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Haloalkanes and Haloarenes Set-8 MCQ Online Practice Test | Class 12 | Quiz Tweek

Haloalkanes and Haloarenes Set-8

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

Haloalkanes and haloarenes are pivotal in organic chemistry because they connect structure (sterics/electronics) to reactivity through SN1/SN2/E1/E2 and SNAr/addition–elimination mechanisms. Board and competitive exams frequently test how ring activation (e.g., nitro groups) and solvent/nucleophile strength control product formation and stereochemistry, so mastering these patterns is essential for scoring confidently.

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Q1. Which of the following haloalkanes will react fastest with $CN^-$ in dry acetone at 25°C by an $S_N2$ mechanism?

(A)

$CH_3CHClCH_3$ (2-chloropropane)

(B)

$CH_3Cl$ (methyl chloride)

(C)

$(CH_3)_3CCl$ (tert‑butyl chloride)

(D)

$CH_3CH_2Cl$ (ethyl chloride)

Q2. Which of the following halobenzenes will undergo nucleophilic aromatic substitution most rapidly with aqueous $OH^-$ at 60°C (addition–elimination $S_NAr$ pathway)?

(A)

$C_6H_5Cl$ (chlorobenzene)

(B)

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

(C)

$1$ ‑chloro‑ $2$ ‑nitrobenzene ( $o$ ‑nitrochlorobenzene)

(D)

$1$ ‑chloro‑ $4$ ‑nitrobenzene ( $p$ ‑nitrochlorobenzene)

Q3. In the free radical chlorination of isobutane $(CH_3)_3CH$ the observed product ratio is tertiary chloride : primary chloride = $3:1$ . Using the relation

\frac{P_t}{P_p}=\frac{k_t\,n_t}{k_p\,n_p},

(where $n_t$ and $n_p$ are numbers of equivalent tertiary and primary H atoms), estimate the relative reactivity $\dfrac{k_t}{k_p}$ of a tertiary H to a primary H. ( $n_t=1,\;n_p=9$ for isobutane.)

(A)

$27$

(B)

$9$

(C)

$3$

(D)

$\dfrac{1}{3}$

Q4. Trans‑1‑bromo‑2‑methylcyclohexane is treated with $NaOCH_3$ in methanol under $S_N2$ conditions. What is the major stereochemical product?

(A)

Inversion at C‑1 giving trans‑1‑methoxy‑2‑methylcyclohexane with OMe axial

(B)

Retention at C‑1 giving trans‑1‑methoxy‑2‑methylcyclohexane with OMe equatorial

(C)

Inversion at C‑1 giving cis‑1‑methoxy‑2‑methylcyclohexane with OMe equatorial

(D)

Inversion at C‑1 giving cis‑1‑methoxy‑2‑methylcyclohexane with OMe axial

Q5. Which of the following dichloronitrobenzenes will undergo nucleophilic substitution by $CH_3O^-$ in methanol most rapidly (i.e., give fastest $S_NAr$ via Meisenheimer intermediate)?

(A)

$1$ ‑chloro‑ $3,5$ ‑dinitrobenzene

(B)

$1$ ‑chloro‑ $2,4$ ‑dinitrobenzene

(C)

$1$ ‑chloro‑ $2,6$ ‑dinitrobenzene

(D)

$1$ ‑chloro‑ $3,4$ ‑dinitrobenzene

Q6. In the SN2 reaction $CH_3CH_2Br + CN^- \rightarrow CH_3CH_2CN + Br^-$ the rate law is $r = k[CH_3CH_2Br][CN^-]$ . If initially $[CH_3CH_2Br]=0.02\ \mathrm{M}$ and $[CN^-]=0.04\ \mathrm{M}$ giving rate $r_0$ , what is the initial rate when $[CH_3CH_2Br]$ is doubled and $[CN^-]$ is reduced to $0.01\ \mathrm{M}$ ?

(A)

$2r_0$ (doubled)

(B)

$\tfrac{1}{2}r_0$ (halved)

(C)

$r_0$ (unchanged)

(D)

$4r_0$ (quadrupled)

Q7. Consider the alkyl bromides (i) $CH_3CH_2Br$ (1°), (ii) $(CH_3)_2CHBr$ (2°), (iii) $(CH_3)_3CBr$ (3°). Each reacts with $N_3^-$ at 25°C in (A) acetone (polar aprotic) and (B) ethanol (polar protic). On changing solvent from acetone to ethanol, how does the rate of nucleophilic substitution by $N_3^-$ change for these substrates?

(A)

Rate increases for all three substrates.

(B)

Rate decreases for all three substrates.

(C)

Rate decreases for (i) and (ii), but increases for (iii).

(D)

Rate decreases only for (i); increases for (ii) and (iii).

Q8. When $(R)$ -1-bromo-1-phenylethane, i.e. $(R)-C_6H_5CH(Br)CH_3$ , reacts with $NaCN$ (25°C) in (i) ethanol and (ii) DMSO, which stereochemical outcome is expected in each solvent and why?

(A)

In ethanol: nearly racemic product (SN1 predominates); in DMSO: predominantly inverted enantiomer (SN2 predominates).

(B)

In ethanol: predominantly inverted enantiomer (SN2); in DMSO: racemic product (SN1).

