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Haloalkanes and Haloarenes Set-1 MCQ Online Practice Test | Class 12 | Quiz Tweek

Haloalkanes and Haloarenes Set-1

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

The chapter Haloalkanes and Haloarenes is central to understanding reaction mechanisms (SN1/SN2, E1/E2) and the special behavior of aryl halides towards nucleophiles (SNAr, benzyne pathway). It is heavily tested in both board and competitive exams because questions often mix structure-reactivity relationships, kinetics, solvent effects, and regioselectivity—all of which are directly linked to the CBSE/JEE/NEET syllabus expectations.

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Q1. Among the following alkyl halides, which will have the highest second‑order rate constant for nucleophilic substitution by $CN^-$ in DMSO (SN2 pathway)?

(A)

$CH_3Br$

(B)

$C_6H_5CH_2Br$

(C)

$CH_3CH_2CH_2Br$

(D)

$(CH_3)_2CHBr$

Q2. Which of the following haloarenes will undergo nucleophilic aromatic substitution (addition–elimination, SNAr) with $OH^-$ under mild conditions (room temperature) to give the corresponding phenol most rapidly?

(A)

$p$ ‑nitrochlorobenzene

(B)

chlorobenzene

(C)

$o$ ‑nitrochlorobenzene

(D)

$2,4$ ‑dinitrochlorobenzene

Q3. For the substrate $CH_3CHBrCH_2CH_3$ (2‑bromobutane), which set of reaction conditions will give predominantly the elimination (E2) product at room temperature?

(A)

$t$ ‑BuOK in $t$ ‑BuOH

(B)

$NaI$ in acetone

(C)

$AgNO_3$ in ethanol

(D)

$NaCN$ in DMSO

Q4. A sample of (1‑phenylethyl) bromide ( $C_6H_5CHBrCH_3$ ) shows the following kinetics: (i) with $CN^-$ in DMSO: $rate = k[\text{R–Br}][\text{CN}^-]$ ; (ii) hydrolysis in water: $rate = k'[\text{R–Br}]$ . Which statement best explains these observations?

(A)

Both reactions proceed by concerted bimolecular (SN2) displacement because the benzylic carbon is sterically accessible.

(B)

Both reactions proceed via formation of a benzylic carbocation (SN1), so each should show first‑order kinetics.

(C)

Reaction with $CN^-$ in DMSO follows a concerted SN2 pathway (second‑order), whereas hydrolysis in water follows SN1 through a resonance‑stabilized benzylic carbocation; the latter will lead to racemization of stereochemistry.

(D)

Reaction with $CN^-$ proceeds via a single‑electron transfer (radical) mechanism in DMSO, and hydrolysis proceeds by a concerted nucleophilic attack by water.

Q5. Which of the following haloarenes will be most rapidly converted to the methoxy derivative ( $\mathrm{ArOCH_3}$ ) when treated with $CH_3O^-$ in methanol at 25°C via nucleophilic aromatic substitution (SNAr)?

(A)

$2,4$ ‑dinitrochlorobenzene

(B)

$4$ ‑nitrochlorobenzene

(C)

$2$ ‑nitrochlorobenzene

(D)

$chlorobenzene$

Q6. The hydrolysis of tert‑butyl chloride ( $(CH_3)_3CCl$ ) in water follows $S_N1$ kinetics with rate constant $k = 1.386\times10^{-3}\ \text{s}^{-1}$ . If the initial concentration is $0.100\ \text{M}$ , the time required for $75\%$ hydrolysis is approximately:

(A)

$5.00\times10^{2}\ \text{s}$

(B)

$2.00\times10^{3}\ \text{s}$

(C)

$1.00\times10^{3}\ \text{s}$

(D)

$1.44\times10^{3}\ \text{s}$

Q7. Arrange the following halides in decreasing order of reactivity towards $S_N2$ with $I^-$ in acetone: benzyl chloride ( $C_6H_5CH_2Cl$ ), 2‑chlorobutane ( $CH_3CHClCH_2CH_3$ ), neopentyl chloride ( $(CH_3)_3CCH_2Cl$ ) and tert‑butyl chloride ( $(CH_3)_3CCl$ ).

(A)

$C_6H_5CH_2Cl \;>\; CH_3CHClCH_2CH_3 \;>\; (CH_3)_3CCH_2Cl \;>\; (CH_3)_3CCl$

(B)

$CH_3CHClCH_2CH_3 \;>\; C_6H_5CH_2Cl \;>\; (CH_3)_3CCH_2Cl \;>\; (CH_3)_3CCl$

(C)

$C_6H_5CH_2Cl \;>\; (CH_3)_3CCH_2Cl \;>\; CH_3CHClCH_2CH_3 \;>\; (CH_3)_3CCl$

(D)

$(CH_3)_3CCl \;>\; CH_3CHClCH_2CH_3 \;>\; (CH_3)_3CCH_2Cl \;>\; C_6H_5CH_2Cl$

Q8. Which of the following halobenzenes will undergo nucleophilic aromatic substitution by $OH^-$ most readily under mild conditions (e.g., moderate temperature, polar aprotic/aqueous solvent)?

(A)

Chlorobenzene ( $C_6H_5Cl$ )

(B)

1‑Chloro‑4‑nitrobenzene ( $p$ ‑Cl‑ $C_6H_4$ ‑ $NO_2$ )

(C)

1‑Chloro‑3‑nitrobenzene ( $m$ ‑Cl‑ $C_6H_4$ ‑ $NO_2$ )

(D)

1‑Chloro‑2,4‑dinitrobenzene ( $1$ ‑Cl‑ $2,4$ ‑ $(NO_2)_2C_6H_3$ )

Q9. Assertion (A): Aryl halides such as chlorobenzene ( $C_6H_5Cl$ ) do not undergo $S_N2$ reactions at the ring carbon under normal conditions. Reason (R): Halogen substituents in aromatic rings are ortho/para directing in electrophilic aromatic substitution due to their lone‑pair resonance donation (+R effect).

(A)

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

(B)

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

(C)

A is true but R is false.

(D)

A is false but R is true.

Q10. $m$ ‑Chloronitrobenzene ( $1$ ‑chloro‑3‑nitrobenzene) does not undergo nucleophilic substitution by $OH^-$ under mild aqueous conditions, yet when treated with molten $NaNH_2$ at $250^\circ\text{C}$ followed by acid workup it gives an amino‑substituted benzene. Which statement best explains this observation?

(A)

Under mild conditions the $m$ ‑nitro group cannot stabilise the negative charge in a Meisenheimer complex at the carbon bearing Cl, so addition–elimination ( $S_NAr$ ) is disfavoured; under forcing conditions ( $NaNH_2$ , high $T$ ) elimination–addition (benzyne) pathway operates: base abstracts an adjacent H to give benzyne which is trapped by $NH_2^-$ to give amination.

(B)

Under mild conditions $OH^-$ reduces the nitro group to a less activating species preventing substitution; at high temperature $NaNH_2$ first reduces the nitro to an amino group which then displaces chlorine intramolecularly.

(C)

The meta‑nitro group activates the ring for $S_NAr$ by −I effect so substitution should occur under mild conditions; high temperature simply accelerates the same addition–elimination route producing the observed product.

(D)

Under mild conditions chloride departs to form an aryl cation stabilized by the nitro group enabling substitution; at high temperature the reaction proceeds by a radical single‑electron transfer (SET) giving the amino product.

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