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Amines Set-8 MCQ Online Practice Test | Class 12 | Quiz Tweek

Amines Set-8

Practice Important MCQs for Amines — Chemistry, Class 12.

Amines are among the most important nitrogen compounds in Class 12 Chemistry because they link fundamental concepts (basicity, resonance, inductive effects, solvation) with key reactions (diazotization, Hofmann elimination, Gabriel synthesis, and selective functional group interconversions). Board and competitive exams repeatedly test how substitution patterns and reaction conditions control basicity and product formation—so mastering these MCQs strengthens both conceptual accuracy and speed.

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Q1. Aniline ( $C_6H_5NH_2$ ) has $pK_a$ of its conjugate acid ( $C_6H_5NH_3^+$ ) equal to $4.6$ . In an aqueous solution of pH $2.0$ , what percentage of aniline exists in the protonated form $C_6H_5NH_3^+$ ? Use Henderson–Hasselbalch:

\mathrm{pH}=pK_a+\log\frac{[B]}{[BH^+]}

(A)

$99.0\%$

(B)

$99.75\%$

(C)

$97.5\%$

(D)

$90.0\%$

Q2. Exhaustive methylation of 2-aminobutane (sec-butylamine, $CH_3CH(NH_2)CH_2CH_3$ ) followed by treatment with $Ag_2O$ and heat (Hofmann elimination) yields which major alkene?

(A)

(E)-2-Butene ( $CH_3CH=CHCH_3$ , trans-2-butene)

(B)

(Z)-2-Butene ( $CH_3CH=CHCH_3$ , cis-2-butene)

(C)

A mixture in which 2-butene (both isomers) predominates

(D)

1-Butene ( $CH_2=CHCH_2CH_3$ )

Q3. Arrange the basicity in aqueous solution (strongest to weakest) for the following amines: $o$ -aminophenol, aniline ( $C_6H_5NH_2$ ), $p$ -aminophenol. Consider electronic (+R/–I) effects and intramolecular hydrogen bonding.

(A)

$p$ -aminophenol ;>; aniline ;>; $o$ -aminophenol

(B)

$o$ -aminophenol ;>; aniline ;>; $p$ -aminophenol

(C)

aniline ;>; $p$ -aminophenol ;>; $o$ -aminophenol

(D)

$p$ -aminophenol ;>; $o$ -aminophenol ;>; aniline

Q4. Statement A and R relate to electrophilic chlorination of aniline.
A: When aniline ( $C_6H_5NH_2$ ) is treated with $Cl_2$ in the presence of aqueous $NaOH$ , the major product is $2,4,6$ -trichloroaniline ( $2,4,6$ - $\mathrm{C_6H_2Cl_3NH_2}$ ).
R: The reaction proceeds via formation of the anilinium ion under these conditions, which directs chlorination to ortho and para positions.

(A)

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

(B)

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

(C)

A is true and R is false.

(D)

A is false and R is true.

Q5. For the series $NH_3$ , $CH_3NH_2$ , $(CH_3)_2NH$ , $(CH_3)_3N$ , which of the following correctly gives the order of basicity (strongest → weakest) in the gas phase and in aqueous solution, respectively? (Consider intrinsic proton affinity in gas phase and solvation/steric effects in water.)

(A)

Gas phase: $(CH_3)_3N > (CH_3)_2NH > CH_3NH_2 > NH_3$ ; Aqueous: $(CH_3)_2NH > CH_3NH_2 > (CH_3)_3N > NH_3$

(B)

Gas phase: $(CH_3)_2NH > (CH_3)_3N > CH_3NH_2 > NH_3$ ; Aqueous: $(CH_3)_3N > (CH_3)_2NH > CH_3NH_2 > NH_3$

(C)

Gas phase: $CH_3NH_2 > (CH_3)_2NH > (CH_3)_3N > NH_3$ ; Aqueous: $(CH_3)_2NH > (CH_3)_3N > CH_3NH_2 > NH_3$

(D)

Gas phase: $(CH_3)_3N > CH_3NH_2 > (CH_3)_2NH > NH_3$ ; Aqueous: $CH_3NH_2 > (CH_3)_2NH > (CH_3)_3N > NH_3$

Q6. The base dissociation constant $K_b$ for methylamine, $CH_3NH_2$ , is $4.4\times10^{-4}$ . What percentage of methylamine exists as its protonated form $CH_3NH_3^+$ in an aqueous solution at pH $9.00$ (25°C)?

(A)

$2.22\%$

(B)

$10.0\%$

(C)

$1.11\%$

(D)

$4.44\%$

Q7. Which of the following sequences converts aniline ( $C_6H_5NH_2$ ) to N-ethylaniline ( $C_6H_5NHCH_2CH_3$ ) with minimal formation of N,N-diethyl derivatives?