(C)

Both solvents give racemic product due to rapid benzylic carbocation formation in both media.

(D)

Both solvents give retention of configuration because the phenyl ring participates as a neighbouring group.

Q9. Which of the following best explains why $1$ -chloro- $2,4$ -dinitrobenzene undergoes nucleophilic aromatic substitution with methoxide ( $CH_3O^-$ ) at room temperature, whereas chlorobenzene requires very harsh conditions (e.g., $NaNH_2$ , high temperature) to substitute?

(A)

The C–Cl bond in the dinitro derivative is intrinsically much weaker than in chlorobenzene, so it breaks more easily.

(B)

Steric repulsion between the nitro groups and chlorine in the dinitro compound forces chlorine out, facilitating substitution.

(C)

Methoxide reacts via a radical chain with the dinitro compound but not with chlorobenzene.

(D)

The nitro groups strongly withdraw electron density and stabilize the negatively charged Meisenheimer (σ) intermediate formed on nucleophilic addition, lowering the activation barrier for the addition–elimination pathway.

Q10. For solvolysis in ethanol at $298\ \mathrm{K}$ , the activation energy for ionization of benzyl chloride is $15\ \mathrm{kJ\ mol^{-1}}$ lower than that for n-propyl chloride. Assuming identical pre‑exponential factors, estimate the ratio $k_{\text{benzyl}}/k_{\text{n-propyl}}$ at $298\ \mathrm{K}$ (use $R=8.314\ \mathrm{J\ mol^{-1}K^{-1}}$ ). Choose the nearest value.

(A)

$1.5\times 10^{1}$

(B)

$4.3\times 10^{2}$

(C)

$3.0\times 10^{3}$

(D)

$2.7\times 10^{5}$

Q11. Among the following haloalkanes, which will undergo nucleophilic substitution (SN2) most rapidly with hydroxide ion $OH^-$ in acetone at $298\,$ K?

(A)

$CH_3CH_2Br$ (1‑bromopropane)

(B)

$CH_3CH(Br)CH_3$ (2‑bromopropane)

(C)

$CH_3Br$ (methyl bromide)

(D)

$(CH_3)_3CBr$ (tert‑butyl bromide)

Q12. Which of the following will undergo nucleophilic aromatic substitution (addition–elimination, SNAr) most readily with methoxide $CH_3O^-$ in DMSO, taking into account resonance and inductive effects?

(A)

$p$ ‑NO $_2$ ‑C $_6$ H $_4$ Cl\ ( $p$ ‑nitrochlorobenzene)

(B)

$C_6H_5Cl$ \ (chlorobenzene)

(C)

$m$ ‑NO $_2$ ‑C $_6$ H $_4$ Cl\ ( $m$ ‑nitrochlorobenzene)

(D)

$p$ ‑OCH $_3$ ‑C $_6$ H $_4$ Cl\ ( $p$ ‑methoxychlorobenzene)

Q13. When $2$ ‑bromobutane ( $CH_3CHBrCH_2CH_3$ ) is treated with potassium tert‑butoxide ( $t$ ‑BuOK) in tert‑butanol at $60^\circ\mathrm{C}$ , which alkene will be formed predominantly?

(A)

trans‑2‑butene ( $trans$ ‑ $CH_3CH=CHCH_3$ )

(B)

cis‑2‑butene ( $cis$ ‑ $CH_3CH=CHCH_3$ )

(C)

an approximately equal mixture of cis‑ and trans‑2‑butene

(D)

1‑butene ( $CH_2=CHCH_2CH_3$ )

Q14. Consider the following two statements. Statement‑I: Chlorobenzene does not undergo nucleophilic substitution with aqueous $NaOH$ under mild conditions, but can be converted into aniline on treatment with $NaNH_2$ in liquid $NH_3$ followed by hydrolysis. Statement‑II: Under the $NaNH_2$ /liquid $NH_3$ conditions the reaction proceeds via elimination of $HCl$ to give a benzyne (aryne) intermediate, which is then attacked by nucleophile to give substitution. Which option is correct?

(A)

Statement‑I is true, Statement‑II is false

(B)

Statement‑I is true and Statement‑II is true, and Statement‑II correctly explains Statement‑I

(C)

Statement‑I is false, Statement‑II is true

(D)

Both Statement‑I and Statement‑II are false

Q15. Despite the stronger $C$ – $F$ bond, $p$ ‑nitrofluorobenzene ( $p$ ‑NO $_2$ ‑C $_6$ H $_4$ F) often reacts faster with methoxide $CH_3O^-$ (SNAr, addition–elimination) than $p$ ‑nitrochlorobenzene. Which of the following best explains this observation?

(A)

The $C$ – $F$ bond is weaker than the $C$ – $Cl$ bond, so $F^-$ departs more easily

(B)

Fluoride is a better nucleophile than chloride, which accelerates the substitution of the aryl halide

(C)

In SNAr the rate‑determining step is nucleophilic addition to form the Meisenheimer complex; the strong $-I$ effect of $F$ increases ring electron deficiency and better stabilizes the intermediate, so $p$ ‑nitrofluorobenzene reacts faster

(D)

Methoxide forms stronger hydrogen bonds with fluoride in the transition state, lowering the activation energy

...and 5 more challenging questions available in the interactive simulator.

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