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Amines Set-7 MCQ Online Practice Test | Class 12 | Quiz Tweek

Amines Set-7

Practice Important MCQs for Amines — Chemistry, Class 12.

Amines are a high-yield topic because they connect structure with basicity, reactivity, and stereochemical outcomes. Board and competitive exams commonly test trends in basic strength (gas vs aqueous), resonance/inductive effects of substituents on aniline, and classic transformations such as diazotization and Gabriel/Hofmann reactions—so mastering these concepts directly improves scoring.

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Q1. A 0.10 M aqueous solution of aniline ( $C_6H_5NH_2$ ) has $K_b = 4.3\times10^{-10}$ . At pH $3.0$ , what percentage of aniline molecules are present as the protonated form $C_6H_5NH_3^+$ ? (Use $K_w=10^{-14}$ and assume activity ≈ concentration.)

(A)

2.3%

(B)

92.5%

(C)

97.7%

(D)

50.0%

Q2. Arrange the following amines in decreasing order of basicity in aqueous solution: (i) $CH_3NH_2$ , (ii) $C_6H_5CH_2NH_2$ (benzylamine), (iii) $C_6H_5NH_2$ (aniline), (iv) $NH_3$ . Which order is correct?

(A)

$CH_3NH_2 > C_6H_5CH_2NH_2 > NH_3 > C_6H_5NH_2$

(B)

$C_6H_5CH_2NH_2 > CH_3NH_2 > NH_3 > C_6H_5NH_2$

(C)

$NH_3 > CH_3NH_2 > C_6H_5CH_2NH_2 > C_6H_5NH_2$

(D)

$CH_3NH_2 > NH_3 > C_6H_5CH_2NH_2 > C_6H_5NH_2$

Q3. Consider these substituted anilines in aqueous solution: (A) $p$ -anisidine ( $p$ -methoxyaniline, $p$ - $OCH_3$ ), (B) aniline ( $C_6H_5NH_2$ ), (C) $m$ -nitroaniline, (D) $p$ -nitroaniline. Which of the following gives the correct order of decreasing basicity?

(A)

$p$ -anisidine $> m$ -nitroaniline $> aniline$ > p$-nitroaniline

(B)

aniline $> p$ -anisidine $> m$ -nitroaniline $> p$ -nitroaniline

(C)

$p$ -nitroaniline $> m$ -nitroaniline $>$ aniline $> p$ -anisidine

(D)

$p$ -anisidine $> aniline$ > m $-nitroaniline$ > p$-nitroaniline

Q4. For the series of amines $NH_3$ , $CH_3NH_2$ , $(CH_3)_2NH$ , $(CH_3)_3N$ , which one of the following pairs correctly gives the order of basicity in (i) gas phase and (ii) aqueous solution?

(A)

(i) $NH_3 > CH_3NH_2 > (CH_3)_2NH > (CH_3)_3N$ ; (ii) $(CH_3)_3N > (CH_3)_2NH > CH_3NH_2 > NH_3$

(B)

(i) $(CH_3)_3N > (CH_3)_2NH > CH_3NH_2 > NH_3$ ; (ii) $(CH_3)_2NH > CH_3NH_2 > (CH_3)_3N > NH_3$

(C)

(i) $(CH_3)_2NH > (CH_3)_3N > CH_3NH_2 > NH_3$ ; (ii) $CH_3NH_2 > (CH_3)_2NH > (CH_3)_3N > NH_3$

(D)

(i) $(CH_3)_3N > CH_3NH_2 > (CH_3)_2NH > NH_3$ ; (ii) $(CH_3)_3N > (CH_3)_2NH > CH_3NH_2 > NH_3$

Q5. The primary amine $X$ has structure $CH_3-CH(CH_3)-CH_2-CH_2-NH_2$ (2‑methylbutan‑1‑amine). Exhaustive methylation of $X$ followed by Ag_2O/heat (Hofmann elimination) affords a major alkene. Which alkene is expected as the major product?

(A)

$2$ ‑methyl‑2‑butene

(B)

$1$ ‑butene

(C)

$2$ ‑methyl‑1‑butene

(D)

$3$ ‑methyl‑1‑butene

Q6. The $pK_a$ of the conjugate acid of methylamine ( $CH_3NH_3^+$ ) is $10.64$ at $25^\circ\mathrm{C}$ . Using $K_w=1.0\times10^{-14}$ , the $K_b$ of methylamine ( $CH_3NH_2$ ) is approximately:

(A)

$2.29\times10^{-4}$

(B)

$4.37\times10^{-3}$

(C)

$4.37\times10^{-4}$

(D)

$2.29\times10^{-5}$

Q7. A $1.00\ \mathrm{L}$ aqueous solution contains $0.010\ \mathrm{mol}$ each of aniline ( $C_6H_5NH_2$ ) and methylamine ( $CH_3NH_2$ ). Given $pK_a(C_6H_5NH_3^+)=4.60$ and $pK_a(CH_3NH_3^+)=10.64$ , at $pH\;8.00$ which amine has the higher concentration of the unprotonated base $[B]$ and by approximately what factor (ratio $[B]_{aniline}/[B]_{methylamine}$ )?

(A)

$\text{Aniline by approximately }4.38\times10^{2}$

(B)

$\text{Methylamine by approximately }4.38\times10^{2}$

(C)

$\text{Aniline by approximately }2.3$

(D)

$\text{Methylamine by approximately }2.3\times10^{-3}$

Q8. Optically active (R)-1-phenylethylamine ( $\mathrm{PhCH(NH_2)CH_3}$ ) is treated with $NaNO_2/HCl$ at $0^\circ\mathrm{C}$ and then warmed; the isolated product is 1-phenylethanol which is racemic (no optical activity). Which explanation best accounts for this observation?

