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Amines Set-4 MCQ Online Practice Test | Class 12 | Quiz Tweek

Amines Set-4

Practice Important MCQs for Amines — Chemistry, Class 12.

The chapter “Amines” is crucial because it links fundamental concepts of structure, basicity, and reaction mechanisms with high-scoring board and competitive exam problems. Questions from this chapter commonly test how pK values relate, how substituents and resonance/inductive/steric effects control basicity and orientation in reactions, and how amines behave under diazotization conditions. Strong mastery here improves accuracy in both numerical and reasoning-based MCQs.

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Q1. A 1.0 L aqueous solution contains $0.10\ \mathrm{M}$ methylamine ( $CH_3NH_2$ ). To this solution $0.020\ \mathrm{mol}$ of HCl is added. Given $pK_b(CH_3NH_2)=3.34$ , calculate the pH of the resulting solution (assume volumes additive and neglect activity coefficients).

(A)

$11.90$

(B)

$10.80$

(C)

$11.26$

(D)

$12.00$

Q2. Arrange the following amines in order of decreasing basicity in aqueous solution: (i) $C_3H_7NH_2$ (n‑propylamine), (ii) $C_6H_5NH_2$ (aniline), (iii) $(C_2H_5)_2NH$ (diethylamine), (iv) $(CH_3)_3N$ (trimethylamine).

(A)

$(CH_3)_3N > (C_2H_5)_2NH > C_3H_7NH_2 > C_6H_5NH_2$

(B)

$(C_2H_5)_2NH > C_3H_7NH_2 > (CH_3)_3N > C_6H_5NH_2$

(C)

$C_3H_7NH_2 > (CH_3)_3N > (C_2H_5)_2NH > C_6H_5NH_2$

(D)

$(CH_3)_3N > C_3H_7NH_2 > (C_2H_5)_2NH > C_6H_5NH_2$

Q3. When aniline ( $C_6H_5NH_2$ ) is nitrated using a mixture of concentrated $HNO_3$ and $H_2SO_4$ (without protecting the –NH $_2$ ), which is the major nitro-substituted product formed?

(A)

o‑Nitroaniline ( $2$ ‑nitroaniline)

(B)

p‑Nitroaniline ( $4$ ‑nitroaniline)

(C)

m‑Nitroaniline ( $3$ ‑nitroaniline)

(D)

2,4‑Dinitroaniline

Q4. Statement I: N,N‑Dimethylaniline is a weaker base than aniline in aqueous solution at 25°C.
Statement II: This is mainly because steric hindrance from the two methyl groups in N,N‑dimethylaniline reduces solvation/stabilization of its protonated form more than the +I effect of the methyl groups increases intrinsic basicity.
Which of the following is correct?

(A)

Both Statement I and Statement II are true, and Statement II correctly explains Statement I.

(B)

Both Statement I and Statement II are true, but Statement II does not correctly explain Statement I.

(C)

Statement I is true but Statement II is false.

(D)

Statement I is false and Statement II is true.

Q5. To synthesize $p$ ‑bromoaniline ( $4$ ‑bromoaniline) from aniline ( $C_6H_5NH_2$ ) with para‑selectivity, which of the following minimal sequences of steps is most appropriate?

(A)

Direct bromination of aniline with $Br_2$ (aq) at room temperature.

(B)

Acetylate aniline to acetanilide (e.g. with $(CH_3CO)_2O$ or $CH_3COCl$ ), brominate the acetanilide with $Br_2$ to give $p$ ‑bromoacetanilide, then hydrolyze the amide to obtain $p$ ‑bromoaniline.

(C)

Acetylate aniline to acetanilide, brominate the acetanilide to give $o$ ‑bromoacetanilide, then hydrolyze the amide to obtain $o$ ‑bromoaniline.

(D)

Protonate aniline to its anilinium salt (HCl) and then brominate with $Br_2$ (aq) to obtain $p$ ‑bromoaniline.

Q6. If aniline has $pK_b = 9.40$ at $25^\circ\text{C}$ , what is the $pK_a$ of its conjugate acid $\,\mathrm{C_6H_5NH_3^+}\,$ ? (Use

pK_a + pK_b = 14.00

(A)

$5.60$

(B)

$9.40$

(C)

$4.60$

(D)

$14.00$

Q7. The following reaction sequence is carried out:

\mathrm{C_6H_5NH_2} \xrightarrow{(i)\ CH_3COCl,\ \text{pyridine}} \xrightarrow{(ii)\ Br_2} \xrightarrow{(iii)\ H_3O^+,\ \Delta}

What is the major final organic product?

