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Haloalkanes and Haloarenes Set-7 MCQ Online Practice Test | Class 12 | Quiz Tweek

Haloalkanes and Haloarenes Set-7

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

“Haloalkanes and Haloarenes” is a high-scoring chapter because it connects core concepts of substitution/elimination chemistry (SN1/SN2/E1/E2), factors controlling reaction rates (substrate/solvent/nucleophile), and aromatic substitution mechanisms (SNAr, benzyne). In CBSE boards and JEE/NEET, questions frequently test mechanism recognition, relative reactivity orders, and stereochemical outcomes—making this chapter essential for both conceptual clarity and exam strategy.

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Q1. Which of the following haloalkanes will undergo nucleophilic substitution with $CN^-$ in acetone (polar aprotic) predominantly via the $S_N2$ pathway and at the fastest rate?

(A)

$CH_3CH_2CH_2CH_2Cl$ (1‑chlorobutane)

(B)

$CH_3CH_2CH_2CH_2Br$ (1‑bromobutane)

(C)

$CH_3CH(CH_3)CH_2Br$ (2‑bromobutane)

(D)

$(CH_3)_3CBr$ (tert‑butyl bromide)

Q2. Which of the following aryl halides will undergo nucleophilic aromatic substitution (addition–elimination, $S_{N}\!Ar$ ) with $OH^-$ most rapidly at moderate temperature ( $\approx 60^\circ\mathrm{C}$ )?

(A)

$2,4$ ‑dinitrochlorobenzene $\;(C_6H_3Cl(NO_2)_2)$

(B)

$p$ ‑nitrochlorobenzene $\;(C_6H_4ClNO_2)$

(C)

chlorobenzene $\;(C_6H_5Cl)$

(D)

$o$ ‑fluoronitrobenzene $\;(C_6H_4FNO_2)$

Q3. Which of the following correctly matches the substrate and the typical nucleophilic aromatic substitution mechanism under the stated conditions?

(A)

Chlorobenzene $\;(C_6H_5Cl)$ with $OH^-$ (warm aqueous) — addition–elimination ( $S_{N}\!Ar$ )

(B)

$p$ ‑Nitrochlorobenzene $\;(C_6H_4ClNO_2)$ with $NaNH_2$ (fused) — addition–elimination ( $S_{N}\!Ar$ )

(C)

Chlorobenzene $\;(C_6H_5Cl)$ with $NaNH_2$ (fused) — addition–elimination ( $S_{N}\!Ar$ )

(D)

$p$ ‑Nitrochlorobenzene $\;(C_6H_4ClNO_2)$ with $OH^-$ (moderate $T$ ) — addition–elimination ( $S_{N}\!Ar$ ); Chlorobenzene $\;(C_6H_5Cl)$ with $NaNH_2$ (fused) — benzyne mechanism

Q4. Experimental data show that for nitro‑substituted aryl halides undergoing addition–elimination ( $S_{N}\!Ar$ ) the observed reactivity order of halogens is $F>Cl>Br>I$ , whereas under conditions that proceed via a benzyne intermediate the observed order is $I>Br>Cl>F$ . Which statement best explains this contrasting order?

(A)

In both mechanisms leaving‑group ability depends only on C–X bond strength; solvent differences cause the observed reversal.

(B)

Fluorine is always the best leaving group in aromatic substitution because of its small size and high electronegativity.

(C)

In addition–elimination ( $S_{N}\!Ar$ ) the nucleophile first adds to form a negatively charged Meisenheimer complex which is stabilized by strong $-I$ effects (so $F$ stabilizes the intermediate), making aryl–F most reactive; in the benzyne route the rate‑determining step involves removal of $X^-$ (departure correlated with weaker C–X bond), so $I$ is best.

(D)

Benzyne mechanism requires strong electron‑donating groups, which reverses the leaving‑group order to $I>Br>Cl>F$ .

