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Haloalkanes and Haloarenes Set-9 MCQ Online Practice Test | Class 12 | Quiz Tweek

Haloalkanes and Haloarenes Set-9

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

“Haloalkanes and Haloarenes” is crucial because it links core ideas of substitution vs elimination, SN1/SN2/SNAr mechanisms, stereochemistry, and reactivity trends—all of which appear repeatedly in CBSE boards and are heavily tested in JEE/NEET through mechanism-based reasoning and option-based inference.

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Q1. A student studies SN2 reactions of methyl bromide $\mathrm{CH_3Br}$ with halide ions $\mathrm{F^-},\ \mathrm{Cl^-},\ \mathrm{Br^-},\ \mathrm{I^-}$ at equal concentrations in dimethyl sulfoxide (DMSO, an aprotic solvent). Which of the following sequences (fastest → slowest) correctly represents the expected order of reaction rates with $\mathrm{CH_3Br}$ in DMSO?

(A)

$\mathrm{I^-} > \mathrm{Br^-} > \mathrm{Cl^-} > \mathrm{F^-}$

(B)

$\mathrm{F^-} > \mathrm{Cl^-} > \mathrm{Br^-} > \mathrm{I^-}$

(C)

$\mathrm{Cl^-} > \mathrm{Br^-} > \mathrm{I^-} > \mathrm{F^-}$

(D)

$\mathrm{Br^-} > \mathrm{I^-} > \mathrm{Cl^-} > \mathrm{F^-}$

Q2. Which of the following haloarenes will undergo nucleophilic aromatic substitution (addition–elimination, SNAr) by $\mathrm{NaOCH_3}$ in methanol at room temperature most rapidly?

(A)

Chlorobenzene $\,\mathrm{(C_6H_5Cl)}$

(B)

1-Chloro-3-nitrobenzene $\,\mathrm{(m\text{-}ClC_6H_4NO_2)}$

(C)

1-Chloro-4-nitrobenzene $\,\mathrm{(p\text{-}ClC_6H_4NO_2)}$

(D)

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

Q3. When 2-bromobutane $\,\mathrm{CH_3CHBrCH_2CH_3}\,$ is treated with excess sodium methoxide $\,\mathrm{NaOCH_3}\,$ in methanol, which pair correctly describes the major product at (i) 25 °C and (ii) 60 °C?

(A)

(i) 2-Methoxybutane (substitution, SN2) ; (ii) Predominantly 2-butene (Zaitsev, E-2-butene) (elimination, E2)

(B)

(i) 1-Methoxybutane (rearranged substitution) ; (ii) 1-Butene (elimination at terminal position)

(C)

(i) Predominantly 2-butene (elimination, E2) (Zaitsev) ; (ii) 2-Methoxybutane (substitution)

(D)

(i) Mixture from SN1/E1 with extensive carbocation rearrangement ; (ii) Polymeric by-products (no defined elimination product)

Q4. Arrange the following in decreasing rate of solvolysis (SN1) in ethanol (fastest → slowest): benzyl chloride $\,\mathrm{C_6H_5CH_2Cl}\,$ , tert-butyl chloride $\,\mathrm{(CH_3)_3CCl}\,$ , 1-chlorobutane $\,\mathrm{CH_3CH_2CH_2CH_2Cl}\,$ , vinyl chloride $\,\mathrm{CH_2=CHCl}\,$ .

(A)

$\mathrm{(CH_3)_3CCl} > \mathrm{C_6H_5CH_2Cl} > \mathrm{CH_3CH_2CH_2CH_2Cl} > \mathrm{CH_2=CHCl}$

(B)

$\mathrm{C_6H_5CH_2Cl} > \mathrm{CH_3CH_2CH_2CH_2Cl} > \mathrm{(CH_3)_3CCl} > \mathrm{CH_2=CHCl}$

(C)

$\mathrm{C_6H_5CH_2Cl} > \mathrm{(CH_3)_3CCl} > \mathrm{CH_3CH_2CH_2CH_2Cl} > \mathrm{CH_2=CHCl}$

(D)

$\mathrm{CH_2=CHCl} > \mathrm{C_6H_5CH_2Cl} > \mathrm{(CH_3)_3CCl} > \mathrm{CH_3CH_2CH_2CH_2Cl}$

Q5. Toluene $\,\mathrm{C_6H_5CH_3}\,$ is subjected to two different bromination conditions:

(i) $\mathrm{Br_2/FeBr_3}$ (electrophilic aromatic substitution) and

(ii) N-bromosuccinimide (NBS) in $\mathrm{CCl_4}$ under hv (radical, benzylic bromination).

