Amines Set-3

Q1. (At pH 7.4, what percentage of methylamine $CH_3NH_2$ (given $pK_b = 3.34$) exists as its protonated form $CH_3NH_3^+$? (Use $pK_a + pK_b = 14$ and $pH = pK_a + \log\frac{[\text{base}]}{[\text{acid}]}$).)

Q2. (Arrange the following amines in decreasing order of their initial rate of reaction with methyl iodide $CH_3I$ (nucleophilic methylation) under identical conditions: $CH_3NH_2$ (methylamine), $(CH_3)_2NH$ (dimethylamine), $(CH_3)_3N$ (trimethylamine).)

Q3. (Three amines X, Y and Z were treated with benzenesulfonyl chloride ($C_6H_5SO_2Cl$, Hinsberg reagent) and then shaken with aqueous NaOH. Observations: X dissolves in NaOH and on acidification a white solid precipitates; Y gives a sulfonyl derivative that is insoluble in NaOH (no change on acidification); Z does not give a sulfonyl derivative but on addition of HCl the amine dissolves as a water‑soluble salt. Which assignment is correct?)

Q4. (Assertion (A): Tertiary amines readily form diazonium salts on treatment with $NaNO_2$ and $HCl$ at $0$–$5^\circ$C.
Reason (R): Formation of diazonium salts requires an $-NH_2$ group (a primary amine); tertiary amines lack an N–H bond and hence cannot form diazonium ions.)

Q5. (At 25°C in aqueous solution, $(CH_3)_2NH$ is observed to be a stronger base than $(CH_3)_3N$. Which of the following best explains this observation?)

Q6. A solution contains equimolar amounts of the following amines. At pH 7.0 which species will be present predominantly in its protonated form? (Given conjugate-acid $pK_a$ values: methylammonium $=10.64$, anilinium $=4.60$, ammonium $=9.25$, $p$-nitroanilinium $=1.00$)

Q7. Arrange the following amines in decreasing order of basicity (most → least) in (i) the gas phase and (ii) aqueous solution: $NH_3,\; CH_3NH_2,\; (CH_3)_2NH,\; (CH_3)_3N$.

Q8. Which of the following reaction sequences will convert aniline ($C_6H_5NH_2$) into $p$‑nitroaniline ($p$‑NO$_2$‑$C_6H_4$NH$_2$) as the major product with minimal ortho substitution?

Q9. Considering both electronic (+I/hyperconjugation) effects and steric/solvation effects, which of the following gives the correct order of basicity (most basic → least) in aqueous solution?
Aniline ($C_6H_5NH_2$), o‑methylaniline (2‑CH$_3$‑$C_6H_4$NH$_2$), p‑methylaniline (4‑CH$_3$‑$C_6H_4$NH$_2$)

Q10. A mixture containing aniline ($C_6H_5NH_2,\; pK_a(\text{anilinium})=4.60$), benzylamine ($C_6H_5CH_2NH_2,\; pK_a(\text{benzylammonium})=9.34$) and benzoic acid ($C_6H_5COOH,\; pK_a=4.20$) is dissolved in diethyl ether. The solution is first shaken with 5% aqueous NaHCO$_3$ and the aqueous layer is set aside. The separated organic layer is then shaken with a buffered aqueous solution at pH 7.0. Which compounds will be found in (i) the first aqueous extract and (ii) the second aqueous extract, respectively?