(A)

Reaction proceeds with retention of configuration via a concerted displacement by nitrite.

(B)

Reaction proceeds by an $S_N2$ displacement at the stereocentre giving inversion of configuration.

(C)

Nitrosation produces a stable aliphatic diazonium that undergoes internal return preserving configuration.

(D)

The aliphatic diazonium decomposes with loss of $N_2$ to a planar carbocation; water attacks from either face giving racemization.

Q9. A diethyl ether solution contains equal amounts of three amines: methylamine ( $CH_3NH_2$ , $pK_a=10.64$ ), aniline ( $C_6H_5NH_2$ , $pK_a=4.60$ ) and $p$ -nitroaniline ( $p$ -NO $_2$ –C $_6$ H $_4$ –NH $_2$ , $pK_a=1.00$ ). Which sequence of aqueous extractions (with indicated pH) will selectively separate them so that methylamine is removed first, then aniline, leaving $p$ -nitroaniline in the organic layer?

(A)

Extract with $1\ \mathrm{M}$ HCl ( $pH\approx0$ ) to remove methylamine; then basify and extract again to separate aniline.

(B)

Wash the ether layer with water at $pH\approx7$ to extract methylamine as its ammonium salt; then acidify the remaining organic layer to $pH\approx2.5$ to extract aniline as its ammonium salt (leaving $p$ -nitroaniline in ether); finally basify the aqueous extracts to recover the amines.

(C)

First extract with aqueous solution at $pH\approx2$ to remove $p$ -nitroaniline, then extract the remaining organic layer with $1\ \mathrm{M}$ NaOH to remove aniline.

(D)

Wash with brine, then extract with $1\ \mathrm{M}$ NaOH to selectively protonate aniline and methylamine.

Q10. Assertion (A): In the gas phase the basicity of amines increases with alkyl substitution (i.e. tertiary $>$ secondary $>$ primary $>$ ammonia), but in aqueous solution secondary amines are generally more basic than tertiary amines. Reason (R): In the gas phase intrinsic electronic effects (electron-donating alkyl groups) dominate; in aqueous solution solvation stabilizes protonated primary and secondary ammonium ions by hydrogen bonding, while tertiary ammonium ions are sterically hindered and less well solvated.

(A)

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

(B)

(A) is true but (R) is false.

(C)

Both (A) and (R) are true, and (R) correctly explains (A).

(D)

(A) is false but (R) is true.

Q11. Given methylamine $\mathrm{CH_3NH_2}$ with $pK_b=3.36$ at $25^\circ\mathrm{C}$ , what percentage of methylamine exists as its conjugate acid $\mathrm{CH_3NH_3^+}$ in a solution of pH $10.00$ ? (Assume $pK_w=14.00$ .)

(A)

$18.7\%$

(B)

$61.7\%$

(C)

$81.3\%$

(D)

$50.0\%$

Q12. Which sequence of steps will give predominantly p-nitroaniline from aniline ( $\mathrm{C_6H_5NH_2}$ ) while minimizing oxidation and ortho/para mixture?

(A)

Direct nitration of aniline with $\mathrm{HNO_3/H_2SO_4}$ (no protection), then isolate product

(B)

Acetylation of aniline with $\mathrm{(CH_3CO)_2O}$ (→ acetanilide) $\rightarrow$ Nitration with $\mathrm{HNO_3/H_2SO_4}$ (gives mainly p-nitroacetanilide) $\rightarrow$ Hydrolysis of the acetamide to yield p-nitroaniline

(C)

Diazotization of aniline ( $\mathrm{NaNO_2/HCl,\ 0{-}5^\circ C}$ ) followed by a Sandmeyer-type procedure to introduce $\mathrm{NO_2}$

(D)

First form aniline hydrochloride (treat with HCl) then carry out nitration and finally basify

Q13. Four amines A–D have $pK_b$ values at $25^\circ\mathrm{C}$ as follows: A: $3.36$ , B: $9.40$ , C: $4.00$ , D: $7.00$ . In a solution at pH $7.00$ , which of these amines will exist more than $90\%$ as their protonated forms ( $BH^+$ )? (Assume $pK_w=14.00$ .)

(A)

Only A and C

(B)

Only A

(C)

A, C and D

(D)

All four (A, B, C and D)

Q14. A solution contains $0.100\ \mathrm{M}$ aniline and $0.300\ \mathrm{M}$ methylamine. At pH $9.00$ , calculate the ratio $\dfrac{[\mathrm{Anilinium^+}]}{[\mathrm{Methylammonium^+}]}$ . Given $pK_b(\text{aniline})=9.40$ and $pK_b(\text{methylamine})=3.36$ at $25^\circ\mathrm{C}$ (assume $pK_w=14.00$ ).

(A)

$1.36\times 10^{-3}$

(B)

$1.36\times 10^{-7}$

(C)

$2.72\times 10^{-5}$

(D)

$1.36\times 10^{-5}$

Q15. Assertion (A): Primary aromatic amines form isolable diazonium salts on treatment with $\mathrm{NaNO_2/HCl}$ at $0{-}5^\circ\mathrm{C}$ , whereas primary aliphatic amines do not give isolable diazonium salts under these conditions.
Reason (R): Aryldiazonium ions are resonance-stabilized by the aromatic ring, but alkyldiazonium ions lack such resonance stabilization and readily decompose (e.g. $\mathrm{R{-}N_2^+ \rightarrow R^+ + N_2}$ ), the resulting carbocations being trapped by nucleophiles (such as water) to give alcohols.

(A)

A is true but R is false

(B)

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

(C)

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

(D)

A is false but R is true

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