Q5. Three isomeric alkyl bromides (formula $C_4H_9Br$ ) labelled P, Q and R give the following experimental observations: (i) P reacts rapidly with $NaI$ in acetone (Finkelstein), (ii) Q gives an immediate white precipitate with $AgNO_3$ in ethanol, (iii) R gives predominantly alkene on treatment with $KOC(CH_3)_3$ in $(CH_3)_3COH$ . Which assignment of P, Q, R to the structures below is consistent with these observations?
Structures: $1$ ‑bromobutane $\;(CH_3CH_2CH_2CH_2Br)$ , $2$ ‑bromobutane $\;(CH_3CHBrCH_2CH_3)$ , $2$ ‑bromo‑2‑methylpropane $\;((CH_3)_3CBr)$ .

(A)

P = $CH_3CH_2CH_2CH_2Br$ , Q = $CH_3CHBrCH_2CH_3$ , R = $(CH_3)_3CBr$

(B)

P = $CH_3CH_2CH_2CH_2Br$ , Q = $(CH_3)_3CBr$ , R = $CH_3CHBrCH_2CH_3$

(C)

P = $(CH_3)_3CBr$ , Q = $CH_3CH_2CH_2CH_2Br$ , R = $CH_3CHBrCH_2CH_3$

(D)

P = $CH_3CHBrCH_2CH_3$ , Q = $(CH_3)_3CBr$ , R = $CH_3CH_2CH_2CH_2Br$

Q6. In a kinetic study of the nucleophilic substitution of $1$ ‑bromobutane ( $\mathrm{CH_3CH_2CH_2CH_2Br}$ ) by azide ion ( $\mathrm{N_3^-}$ ) in DMSO at 298 K, the initial rate doubles when $[\mathrm{N_3^-}]$ is doubled (with $[\mathrm{RX}]$ constant) and also doubles when $[\mathrm{RX}]$ is doubled (with $[\mathrm{N_3^-}]$ constant). Which statement correctly describes the rate law and likely mechanism?

(A)

Rate law $rate = k[\mathrm{RX}]$ — SN1 mechanism.

(B)

Rate law $rate = k[\mathrm{RX}]^2$ — involves two substrate molecules in the rate‑determining step.

(C)

Rate law $rate = k[\mathrm{RX}][\mathrm{N_3^-}]$ — bimolecular SN2 process.

(D)

Rate law $rate = k[\mathrm{N_3^-}]$ — rate depends only on nucleophile concentration.

Q7. For nucleophilic substitution by sodium methoxide in methanol at 298 K, rank the initial SN2 reactivity (fastest → slowest) among the following bromides: (i) $1$ ‑bromopropane $\mathrm{CH_3CH_2CH_2Br}$ , (ii) $2$ ‑bromopropane $\mathrm{CH_3CHBrCH_3}$ , (iii) benzyl bromide $\mathrm{C_6H_5CH_2Br}$ . Which order is correct?

(A)

$\mathrm{C_6H_5CH_2Br} > \mathrm{CH_3CH_2CH_2Br} > \mathrm{CH_3CHBrCH_3}$

(B)

$\mathrm{CH_3CHBrCH_3} > \mathrm{C_6H_5CH_2Br} > \mathrm{CH_3CH_2CH_2Br}$

(C)

$\mathrm{CH_3CH_2CH_2Br} > \mathrm{CH_3CHBrCH_3} > \mathrm{C_6H_5CH_2Br}$

(D)

$\mathrm{C_6H_5CH_2Br} > \mathrm{CH_3CHBrCH_3} > \mathrm{CH_3CH_2CH_2Br}$

Q8. Consider nucleophilic aromatic substitution of $p$ ‑nitrohalobenzenes $p\text{-}\mathrm{NO_2C_6H_4X}$ ( $X=\mathrm{F}$ or $\mathrm{Cl}$ ) by methoxide ( $\mathrm{CH_3O^-}$ ). Which statement correctly predicts the relative rates and the principal reason?

(A)

$p$ ‑Nitrochlorobenzene reacts faster because $\mathrm{Cl}$ is a better leaving group on an aromatic ring.

(B)

$p$ ‑Nitrofluorobenzene reacts faster because the C–F bond is weaker than C–Cl in polar solvents.

(C)

Both react at comparable rates because the strong $p$ ‑NO $_2$ group dominates and equalizes reactivity.

