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Haloalkanes and Haloarenes Set-6

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

“Haloalkanes and Haloarenes” is a core Class 12 Chemistry chapter because it connects structure (sterics, inductive/resonance effects), mechanism (SN1, SN2, E1, E2, SNAr), and stereochemistry (inversion/retention) with reactivity trends. These ideas directly appear in board exams and competitive tests, especially through rate/major-product questions and assertion–reason problems.

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Q1. For the bimolecular nucleophilic substitution (SN2) between $CH_3Br$ and $CN^-$ in acetone the rate law is $rate = k[CH_3Br][CN^-]$ . If $k = 2.5\times10^{-2}\ \mathrm{L\,mol^{-1}s^{-1}}$ , and initial concentrations are $[CH_3Br]=0.10\ \mathrm{M}$ and $[CN^-]=0.20\ \mathrm{M}$ , the initial rate (in $\mathrm{mol\,L^{-1}s^{-1}}$ ) is:

(A)

$1.0\times10^{-3}\ \mathrm{mol\,L^{-1}s^{-1}}$

(B)

$2.5\times10^{-4}\ \mathrm{mol\,L^{-1}s^{-1}}$

(C)

$5.0\times10^{-4}\ \mathrm{mol\,L^{-1}s^{-1}}$

(D)

$5.0\times10^{-5}\ \mathrm{mol\,L^{-1}s^{-1}}$

Q2. Which haloarene will undergo nucleophilic aromatic substitution (addition–elimination via a Meisenheimer complex) most readily with methoxide ( $CH_3O^-$ ) in methanol at room temperature?

(A)

$p$ -Nitrochlorobenzene

(B)

Chlorobenzene

(C)

$m$ -Nitrochlorobenzene

(D)

$o$ -Nitrochlorobenzene

Q3. When 3-bromo-2-methylbutane ( $CH_3\!-\!CH(CH_3)\!-\!CH(Br)\!-\!CH_3$ ) is hydrolysed under conditions that favour an SN1 pathway (aqueous ethanol, mild acid), which alcohol will be produced predominantly?

(A)

$CH_3\!-\!CH(CH_3)\!-\!CH(OH)\!-\!CH_3$ (3-hydroxy-2-methylbutane)

(B)

$CH_3\!-\!CH_2\!-\!CH(OH)\!-\!CH_2\!-\!CH_3$ (3-pentanol)

(C)

$CH_3\!-\!CH_2\!-\!CH_2\!-\!CH(OH)\!-\!CH_3$ (2-pentanol)

(D)

$CH_3\!-\!C(OH)(CH_3)\!-\!CH_2\!-\!CH_3$ (2-methyl-2-butanol)

Q4. Neopentyl chloride ( $(CH_3)_3CCH_2Cl$ ) is unusually resistant to both SN1 and SN2 reactions. Which statement best explains this behaviour?

(A)

The C–Cl bond is unusually strong due to conjugation with the adjacent quaternary carbon.

(B)

Steric hindrance around the primary carbon blocks backside attack (SN2) and ionization to give a stable carbocation (SN1) is disfavoured, so both pathways are slow.

(C)

The three methyl groups donate electron density making the carbon too electron-rich for nucleophilic attack.

(D)

Neopentyl chloride rapidly undergoes elimination to give an alkene, so substitution pathways are not observed.

Q5. Arrange the following haloarenes in decreasing order of rate of nucleophilic aromatic substitution by methoxide ( $CH_3O^-$ ) at 25°C via the addition–elimination (SNAr) pathway: (i) $1$ -chloro- $2,4$ -dinitrobenzene, (ii) chlorobenzene, (iii) $2$ -chloronitrobenzene.

(A)

(ii) $>$ (iii) $>$ (i)

(B)

(iii) $>$ (i) $>$ (ii)

(C)

(i) $>$ (iii) $>$ (ii)

(D)

(i) $>$ (ii) $>$ (iii)

Q6. For the SN2 reaction between $CH_3CH_2CH_2Br$ and $CN^-$ the rate law is rate = $k[CH_3CH_2CH_2Br][CN^-]$ . If initial concentrations are $[CH_3CH_2CH_2Br]=0.100\,$ M and $[CN^-]=0.200\,$ M $giving initial rate R$ , what will be the initial rate when $[CN^-]$ is increased to $0.600\,$ M while substrate concentration is unchanged?

(A)

$\dfrac{R}{3}$

(B)

$3R$

(C)

$9R$

(D)

$R$

Q7. Which of the following aryl halides will undergo nucleophilic aromatic substitution with aqueous $NaOH$ under mild conditions by the addition–elimination (Meisenheimer) mechanism?

(A)

$C_6H_5Cl$ (chlorobenzene)

(B)

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

(C)

$1$ -chloro- $4$ -methoxybenzene ( $p$ -methoxychlorobenzene)

(D)

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

Q8. 2-Bromobutane ( $CH_3CHBrCH_2CH_3$ ) is treated separately with (i) $KCN$ in acetone at $25^\circ$ C and (ii) $t$ -BuOK in tert-butanol at $25^\circ$ C. Identify the major organic product in each case.