(A)

$p\text{-}\mathrm{BrC_6H_4NH_2}$

(B)

$o\text{-}\mathrm{BrC_6H_4NH_2}$

(C)

$2,4\text{-}\mathrm{Br_2C_6H_3NH_2}$

(D)

$p\text{-}\mathrm{BrC_6H_4NHCOCH_3}$

Q8. Arrange the isomeric nitroanilines in order of increasing basicity in aqueous solution:
$\;o\text{-}\mathrm{NO_2C_6H_4NH_2},\; m\text{-}\mathrm{NO_2C_6H_4NH_2},\; p\text{-}\mathrm{NO_2C_6H_4NH_2}.$

(A)

$m\text{-}\mathrm{NO_2C_6H_4NH_2} < p\text{-}\mathrm{NO_2C_6H_4NH_2} < o\text{-}\mathrm{NO_2C_6H_4NH_2}$

(B)

$p\text{-}\mathrm{NO_2C_6H_4NH_2} < o\text{-}\mathrm{NO_2C_6H_4NH_2} < m\text{-}\mathrm{NO_2C_6H_4NH_2}$

(C)

$m\text{-}\mathrm{NO_2C_6H_4NH_2} < o\text{-}\mathrm{NO_2C_6H_4NH_2} < p\text{-}\mathrm{NO_2C_6H_4NH_2}$

(D)

$o\text{-}\mathrm{NO_2C_6H_4NH_2} < p\text{-}\mathrm{NO_2C_6H_4NH_2} < m\text{-}\mathrm{NO_2C_6H_4NH_2}$

Q9. Which pair of amines (X, Y) shows that X is more basic than Y in the gas phase but X is less basic than Y in aqueous solution at $25^\circ\text{C}$ ?

(A)

$\mathrm{C_6H_5NH_2,\; C_6H_5CH_2NH_2}$

(B)

$(CH_3)_3N,\; CH_3NH_2$

(C)

$(CH_3)_2NH,\; (CH_3)_3N$

(D)

$CH_3NH_2,\; (CH_3)_2NH$

Q10. Which of the following amines will be converted most readily into the corresponding diazonium salt by treatment with $\mathrm{NaNO_2}/\mathrm{HCl}$ at $0$ – $5^\circ\text{C}$ (i.e., which forms the diazonium most easily)?

(A)

$p\text{-}\mathrm{CH_3OC_6H_4NH_2}$ (p-methoxyaniline)

(B)

$\mathrm{C_6H_5NH_2}$ (aniline)

(C)

$\mathrm{C_6H_5CH_2NH_2}$ (benzylamine)

(D)

$p\text{-}\mathrm{NO_2C_6H_4NH_2}$ (p-nitroaniline)

Q11. A 0.100 M aqueous solution of methylamine ( $CH_3NH_2$ ) at 25°C has $pK_b = 3.35$ . Using the approximation $x \ll 0.100$ , estimate the percentage of methylamine present as its protonated form $CH_3NH_3^+$ .

(A)

0.67%

(B)

6.7%

(C)

43%

(D)

25%

Q12. To prepare p-nitroaniline ( $p$ - $NO_2$ - $C_6H_4$ - $NH_2$ ) from benzene with minimum side products, which of the following sequences of steps and reagents is the most appropriate?

(A)

Benzene —(HNO_3/H_2SO_4)→ nitrobenzene —(Sn/HCl)→ aniline —(HNO_3/H_2SO_4)→ nitration of aniline (gives mixture)

(B)

Benzene —(Cl_2/FeCl_3)→ chlorobenzene —(NH_3, high T, high P)→ aniline —(HNO_3/H_2SO_4)→ nitroaniline

(C)

Benzene —(HNO_3/H_2SO_4)→ nitrobenzene —(HNO_3/H_2SO_4)→ dinitrobenzene —(H_2/Pd)→ mixture of aminonitrobenzenes

(D)

Benzene —(HNO_3/H_2SO_4)→ nitrobenzene —(Fe/HCl or H_2/Pd)→ aniline —(Ac_2O or CH_3COCl)→ acetanilide —(HNO_3/H_2SO_4)→ p-nitroacetanilide —(H_3O^+)→ p-nitroaniline

Q13. Considering inductive (+I) effects and solvation of protonated species in water, which of the following amines is the most basic in aqueous solution?

(A)

$(C_2H_5)_2NH$ (diethylamine)

(B)

$(C_2H_5)_3N$ (triethylamine)

(C)

$C_2H_5NH_2$ (ethylamine)

(D)

$NH_3$ (ammonia)

Q14. Arrange the isomeric aminobenzoic acids in order of decreasing basicity (availability of the –NH_2 lone pair for protonation) of the amino group in aqueous solution: 2-aminobenzoic acid (o-), 3-aminobenzoic acid (m-), 4-aminobenzoic acid (p-). Which order is correct?

(A)

o- > m- > p-

(B)

p- > o- > m-

(C)

m- > o- > p-

(D)

o- > p- > m-

Q15. A primary aliphatic amine X on treatment with NaNO_2/HCl at 0–5°C followed by warming gives 2-butanol as the major product. Which of the following is X?

(A)

$CH_3CH_2CH_2CH_2NH_2$ (1-aminobutane)

(B)

$CH_3CH(NH_2)CH_2CH_3$ (2-aminobutane)

(C)

$(CH_3)_3CNH_2$ (tert-butylamine)

(D)

$CH_3CH_2CH(CH_3)NH_2$ (2-methyl-1-propanamine, isobutylamine)

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