Amines – JEE Main ChemistryPractice Questions & PYQs

The basicity of amines in an aqueous solution depends on the electron density available on the nitrogen atom and the steric accessibility of the lone pair for bonding with a proton. Aromatic amines, such as aniline, exhibit much lower basicity because the phenyl group exerts an $-I$ effect that reduces electron density on the nitrogen atom. When comparing aliphatic amines, the order of basic strength is $2^{\circ} > 1^{\circ} > 3^{\circ}$, where $3^{\circ}$ amines are less basic than expected due to steric crowding that hinders proton approach, making the $2^{\circ}$ dimethylamine the strongest base among these choices.

The reaction of ethylamine with chloroform in the presence of potassium hydroxide is a classic carbylamine reaction, which serves as a chemical test to detect primary amines. This transformation results in the formation of ethyl isocyanide, accompanied by potassium chloride and water as byproducts. Based on the balanced stoichiometry of the reaction represented by
$\mathrm{CH_3CH_2NH_2} + \mathrm{CHCl_3} + 3\,\mathrm{KOH} \rightarrow \mathrm{C_2H_5NC} + 3\,\mathrm{KCl} + 3\,\mathrm{H_2O}$
the compound (A) is $\mathrm{C_2H_5NC}$ and the compound (B) is $3\,\mathrm{KCl}$.

The empirical formula of the compound is determined by converting the percentage composition of each element into its relative molar proportion. Dividing the given percentages by their respective atomic masses yields values of 1.66 for carbon (20/12), 6.67 for hydrogen (6.67/1), 3.33 for nitrogen (46.67/14), and 1.66 for oxygen (26.64/16). When these values are divided by the smallest result, 1.66, we obtain a simple integer ratio of 1:4:2:1 for C, H, N, and O, respectively, which identifies the empirical formula as $\mathrm{CH}_4\mathrm{N}_2\mathrm{O}$. Since the molecular mass of 60 equals the formula mass of this empirical unit, the molecular formula is identical, identifying the compound as urea, $\mathrm{NH}_2\mathrm{CONH}_2$.

Heating urea causes it to undergo thermal decomposition, during which it releases ammonia gas and leaves behind a solid residue of biuret, represented by the formula $\mathrm{NH}_2\mathrm{CONHCONH}_2$. Biuret contains peptide-like $-\mathrm{CONH}-$ linkages, which are detected by the biuret test. This test involves the formation of a violet-colored complex when the residue reacts with an alkaline copper sulfate solution, confirming the structural identity of the original compound.

The Wurtz reaction is designed exclusively for the synthesis of higher hydrocarbons through the coupling of alkyl halides in the presence of sodium metal, following the chemical equation $RX + 2Na + XR \longrightarrow R-R + 2NaX$. Since this transformation is strictly a method for carbon-carbon bond formation to produce alkanes, it possesses no role in the manipulation or synthesis of nitrogen-containing amine functional groups. In contrast, the Hinsberg, Hofmann, and Curtius reactions are all specialized procedures specifically utilized for the synthesis, separation, or structural identification of various amine species.

The basicity of nitrogen-containing compounds is governed by the availability of the lone pair on the nitrogen atom to act as a proton acceptor. In benzylamine ($C_6H_5CH_2NH_2$), the lone pair is localized and fully available because it is separated from the benzene ring by a methylene group, whereas in aniline, the lone pair is delocalized into the aromatic ring via resonance, thereby reducing its basic character. Compounds such as acetanilide and p-nitroaniline feature electron-withdrawing groups or additional conjugation that further decrease the electron density at the nitrogen site, confirming benzylamine as the most basic compound.

The acid-catalyzed hydrolysis of isocyanides, such as ethyl isocyanide, involves the cleavage of the bond between the nitrogen atom and the terminal carbon atom. As water interacts with the molecule, the ethyl group remains attached to the nitrogen to yield a primary amine, while the isocyanide carbon is oxidized to form methanoic acid. Because the reaction occurs in an acidic medium, the resulting ethylamine is protonated to form an ethylamine salt, resulting in a final mixture containing an ethylamine salt and methanoic acid according to the reaction $\mathrm{CH}_3\mathrm{CH}_2\mathrm{NC} + 2\mathrm{H}_2\mathrm{O} \xrightarrow{\mathrm{H}^+} \mathrm{CH}_3\mathrm{CH}_2\mathrm{NH}_3^+ + \mathrm{HCOOH}$.

The basicity of amines is primarily determined by the availability of the lone pair of electrons on the nitrogen atom for protonation. Methyl groups attached to the nitrogen act as electron-donating substituents through the $+\mathrm{I}$ inductive effect, which effectively increases the electron density around the nitrogen center. Because adding more alkyl groups strengthens this electron-donating effect, the basic strength increases accordingly, resulting in the order $\mathrm{NH}_3 < \mathrm{CH}_3\mathrm{NH}_2 < (\mathrm{CH}_3)_2\mathrm{NH}$.

The carbylamine reaction is a specialized chemical test used to identify primary amines through the formation of a distinctively foul-smelling substance. When a primary amine is heated with chloroform and ethanolic potassium hydroxide, it converts into an isocyanide, marking a definitive result for this diagnostic process. This chemical transformation occurs according to the stoichiometry $\mathrm{C}_2\mathrm{H}_5\mathrm{NH}_2 + \mathrm{CHCl}_3 + 3\mathrm{KOH} \rightarrow \mathrm{C}_2\mathrm{H}_5\mathrm{N} \equiv \mathrm{C} + 3\mathrm{KCl} + 3\mathrm{HCl}$.