chloride has an $sp^3$ benzylic carbon and the transition state/carbocation is resonance‑stabilised, so substitution by $\mathrm{OH^-}$ is facile.

(D)

Chloride is a much poorer leaving group on an aromatic ring because it is tightly held by the ring; on an $sp^3$ benzylic carbon the chloride is loosely held, so displacement by $\mathrm{OH^-}$ is easy.

Q5. Which of the following haloarenes will react most rapidly with methoxide ion ( $\mathrm{CH_3O^-}$ ) by the addition–elimination ( $S_{N}Ar$ ) mechanism at 25°C?

(A)

1‑Chloro‑2,4‑dinitrobenzene, $\mathrm{ClC_6H_3(NO_2)_2}$

(B)

1‑Fluoro‑2,4‑dinitrobenzene, $\mathrm{FC_6H_3(NO_2)_2}$

(C)

1‑Fluoro‑4‑nitrobenzene, $\mathrm{FC_6H_4NO_2}$

(D)

1‑Chloro‑4‑nitrobenzene, $\mathrm{ClC_6H_4NO_2}$

Q6. Among the following haloalkanes, which will undergo the fastest bimolecular nucleophilic substitution (SN2) with $CN^{-}$ in DMSO at room temperature?

(A)

$(CH_3)_3CBr$ (tert‑butyl bromide)

(B)

$CH_3CH(Br)CH_3$ (2‑bromopropane)

(C)

$CH_3Br$ (methyl bromide)

(D)

$CH_3CH_2CH_2Br$ (1‑bromopropane)

Q7. When treated with aqueous $OH^{-}$ at room temperature, which of the following halides will undergo nucleophilic substitution most readily?

(A)

$C_6H_5CH_2Cl$ (benzyl chloride)

(B)

$C_6H_5Cl$ (chlorobenzene)

(C)

$CH_3CH(Cl)CH_3$ (2‑chloropropane)

(D)

$CH_3Cl$ (methyl chloride)

Q8. Which of the following aryl chlorides will undergo nucleophilic aromatic substitution (addition–elimination, Meisenheimer pathway) by $CH_3O^{-}$ most readily under mild conditions?

(A)

$o$ ‑NO $_2$ C $_6$ H $_4$ Cl ( $o$ ‑nitrochlorobenzene)

(B)

$m$ ‑NO $_2$ C $_6$ H $_4$ Cl ( $m$ ‑nitrochlorobenzene)

(C)

$C_6H_5Cl$ (chlorobenzene)

(D)

both $o$ ‑ and $p$ ‑NO $_2$ C $_6$ H $_4$ Cl ( $o$ ‑ and $p$ ‑nitrochlorobenzene)

Q9. Which experimental condition is most likely to give nucleophilic substitution of chlorine in an aryl chloride via the benzyne mechanism rather than the addition–elimination (Meisenheimer) SNAr pathway?

(A)

$p$ ‑NO $_2$ C $_6$ H $_4$ Cl treated with $NaOCH_3$ in $CH_3OH$ at 25°C

(B)

$C_6H_5Cl$ treated with $NaNH_2$ at high temperature (e.g., ≈400°C) followed by acid work‑up

(C)

$o$ ‑diNO $_2$ C $_6$ H $_3$ Cl treated with $NaOH$ in ethanol at room temperature

(D)

$C_6H_5Cl$ treated with $NaOH$ in water at 25°C

Q10. Which of the following alkyl chlorides will show the lowest reactivity toward bimolecular nucleophilic substitution (SN2) by $I^{-}$ in acetone?

(A)

$(CH_3)_3CCH_2Cl$ (neopentyl chloride)

(B)

$(CH_3)_2CHCH_2Cl$ (isobutyl chloride)

(C)

$C_6H_5CH_2Cl$ (benzyl chloride)

(D)

$CH_3CHClCH_3$ (2‑chloropropane)

Q11. Arrange the following alkyl halides in decreasing reactivity toward an $S_{\mathrm{N}}2$ nucleophile (e.g., $CN^-$ in DMF): (i) benzyl bromide ( $C_6H_5CH_2Br$ ), (ii) 1-bromobutane ( $CH_3CH_2CH_2CH_2Br$ ), (iii) 2-bromobutane ( $CH_3CHBrCH_2CH_3$ ), (iv) neopentyl bromide ( $1$ -bromo-2,2-dimethylpropane, $(CH_3)_3CCH_2Br$ ). Which order is correct?