(A)

(i) $CH_3CH(CN)CH_2CH_3$ (nucleophilic substitution via $SN2$ ); (ii) predominantly trans-2-butene ( $CH_3CH=CHCH_3$ , $trans$ isomer major) via $E2$

(B)

(i) racemic $CH_3CH(CN)CH_2CH_3$ (via $SN2$ ); (ii) predominantly 1-butene ( $CH_2=CHCH_2CH_3$ ) (Hofmann product)

(C)

(i) $CH_3CH(CN)CH_2CH_3$ with retention of configuration; (ii) predominantly cis-2-butene

(D)

(i) elimination to give 2-butene; (ii) nucleophilic substitution to give $CH_3CH(OC(CH_3)_3)CH_2CH_3$ (tert-butyl ether)

Q9. An optically pure sample (100% enantiomeric excess) of a chiral secondary alkyl bromide A is reacted with $NaN_3$ in acetone to give azide B. The observed specific rotation of B is only 40% of that expected for an optically pure enantiomer. Assuming substitution occurs either by $SN2$ (complete inversion) or by $SN1$ (complete racemisation), what percentage of product formation proceeded via $SN2$ ?

(A)

$60\%$ via $SN2$ and $40\%$ via $SN1$

(B)

$20\%$ via $SN2$ and $80\%$ via $SN1$

(C)

$40\%$ via $SN2$ and $60\%$ via $SN1$

(D)

$80\%$ via $SN2$ and $20\%$ via $SN1$

Q10. Which stereoisomer of 1-bromo-2-methylcyclohexane undergoes $E2$ elimination more readily on treatment with a strong base such as $t$ -BuOK to give 1-methylcyclohexene, and why?

(A)

The cis isomer, because both substituents can adopt axial positions in the reactive conformation facilitating anti-periplanar elimination

(B)

The trans isomer, because in its favored conformation the bromine is axial and is anti-periplanar to an axial $\beta$ -hydrogen, allowing facile $E2$

(C)

Both isomers react equally because a ring flip can always produce the required anti-periplanar arrangement

(D)

Neither reacts readily because an equatorial C–Br bond is required for better leaving-group orientation in elimination

Q11. Which of the following alkyl halides will show the highest rate of bimolecular nucleophilic substitution (SN2) with azide ion ( $\mathrm{N_3^-}$ ) in acetone at room temperature?

(A)

2-bromopropane ( $\mathrm{CH_3CHBrCH_3}$ )

(B)

1-bromobutane ( $\mathrm{CH_3CH_2CH_2CH_2Br}$ )

(C)

bromobenzene ( $\mathrm{C_6H_5Br}$ )

(D)

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

Q12. Which of the following aryl halides will undergo nucleophilic aromatic substitution (addition–elimination) with methoxide ( $\mathrm{CH_3O^-}$ ) most readily at $25^\circ\mathrm{C}$ ?

(A)

chlorobenzene ( $\mathrm{C_6H_5Cl}$ )

(B)

p-chloronitrobenzene ( $p$ - $\mathrm{ClC_6H_4NO_2}$ )

(C)

o-chlorotoluene ( $o$ - $\mathrm{ClC_6H_4CH_3}$ )

(D)

2,4-dinitrochlorobenzene ( $2,4$ - $\mathrm{(NO_2)_2C_6H_3Cl}$ )

Q13. When equal molar quantities of ethyl bromide and n-propyl bromide are subjected to Wurtz coupling (sodium, dry ether), which hydrocarbon will be formed in the largest proportion?

(A)

n-pentane ( $\mathrm{C_5H_{12}}$ )

(B)

n-butane ( $\mathrm{C_4H_{10}}$ )

(C)

n-hexane ( $\mathrm{C_6H_{14}}$ )

(D)

All three in equal amounts

Q14. Assertion (A): $1$ -fluoro- $2$ -nitrobenzene reacts with methoxide ( $\mathrm{CH_3O^-}$ ) faster than $1$ -chloro- $2$ -nitrobenzene under identical conditions.
Reason (R): In nucleophilic aromatic substitution via a Meisenheimer (σ) complex the reactivity of halogens often follows $\mathrm{F}>\mathrm{Cl}>\mathrm{Br}$ because the strong electron-withdrawing character of fluorine stabilizes the σ-complex despite fluorine being a poor leaving group in aliphatic substitutions.

(A)

A is true but R is false.

(B)

A is false but R is true.

(C)

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

(D)

Both A and R are true but R is not the correct explanation of A.

Q15. Assertion (A): Solvolysis of $(R)$ -1-phenylethyl bromide ( $\mathrm{Ph\!-\!CH(Br)\!-\!CH_3}$ ) in ethanol proceeds with substantial retention of configuration at the benzylic centre.
Reason (R): The neighbouring phenyl ring can participate to form a bridged "phenonium" ion intermediate; nucleophilic attack on this bridged intermediate occurs predominantly from the same face, resulting in net retention.

(A)

Both A and R are true but R is not the correct explanation of A.

(B)

Both A and R are true and R is 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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