(D)

$p$ ‑Nitrofluorobenzene reacts faster because formation of the Meisenheimer (σ) complex (rate‑determining) is facilitated by the strong −I effect of $\mathrm{F}$ which increases the electrophilicity of the ipso carbon; hence aryl fluorides with ortho/para EWGs are often more reactive toward SNAr than their chloro analogues.

Q9. Pure $(R)$ ‑1‑phenylethyl bromide ( $\mathrm{PhCH(Br)CH_3}$ ; initially 100% ee as $(R)$ ) is subjected to Finkelstein reaction with NaI in acetone. The isolated iodide shows $70\%$ ee with the $(S)$ enantiomer in excess. Assuming substitution occurs either by (i) SN2 giving complete inversion or (ii) SN1 giving a racemic mixture, what percentage of the substitution events proceeded via SN1?

(A)

$15\%$

(B)

$30\%$

(C)

$50\%$

(D)

$70\%$

Q10. Treatment of $cis$ ‑1‑bromo‑2‑methylcyclohexane with potassium tert‑butoxide in tert‑butanol is carried out. Considering the antiperiplanar requirement for an E2 elimination in a cyclohexane chair, which alkene will predominate as the major product?

(A)

$\mathrm{3\text{-}methylcyclohexene}$ (the less‑substituted Hofmann product)

(B)

$\mathrm{1\text{-}methylcyclohexene}$ (the more‑substituted Zaitsev product)

(C)

An approximately equal mixture of $\mathrm{cyclohexene}$ and $\mathrm{1\text{-}methylcyclohexene}$

(D)

$\mathrm{cyclohexene}$ (would require rearrangement; not expected in E2)

Q11. Consider the bimolecular nucleophilic substitution reaction $CH_3Br + OH^- \rightarrow CH_3OH + Br^-$ . whose rate law is $r = k[CH_3Br][OH^-]$ . If $[CH_3Br]$ is doubled and $[OH^-]$ is reduced to one-fourth of its original value, the new rate will be:

(A)

Unchanged

(B)

Twice the original rate

(C)

Half the original rate

(D)

One-fourth the original rate

Q12. Which of the following aryl halides will undergo nucleophilic aromatic substitution (addition–elimination, $S_NAr$ ) with $NaOCH_3$ at room temperature most readily?

(A)

$1$ -chloro- $2,4$ -dinitrobenzene $\;(C_6H_3Cl(NO_2)_2)$

(B)

$1$ -chloro- $3$ -nitrobenzene $\;(C_6H_4ClNO_2)$

(C)

chlorobenzene $\;(C_6H_5Cl)$

(D)

$1$ -chloro- $4$ -methoxybenzene $\;(C_6H_4ClOCH_3)$

Q13. To maximize the rate of an $S_N2$ reaction of a secondary alkyl bromide with azide ion ( $N_3^-$ ), which of the following solvents is most suitable?

(A)

Ethanol

(B)

Acetic acid ( $CH_3COOH$ )

(C)

Water ( $H_2O$ )

(D)

Dimethyl sulfoxide ( $DMSO$ )

Q14. Which reagent sequence will convert chlorobenzene ( $C_6H_5Cl$ ) into aniline ( $C_6H_5NH_2$ ) via formation of a benzyne (aryne) intermediate?

(A)

Aqueous $NaOH$ , $350^\circ\mathrm{C}$ , then acid work-up

(B)

$NaNH_2$ (liq $NH_3$ ), then hydrolysis ( $H_3O^+$ )

(C)

Gaseous $NH_3$ , $200^\circ\mathrm{C}$ (direct nucleophilic substitution)

(D)

Hydrogenation with $H_2/Pd$ catalyst

Q15. Assertion (A): Aryl halides undergo nucleophilic aromatic substitution (addition–elimination, $S_NAr$ ) readily only when strong electron-withdrawing groups (e.g., $NO_2$ ) are present at the ortho or para positions to the leaving group.
Reason (R): This is because electron-withdrawing groups increase the electron density on the aromatic ring, facilitating nucleophilic attack.

(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.

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