(A)

Direct alkylation: $C_6H_5NH_2 \xrightarrow{CH_3CH_2Br,\;K_2CO_3}$ (gives mixture of $C_6H_5NHCH_2CH_3$ and $C_6H_5N(CH_2CH_3)_2$ )

(B)

Eschweiler–Clarke type (formaldehyde + formic acid): $C_6H_5NH_2 \xrightarrow{HCHO,\;HCOOH} C_6H_5NHCH_3$ (gives methylation, not ethylation)

(C)

Gabriel-type approach starting from phthalimide derivatives on the aromatic ring to introduce ethyl at nitrogen (Gabriel method is not applicable to aromatic anilines for N‑ethylation)

(D)

Reductive amination: $C_6H_5NH_2 \xrightarrow{CH_3CHO} C_6H_5N=CHCH_3 \xrightarrow{NaBH_3CN} C_6H_5NHCH_2CH_3$

Q8. The basicity of the amino group in aminobenzoic acids is influenced by both the $-COOH$ group's resonance/inductive effects and intramolecular hydrogen bonding. Arrange the isomeric aminobenzoic acids in order of increasing basicity of the amino group in aqueous solution (least → most).

(A)

$p$ -aminobenzoic acid $<$ $m$ -aminobenzoic acid $<$ $o$ -aminobenzoic acid

(B)

$m$ -aminobenzoic acid $<$ $p$ -aminobenzoic acid $<$ $o$ -aminobenzoic acid

(C)

$o$ -aminobenzoic acid $<$ $p$ -aminobenzoic acid $<$ $m$ -aminobenzoic acid

(D)

$o$ -aminobenzoic acid $<$ $m$ -aminobenzoic acid $<$ $p$ -aminobenzoic acid

Q9. Tertiary amines (for example $N(CH_3)_3$ ) are often stronger bases than primary amines (for example $CH_3NH_2$ ) in the gas phase, but the reverse trend is commonly observed in aqueous solution. Which statement best explains this reversal?

(A)

In the gas phase intrinsic electron-donating effects (hyperconjugation and +I) of alkyl groups stabilize the protonated tertiary amine, so tertiary amines are more basic; in aqueous solution solvation/hydration stabilises smaller protonated species (primary ammonium ions) much more effectively, while steric bulk around tertiary ammonium ions hinders hydration — this reduces the apparent basicity of tertiary amines in water.

(B)

Alkyl groups withdraw electron density by induction in solution, so tertiary amines are intrinsically less able to accept a proton in water.

(C)

Tertiary amines form stronger hydrogen bonds with water in their unprotonated form, which lowers their tendency to be protonated.

(D)

Tertiary amines are more readily oxidised in aqueous media, and this oxidation lowers their basicity compared to primary amines.

Q10. Which of the following para-substituted anilines is the most basic in aqueous solution?

(A)

Aniline, $C_6H_5NH_2$

(B)

$p$ -Methoxyaniline, $p$ - $CH_3O\!-\!C_6H_4\!-\!NH_2$

(C)

$p$ -Aminobenzoic acid, $p$ - $NH_2\!-\!C_6H_4\!-\!COOH$

(D)

$p$ -Nitroaniline, $p$ - $NO_2\!-\!C_6H_4\!-\!NH_2$

Q11. Aniline ( $C_6H_5NH_2$ ) has $pK_b=9.40$ . What percentage of aniline molecules are present as the anilinium ion ( $C_6H_5NH_3^+$ ) at pH $5.40$ ? (Assume $pK_w=14.00$ and 25°C.)

(A)

$6.7\%$

(B)

$20.0\%$

(C)

$13.7\%$

(D)

$9.1\%$

Q12. An equimolar mixture of aniline ( $pK_b=9.40$ ) and $n$ -butylamine ( $pK_b=3.34$ ) is dissolved in diethyl ether. To selectively extract $n$ -butylamine into the aqueous layer such that $>99\%$ of $n$ -butylamine is protonated while $<5\%$ of aniline is protonated, which aqueous solution pH should be used?

(A)

pH $6.0$

(B)

pH $1.0$

(C)

pH $9.0$

(D)

pH $3.0$

Q13. Which of the following amines will react fastest with acetyl chloride ( $CH_3COCl$ ) in pyridine at 25°C to form the corresponding amide (or amidic product)?

(A)

$C_6H_5NH_2$ (aniline)

(B)

$C_6H_5N(CH_3)_2$ (N,N-dimethylaniline)

(C)

$C_6H_5NHCH_3$ (N-methylaniline)

(D)

$C_6H_5CH_2NH_2$ (benzylamine)

Q14. Assertion (A): Aniline ( $C_6H_5NH_2$ ) is a weaker base than methylamine ( $CH_3NH_2$ ) in aqueous solution.
Reason (R): The lone pair on nitrogen in aniline is delocalized into the benzene ring, and the anilinium ion is less stabilized by hydration than methylammonium; both effects lower aniline's basicity in water.

(A)

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

(B)

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

(C)

A is true but R is false.

(D)

A is false but R is true.

Q15. To prepare p-nitroaniline selectively from aniline ( $C_6H_5NH_2$ ), which sequence of steps is most appropriate?

(A)

Direct nitration of aniline with $HNO_3/H_2SO_4$ .

(B)

Acetylate aniline to give acetanilide (e.g. with $(CH_3CO)_2O$ ), nitrate the acetanilide with $HNO_3/H_2SO_4$ to give $p$ -nitroacetanilide, then hydrolyse to obtain $p$ -nitroaniline.

(C)

Diazotize aniline with $NaNO_2/HCl$ (0–5°C) to give the diazonium salt, convert to phenol ( $Cu_2O$ , $H_2O$ ), nitrate the phenol, then reduce the nitro group to an amine.

(D)

Sulfonate aniline with $SO_3/H_2SO_4$ to protect the amino group, nitrate, then desulfonate to yield $p$ -nitroaniline.

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

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