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Haloalkanes and Haloarenes Set-5 MCQ Online Practice Test | Class 12 | Quiz Tweek

Haloalkanes and Haloarenes Set-5

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

This chapter is crucial because it builds the core CBSE/JEE/NEET foundation of reaction mechanisms (SN1, SN2, E1, E2, SNAr), stereochemistry (inversion/retention, cis–trans outcomes), and relative reactivities driven by carbocation stability and electronic/steric effects. Questions from this chapter frequently test concept clarity through application to both aliphatic halides and haloarenes, making it a high-scoring area in board and competitive exams.

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Q1. Among the following haloalkanes, which will react fastest with $NaCN$ in acetone via an $S_N2$ pathway?

(A)

(A) $CH_3CH(Br)CH_2CH_3$ (2-bromobutane)

(B)

(B) $(CH_3)_3CCH_2Br$ (neopentyl bromide)

(C)

(D)

(D) $(CH_3)_3CBr$ (tert‑butyl bromide)

Q2. Which of the following aryl chlorides will undergo nucleophilic aromatic substitution with $CH_3O^-$ in ethanol most readily at room temperature?

(A)

(A) $C_6H_5Cl$ (chlorobenzene)

(B)

(B) $1$ ‑chloro‑2,4‑dinitrobenzene;( $C_6H_3Cl(NO_2)_2$ , nitro at 2 and 4)

(C)

(D)

(D) $1$ ‑chloro‑3‑nitrobenzene;( $m$ ‑nitrochlorobenzene)

Q3. When $3$ ‑bromo‑ $2$ ‑methylbutane ( $CH_3CH(CH_3)CH(Br)CH_3$ ) is treated with $AgNO_3$ in ethanol, which is the major substitution product formed?

(A)

(A) $CH_3CH(CH_3)CH(OCH_2CH_3)CH_3$ (3‑ethoxy‑2‑methylbutane)

(B)

(B) $CH_3CH(OCH_2CH_3)CH(CH_3)CH_3$ (2‑ethoxy‑3‑methylbutane)

(C)

(D)

(D) $CH_3C(OCH_2CH_3)(CH_3)CH_2CH_3$ (2‑ethoxy‑2‑methylbutane, product after 1,2‑hydride shift)

Q4. Which of the following halides will form the corresponding Grignard reagent most readily with magnesium in dry ether?

(A)

(A) $C_6H_5CH_2Cl$ (benzyl chloride)

(B)

(B) $(CH_3)_3CCl$ (tert‑butyl chloride)

(C)

(D)

(D) $(CH_3)_3CCH_2Cl$ (neopentyl chloride)

Q5. Chlorobenzene undergoes nitration much slower than benzene yet gives predominantly ortho and para nitro products. Which statement best explains this observation?

(A)

(A) Chlorine is a strong electron‑donating group by resonance ( $+R$ ), so the ring is overall activated; steric hindrance at ortho positions lowers ortho product.

(B)

(B) Chlorine exerts only an electron‑withdrawing inductive effect ( $-I$ ) and also withdraws by resonance ( $-M$ ), therefore the ring is deactivated and should direct meta.

(C)

(C) Chlorine withdraws electron density by the inductive effect ( $-I$ ), making the ring less reactive overall, but its lone pairs can donate by resonance ( $+R$ ) which raises electron density at ortho and para positions relative to meta; net effect: slower nitration than benzene but ortho/para directing.

(D)

(D) Chlorine stabilizes the sigma‑complex at meta positions by hyperconjugation, hence nitration occurs mostly at ortho/para because meta is sterically inaccessible.

Q6. The hydrolysis of tert‑butyl chloride ( $(\mathrm{CH_3})_3\mathrm{CCl}$ ) in water follows first‑order kinetics with rate constant $k=3.6\times10^{-4}\ \mathrm{s^{-1}}$ . Using $N_t=N_0e^{-kt}$ , what fraction of tert‑butyl chloride remains unreacted after $1$ hour ( $3600\ \mathrm{s}$ )?

(A)

$0.637$

(B)

$0.500$

(C)

$0.274$

(D)

$0.018$

Q7. Treatment of 2‑bromobutane ( $\mathrm{CH_3CHBrCH_2CH_3}$ ) with potassium tert‑butoxide ( $\mathrm{KOtBu}$ ) in tert‑butanol typically gives which major alkene product?

(A)

1‑Butene, $\mathrm{CH_2=CHCH_2CH_3}$

(B)

trans‑2‑Butene, $\mathrm{CH_3CH=CHCH_3}$ (trans)

(C)

cis‑2‑Butene, $\mathrm{CH_3CH=CHCH_3}$ (cis)

(D)

Predominantly a mixture of cis/trans‑2‑butene (Zaitsev product)

Q8. Which of the following chlorobenzenes will undergo nucleophilic aromatic substitution (addition–elimination, $S_N$ Ar) with methoxide ( $\mathrm{CH_3O^-}$ ) most readily at $25^\circ\mathrm{C}$ ?