(A)

$CH_3CH_2CH_2CH_2Br > CH_3CHBrCH_2CH_3 > C_6H_5CH_2Br > (CH_3)_3CCH_2Br$

(B)

$(CH_3)_3CCH_2Br > CH_3CHBrCH_2CH_3 > CH_3CH_2CH_2CH_2Br > C_6H_5CH_2Br$

(C)

$C_6H_5CH_2Br > CH_3CH_2CH_2CH_2Br > CH_3CHBrCH_2CH_3 > (CH_3)_3CCH_2Br$

(D)

$C_6H_5CH_2Br > CH_3CHBrCH_2CH_3 > CH_3CH_2CH_2CH_2Br > (CH_3)_3CCH_2Br$

Q12. Consider the attempted Finkelstein exchange: neopentyl chloride ( $1$ -chloro-2,2-dimethylpropane, $(CH_3)_3CCH_2Cl$ ) + $NaI$ in acetone at room temperature. Which statement best describes whether the halide exchange proceeds at an appreciable rate and why?

(A)

Reaction is negligibly slow — severe steric hindrance prevents backside $S_{\mathrm{N}}2$ attack and the neopentyl carbocation required for $S_{\mathrm{N}}1$ is highly unstable.

(B)

Reaction proceeds readily because precipitation of $NaCl$ in acetone drives the equilibrium to completion, so neopentyl iodide is formed in good yield.

(C)

Reaction proceeds via a radical chain (neopentyl radical formation) under these conditions to give neopentyl iodide efficiently.

(D)

Reaction is slow at room temperature but will proceed at reflux mainly by an $S_{\mathrm{N}}1$ pathway to give neopentyl iodide.

Q13. Benzyl chloride ( $C_6H_5CH_2Cl$ ) at $[S]=0.010\ \mathrm{M}$ reacts with $OH^-$ at $[OH^-]=0.100\ \mathrm{M}$ in water via two parallel pathways: unimolecular ( $S_{\mathrm{N}}1$ ) with $k_1=5.0\times10^{-4}\ \mathrm{s^{-1}}$ and bimolecular ( $S_{\mathrm{N}}2$ ) with $k_2=2.0\ \mathrm{M^{-1}s^{-1}}$ . Using $r_{SN1}=k_1[S]$ and $r_{SN2}=k_2[S][OH^-]$ , which statement correctly describes the relative contributions?

(A)

$S_{\mathrm{N}}1$ predominates because resonance stabilisation makes the benzyl cation form readily.

(B)

$S_{\mathrm{N}}1$ and $S_{\mathrm{N}}2$ contribute roughly equally under the given concentrations.

(C)

Overall rate is negligible under these conditions.

(D)

The bimolecular $S_{\mathrm{N}}2$ pathway accounts for >99% of the observed rate (compute $r_{SN1}$ and $r_{SN2}$ using the given $k$ and concentrations).

Q14. Assertion (A): Benzyl chloride ( $C_6H_5CH_2Cl$ ) reacts with aqueous $NaOH$ faster than isopropyl chloride ( $(CH_3)_2CHCl$ ) under identical conditions. Reason (R): The benzyl cation is resonance-stabilized which lowers the activation energy for heterolytic cleavage of the C–Cl bond. Choose the correct option.

(A)

Both A and R are true and R is the correct explanation of A.

(B)

Both A and R are true but R is NOT the correct explanation of A.

(C)

A is true but R is false.

(D)

A is false but R is true.

Q15. Which of the following halides will react fastest with magnesium metal in dry ether to give the corresponding Grignard reagent?

(A)

benzyl bromide ( $C_6H_5CH_2Br$ )

(B)

bromobenzene ( $C_6H_5Br$ )

(C)

p-nitrobromobenzene ( $p$ - $NO_2$ - $C_6H_4Br$ )

(D)

tert-butyl bromide ( $(CH_3)_3CBr$ )

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