Which pair gives the major products for (i) and (ii), respectively?

(A)

(i) Mixture of ortho- and para-bromotoluene with ortho as major ; (ii) Benzyl bromide $\,\mathrm{C_6H_5CH_2Br}$

(B)

(i) Para-bromotoluene $\,\mathrm{(p\text{-}BrC_6H_4CH_3)}$ as major ; (ii) Benzyl bromide $\,\mathrm{C_6H_5CH_2Br}$

(C)

(i) Benzyl bromide $\,\mathrm{C_6H_5CH_2Br}$ ; (ii) Para-bromotoluene $\,\mathrm{(p\text{-}BrC_6H_4CH_3)}$

(D)

(i) Para-bromotoluene $\,\mathrm{(p\text{-}BrC_6H_4CH_3)}$ ; (ii) Benzal bromide $\,\mathrm{C_6H_5CHBr_2}$

Q6. A solvolysis reaction of tert-butyl chloride $\big((CH_3)_3CCl\big)$ in ethanol follows first-order kinetics with rate constant $k = 1.2\times 10^{-3}\ \mathrm{s^{-1}}$ . How long (in minutes) will it take for $87.5\%$ of the substrate to react?

(A)

$9.63\ \mathrm{min}$

(B)

$28.9\ \mathrm{min}$

(C)

$19.3\ \mathrm{min}$

(D)

$43.3\ \mathrm{min}$

Q7. Two alkyl halides — tert-butyl chloride $\big((CH_3)_3CCl\big)$ (A) and benzyl chloride $\big(C_6H_5CH_2Cl\big)$ (B) — undergo solvolysis (first order) with rate constants $k_A = 1.5\times10^{-4}\ \mathrm{s^{-1}}$ and $k_B = 3.75\times10^{-3}\ \mathrm{s^{-1}}$ . If both are initially $0.10\ \mathrm{M}$ , what is $[A]$ when $[B]$ has dropped to $0.025\ \mathrm{M}$ ?

(A)

$0.090\ \mathrm{M}$

(B)

$0.047\ \mathrm{M}$

(C)

$0.025\ \mathrm{M}$

(D)

$0.0946\ \mathrm{M}$

Q8. Optically pure $(R)$ -2-bromobutane is reacted with excess $N_3^-$ under conditions where $80\%$ of substitution occurs by an $S_N2$ pathway (complete inversion) and $20\%$ occurs by an $S_N1$ pathway (which gives a racemic mixture). What is the absolute configuration of the major enantiomer of the product and its enantiomeric excess (e.e.)?

(A)

$(S)$ , $80\%$ e.e.

(B)

$(R)$ , $80\%$ e.e.

(C)

$(S)$ , $60\%$ e.e.

(D)

racemic

Q9. Among the following haloarenes, which will react fastest with $CH_3O^-$ in $CH_3OH$ at room temperature by the addition–elimination (Meisenheimer) mechanism?

(A)

$1$ -chloro- $2,4$ -dinitrobenzene

(B)

$1$ -bromo- $2,4$ -dinitrobenzene

(C)

$1$ -fluoro- $2,4$ -dinitrobenzene

(D)

chlorobenzene

Q10. Chlorobenzene is nitrated by $HNO_3/H_2SO_4$ much more slowly than benzene, yet chlorobenzene directs incoming electrophiles to the ortho and para positions. Which explanation best accounts for these observations?

