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Haloalkanes and Haloarenes Set-2 MCQ Online Practice Test | Class 12 | Quiz Tweek

Haloalkanes and Haloarenes Set-2

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

The chapter “Haloalkanes and Haloarenes” is crucial because it builds the core ideas of nucleophilic substitution (SN1/SN2), elimination (E1/E2), reactivity trends, and special mechanisms in aromatic systems like SNAr and benzyne. These concepts are heavily tested in both board and competitive exams through reaction-rate/kinetics problems and mechanism-based MCQs.

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Q1. A 0.10 M solution of tertiary butyl chloride ( $t$ - $\mathrm{C_4H_9Cl}$ ) decomposes by an SN1 mechanism with rate constant $k=2.0\times10^{-3}\ \mathrm{s^{-1}}$ . What fraction of the substrate remains after $5.0\ \mathrm{min}$ ?

(A)

$36.8\%$

(B)

$54.9\%$

(C)

$45.1\%$

(D)

$74.1\%$

Q2. Which of the following aryl halides is most likely to undergo nucleophilic aromatic substitution primarily via the benzyne mechanism when treated with $NaNH_2$ in liquid $NH_3$ at high temperature (followed by acidic work-up)?

(A)

$p$ -Dinitrochlorobenzene ( $\mathrm{ClC_6H_3(NO_2)_2}$ )

(B)

$o$ -Nitrochlorobenzene ( $\mathrm{ClC_6H_4NO_2}$ , nitro at ortho)

(C)

$p$ -Nitrochlorobenzene ( $\mathrm{ClC_6H_4NO_2}$ , nitro at para)

(D)

Chlorobenzene ( $\mathrm{C_6H_5Cl}$ )

Q3. An alkyl halide X reacts with sodium azide ( $N_3^-$ ) in acetone and the kinetics follow rate = $k[\mathrm{X}][N_3^-]$ ; moreover the observed reactivity is much higher than for a typical primary alkyl halide. Which of the following structures for X best fits these observations?

(A)

Benzyl chloride ( $\mathrm{C_6H_5CH_2Cl}$ )

(B)

2-Bromobutane ( $\mathrm{CH_3CHBrCH_2CH_3}$ )

(C)

tert-Butyl bromide ( $\mathrm{(CH_3)_3CBr}$ )

(D)

Chlorobenzene ( $\mathrm{C_6H_5Cl}$ )

Q4. Assertion (A): Aryl fluorides bearing strong electron-withdrawing groups at ortho/para positions undergo nucleophilic aromatic substitution faster than the corresponding aryl chlorides under the same conditions.
Reason (R): In the addition–elimination (Meisenheimer) pathway the highly electronegative fluorine, by its strong inductive ( $-I$ ) effect, stabilizes the negative charge developed on the ring in the $\sigma$ -complex, and this stabilization can outweigh fluoride's intrinsically poorer leaving-group ability.

(A)

A is true, R is false.

(B)

A is false, R is true.

(C)

Both A and R are true and R correctly explains A.

(D)

Both A and R are true but R does not correctly explain A.

Q5. Which of the following alkyl halides will undergo halide-exchange fastest with $NaI$ in acetone (Finkelstein reaction, an SN2 process) and why?

(A)

Neopentyl chloride ( $\mathrm{(CH_3)_3CCH_2Cl}$ )

(B)

Benzyl chloride ( $\mathrm{C_6H_5CH_2Cl}$ )

(C)

tert-Butyl chloride ( $\mathrm{(CH_3)_3CCl}$ )

(D)

Isopropyl chloride ( $\mathrm{CH_3CH(Cl)CH_3}$ )

Q6. Among the following alkyl halides, which will undergo the fastest solvolysis (SN1) in ethanol under identical concentrations and temperature?

(A)

$(CH_3)_3CCl$ (tert‑butyl chloride)

(B)

$C_6H_5CH_2Cl$ (benzyl chloride)

(C)

$CH_3CHClCH_3$ (isopropyl chloride)

(D)

$(CH_3)_3CCH_2Cl$ (neopentyl chloride)

Q7. Which of the following chlorobenzenes will undergo nucleophilic aromatic substitution (addition–elimination) by methoxide ( $^-OCH_3$ ) at the carbon bearing the chlorine most rapidly?

(A)

$p$ ‑Nitrochlorobenzene ( $4$ ‑nitrochlorobenzene)

(B)

Chlorobenzene ( $C_6H_5Cl$ )

(C)

$m$ ‑Nitrochlorobenzene ( $3$ ‑nitrochlorobenzene)

(D)

$1$ ‑Chloro‑2,4‑dinitrobenzene

Q8. Kinetic studies at constant temperature for two substrates reacting with sodium methoxide in methanol give:

Substrate X ( $C_6H_5CH_2Cl$ ): rate ∝ [X][NaOCH_3]
Substrate Y ( $(CH_3)_3CCl$ ): rate ∝ [Y] (rate independent of [NaOCH_3])
Which statement correctly assigns the predominant substitution mechanism for X and Y, respectively?

(A)

X undergoes SN2; Y undergoes SN1

(B)

X undergoes SN1; Y undergoes SN2

(C)

Both X and Y undergo SN2

(D)

Both X and Y undergo SN1

Q9. It is observed that $1$ ‑fluoro‑2,4‑dinitrobenzene reacts more rapidly with methoxide than $1$ ‑chloro‑2,4‑dinitrobenzene, even though fluoride is a poorer leaving group in aliphatic nucleophilic substitutions. Which explanation best accounts for this aromatic behavior?

(A)

The C–F bond in these aromatic substrates is weaker than the C–Cl bond, so F leaves more easily.

(B)

Fluorine is larger and produces less steric hindrance for the incoming nucleophile compared to chlorine.

(C)

The strong $-I$ effect of F increases electron deficiency of the ring and stabilises the negatively charged Meisenheimer intermediate, accelerating the addition step; thus fluoro‑aryl is more reactive in SNAr despite F being a poor leaving group in aliphatic SN reactions.

(D)

Fluorine forms hydrogen bonds with methanol that orient the nucleophile for faster attack.

Q10. Which of the following alkyl bromides will show the largest rate enhancement in solvolysis (ethanol) specifically due to neighbouring‑group (anchimeric) participation by an adjacent oxygen lone pair?

(A)

$CH_3C(Br)(OCH_3)CH_3$ (2‑bromo‑2‑methoxypropane)

(B)

$BrCH_2CH(OCH_3)CH_3$ (1‑bromo‑2‑methoxypropane)

(C)

$BrCH_2CH_2CH_2OCH_3$ (1‑bromo‑3‑methoxypropane)

(D)

$C_6H_5CH_2Br$ (benzyl bromide)

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