(A)

Chlorobenzene, $\mathrm{C_6H_5Cl}$

(B)

p‑Methylchlorobenzene, $\mathrm{p\text{-}CH_3C_6H_4Cl}$

(C)

m‑Nitrochlorobenzene, $\mathrm{m\text{-}NO_2C_6H_4Cl}$

(D)

p‑Nitrochlorobenzene, $\mathrm{p\text{-}NO_2C_6H_4Cl}$

Q9. Which one of the following statements about nucleophilic substitution of p‑nitrochlorobenzene ( $\mathrm{p\text{-}NO_2C_6H_4Cl}$ ) by hydroxide ion at room temperature is correct?

(A)

It proceeds by a direct $S_N2$ displacement at the $sp^2$ aromatic carbon.

(B)

It proceeds by an addition–elimination pathway (Meisenheimer complex); the para $-\mathrm{NO_2}$ group stabilizes the negative charge in the intermediate and thus accelerates the reaction.

(C)

It proceeds via the benzyne mechanism under these mild conditions.

(D)

It reacts predominantly by benzylic substitution (attack at a benzylic carbon) rather than aromatic substitution.

Q10. Which of the following alkyl chlorides will undergo the fastest solvolysis (an $S_N1$ process) in aqueous ethanol at $25^\circ\mathrm{C}$ ?

(A)

p‑Nitrobenzyl chloride, $\mathrm{p\text{-}NO_2C_6H_4CH_2Cl}$

(B)

tert‑Butyl chloride, $(\mathrm{CH_3})_3\mathrm{CCl}$

(C)

p‑Methoxybenzyl chloride, $\mathrm{p\text{-}OCH_3C_6H_4CH_2Cl}$

(D)

Benzyl chloride, $\mathrm{C_6H_5CH_2Cl}$

Q11. A sample of optically active $(R)$ -CH $_3$ CH(Br)CH $_2$ CH $_3$ is treated with excess NaI in acetone at room temperature. The major organic product will be:

(A)

$(R)$ -CH $_3$ CH(I)CH $_2$ CH $_3$

(B)

$(S)$ -CH $_3$ CH(I)CH $_2$ CH $_3$

(C)

Racemic mixture of CH $_3$ CH(I)CH $_2$ CH $_3$

(D)

No reaction; starting material recovered

Q12. Which of the following haloarenes will readily undergo nucleophilic aromatic substitution with methoxide ion ( $\mathrm{CH_3O^-}$ ) at 25°C by the addition–elimination (Meisenheimer) mechanism?

(A)

Chlorobenzene ( $\mathrm{C_6H_5Cl}$ )

(B)

m-Nitrochlorobenzene ( $m$ -O $_2$ N–C $_6$ H $_4$ –Cl)

(C)

p-Nitrochlorobenzene ( $p$ -O $_2$ N–C $_6$ H $_4$ –Cl)

(D)

p-Methoxychlorobenzene ( $p$ -MeO–C $_6$ H $_4$ –Cl)

Q13. Racemic 2-bromobutane (±-CH $_3$ CH(Br)CH $_2$ CH $_3$ ) is heated separately with (i) NaOEt in EtOH (60°C) and (ii) t-BuOK in t-BuOH (60°C). The major elimination products from (i) and (ii) respectively are:

(A)

(i) 1-butene, (ii) trans-2-butene

(B)

(i) cis-2-butene, (ii) trans-2-butene

(C)

(i) trans-2-butene, (ii) trans-2-butene

(D)

(i) trans-2-butene, (ii) 1-butene

Q14. When $1,2$ -dibromocyclohexane is treated with excess NaNH $_2$ in liquid NH $_3$ followed by hydrolysis, which stereoisomer gives cyclohexyne (internal alkyne) as the major organic product and why?

(A)

Only trans- $1,2$ -dibromocyclohexane — both bromines can occupy axial positions in a chair conformation allowing two consecutive anti-periplanar E2 eliminations

(B)

Only cis- $1,2$ -dibromocyclohexane — cis geometry favors the required eliminations on the ring

(C)

Both cis and trans give cyclohexyne equally — rapid chair flips make both eliminations feasible

(D)

Neither gives cyclohexyne under these conditions — ring strain prevents alkyne formation

Q15. Among $p$ -fluoronitrobenzene, $p$ -chloronitrobenzene and $p$ -bromonitrobenzene, which reacts fastest with methoxide ion ( $\mathrm{CH_3O^-}$ ) via the addition–elimination (SNAr) pathway and why?

(A)

$p$ } $-$ Bromonitrobenzene — because C–Br is the weakest C–X bond and leaves most easily

(B)

$p$ } $-$ Fluoronitrobenzene — because the strong $-I$ effect of F stabilizes the Meisenheimer intermediate, making aryl–F the most reactive in SNAr despite its strong C–F bond

(C)

$p$ } $-$ Chloronitrobenzene — chlorine gives the best balance of bond strength and leaving ability for SNAr

(D)

All three react at comparable rates under identical conditions

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