(A)

The chlorine atom is a strong electron donor by resonance so it activates the ring overall and directs ortho/para, but steric hindrance slows the reaction compared to benzene.

(B)

Chlorine withdraws electron density by induction ( $-I$ ), deactivating the ring (slower nitration), but it donates electron density by resonance ( $+R$ ) to the ortho/para positions, making the $\sigma$ -complex at ortho/para relatively more stabilized than at meta (hence ortho/para directing yet overall slower).

(C)

Chlorine increases electron density at the meta position through hyperconjugation, so electrophiles preferentially attack ortho/para due to steric relief elsewhere.

(D)

The $-I$ and $+R$ effects of chlorine cancel exactly, but solvent effects make ortho/para substitution easier than meta, producing the observed pattern

Q11. Among the following halo compounds, which will undergo nucleophilic substitution by aqueous $OH^-$ at room temperature most rapidly?

(A)

$C_6H_5Cl$ (chlorobenzene)

(B)

$C_6H_5CH_2Cl$ (benzyl chloride)

(C)

$CH_3CHClCH_3$ (2-chloropropane)

(D)

$CH_3CH_2CH_2CH_2Cl$ (1-chlorobutane)

Q12. When (R)-2-bromobutane ( $CH_3CHBrCH_2CH_3$ ) is treated with $NaOCH_3$ in methanol at $60^\circ\text{C}$ , the major organic product expected is:

(A)

(S)-2-methoxybutane ( $CH_3CH(OCH_3)CH_2CH_3$ ) formed by SN2 inversion

(B)

Racemic 2-methoxybutane formed by SN1

(C)

A mixture in which substitution predominates over elimination

(D)

$(E)$ -but-2-ene ( $CH_3CH=CHCH_3$ ) formed predominantly by elimination

Q13. Which of the following aryl halides undergoes nucleophilic aromatic substitution by $OH^-$ under mild aqueous conditions most readily to give the corresponding phenol?

(A)

$1$ -Chloro-2,4-dinitrobenzene ( $ClC_6H_3(NO_2)_2$ )

(B)

$p$ -Nitrochlorobenzene ( $p$ - $NO_2$ - $C_6H_4Cl$ )

(C)

$m$ -Nitrochlorobenzene ( $m$ - $NO_2$ - $C_6H_4Cl$ )

(D)

$C_6H_5Cl$ (chlorobenzene)

Q14. Consider the following pair of substrates and typical reagents used to probe SN2 vs SN1 reactivity:

\begin{aligned} &ClCH_2C(CH_3)_3 + NaI \xrightarrow{\text{acetone}} ? \\ &(CH_3)_3CCl + AgNO_3 \xrightarrow{\text{EtOH}} ? \end{aligned}

Which one of the following statements about their reactivity is correct?

(A)

Neopentyl chloride ( $ClCH_2C(CH_3)_3$ ) reacts rapidly with $NaI$ in acetone via SN2, while tert‑butyl chloride is unreactive under these SN2 conditions.

(B)

tert‑Butyl chloride ( $(CH_3)_3CCl$ ) reacts rapidly with $AgNO_3$ in ethanol via SN1, and neopentyl chloride reacts even faster by the same mechanism.

(C)

tert‑Butyl chloride undergoes fast solvolysis with $AgNO_3$ /EtOH (SN1), whereas neopentyl chloride is poorly reactive in both $NaI$ /acetone (SN2) and $AgNO_3$ /EtOH (SN1) due to steric hindrance and the inability to form a stable carbocation.

(D)

Neopentyl chloride undergoes SN1 readily because the adjacent quaternary carbon stabilizes the carbocation; therefore it reacts faster than tert‑butyl chloride with $AgNO_3$ /EtOH.

Q15. Assertion (A) and Reason (R):
A: $C_6H_5Cl$ (chlorobenzene) does not undergo nucleophilic substitution by $OH^-$ under mild conditions.
R: The $C_{sp^2}\!-\!Cl$ bond in chlorobenzene has partial double-bond character due to delocalisation of the chlorine lone pair into the ring, and formation of an aryl cation is energetically unfavourable.
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.

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