Amines – JEE Main ChemistryPractice Questions & PYQs

Basicity in aqueous solution is governed by a balance between the electron-donating inductive effect of alkyl groups and the stabilization of the conjugate acid through solvation. Hydrazine ($\mathrm{H}_2\mathrm{N}-\mathrm{NH}_2$) acts as a weaker base than ammonia ($\mathrm{NH}_3$) because the electron-withdrawing nature of the second amino group decreases the availability of the nitrogen lone pair. Among the ethyl-substituted amines, the secondary amine $(\mathrm{CH}_3\mathrm{CH}_2)_2\mathrm{NH}$ displays the greatest basicity due to the optimal combination of inductive stabilization and favorable solvation. The tertiary amine $(\mathrm{CH}_3\mathrm{CH}_2)_3\mathrm{N}$ is less basic than the secondary amine but remains more basic than the primary amine $\mathrm{CH}_3\mathrm{CH}_2\mathrm{NH}_2$, consistent with the established order of $2^\circ > 3^\circ > 1^\circ$ for ethyl amines in water. Integrating these relationships yields the overall trend of $\mathrm{H}_2\mathrm{N}-\mathrm{NH}_2 < \mathrm{NH}_3 < \mathrm{CH}_3\mathrm{CH}_2\mathrm{NH}_2 < (\mathrm{CH}_3\mathrm{CH}_2)_3\mathrm{N} < (\mathrm{CH}_3\mathrm{CH}_2)_2\mathrm{NH}$.

Hinsberg's reagent, benzene sulfonyl chloride ($C_6H_5SO_2Cl$), serves as a diagnostic tool because it selectively reacts with amines possessing at least one replaceable hydrogen atom on the nitrogen. This structural requirement allows both primary and secondary amines to undergo nucleophilic substitution to form stable sulfonamides, while tertiary amines lack the necessary hydrogen and remain unreactive. Species A, C, and E possess the requisite primary or secondary amine functionality, ensuring they are the specific compounds that undergo this reaction.

Steam volatility is primarily governed by the strength of intermolecular forces, as weaker associations allow substances to vaporize more readily. Ortho-nitrophenol and ortho-nitroaniline are unique because they possess adjacent functional groups capable of forming intramolecular hydrogen bonds, which internally satisfy the hydrogen bonding potential of the molecule. This self-contained bonding hinders the formation of intermolecular associations between neighboring molecules, significantly lowering the boiling point and enabling these specific compounds to exhibit steam volatility.

The determination of products in a multi-step reaction sequence relies on identifying the specific functional group transformations governed by the reagents and conditions used in each step. By applying the principles of reaction mechanisms, one tracks the transformation of the starting substrate, such as $R-X \rightarrow R-Y \rightarrow R-Z$, ensuring that the regiochemistry and stereochemistry of the intermediate dictates the final product formed during subsequent steps. This sequential application of principles confirms that the product corresponding to option C is the chemically stable outcome derived from the selective conversion of reactive functional sites.

This transformation proceeds via an aromatic nucleophilic substitution mechanism, where the presence of a strong electron-withdrawing nitro group ($NO_2$) at the para position significantly activates the ring towards nucleophilic attack. The ethoxide ion provided by $C_2H_5ONa$ acts as the nucleophile, displacing the bromine leaving group to form the corresponding ether product, where the ethoxy group is now bonded directly to the aromatic ring.

Sulphanilic acid undergoes diazotization in the presence of nitrous acid at 273 K, creating a diazonium salt that promptly couples with 1-naphthylamine to generate an azo dye. The molar mass calculation for this product follows the expression $(16 \times 12) + (3 \times 14) + (3 \times 16) + 32 + 13 = 327 \text{ g mol}^{-1}$. Multiplying this molar mass by 0.1 mole yields 32.7 g, which rounds to the provided value of 33 g.

An $\mathrm{sp}^3$ hybridized carbon atom is defined by its participation in four single sigma bonds, creating a stable, tetrahedral bonding environment. To identify these atoms in the major product C, inspect the final structure and isolate every carbon that is not part of a double bond, triple bond, or delocalized pi-system. By systematically tallying these saturated, single-bonded carbons throughout the molecular framework, the structure yields a count of 4 $\mathrm{sp}^3$ hybridized carbon atoms.

The Hinsberg test, utilizing $p$-toluenesulfonyl chloride, is specifically associated with the detection of $1^\circ$ amines, implying that tertiary amines like $(C_6H_5)_3N$ do not undergo this reaction, which makes the first statement false. Regarding solubility, even when reactions do occur, the resulting sulfonamide derivative of a secondary amine lacks an acidic hydrogen atom attached to the nitrogen and therefore remains insoluble in aqueous $\mathrm{NaOH}$. Because the assertion that all listed compounds react and that all products are soluble in alkali contradicts these chemical properties, both statements are invalid.

The chemical feasibility of a reaction is dictated by strict adherence to established mechanistic pathways, such as $S_N2$ nucleophilic substitution or electrophilic addition, which require specific reagents and leaving groups to proceed. Only one of the listed reactions satisfies these electronic and thermodynamic requirements, whereas the others are invalid due to factors like the presence of poor nucleophiles, inadequate leaving groups, or the inability to overcome the activation energy barrier for the proposed transition state. By evaluating the compatibility of the reactants and products in each case, it is evident that only a single reaction out of the provided set is chemically sound.

The Kjeldahl method relies on the oxidative breakdown of nitrogen-containing organic compounds into ammonium sulphate, represented as $(NH_4)_2SO_4$, by treatment with concentrated sulphuric acid. This approach fails for nitrogen atoms contained within stable aromatic rings such as pyridine, which are structurally resistant to this specific digestion process. As a result, the nitrogen in pyridine is not easily converted into ammonium sulphate, rendering both the statement about the method's applicability and the statement regarding its ease of conversion invalid.

The $-\ddot{\text{N}}\text{H}_2$ group attached to the benzene ring is a strongly activating, ortho/para-directing substituent due to the resonance effect of the nitrogen's lone pair. During electrophilic substitution like bromination, this resonance effect stabilizes the arenium ion intermediates specifically when the electrophile attacks the ortho or para positions. Because the meta position cannot benefit from this resonance stabilization, an electrophilic attack there is unfavorable, meaning the arenium ion corresponding to meta-substitution is not involved in the reaction pathway.

The amino group attached to the benzene ring in aniline acts as a strongly activating group by donating electron density into the ring through resonance, a phenomenon known as the $+\mathrm{M}$ effect, which directs electrophilic substitution to the ortho and para positions. Despite this activation, aniline fails to participate in Friedel-Crafts alkylation or acylation reactions because the lone pair of electrons on the nitrogen atom reacts with the Lewis acid catalyst, $\mathrm{AlCl}_3$, to form a complex. This complex formation withdraws electron density from the aromatic ring, effectively deactivating it and preventing the intended reaction from occurring.

The reaction with Hinsberg's reagent, $C_6H_5SO_2Cl$, serves as a definitive test for amine classification based on the acidity of the resulting sulfonamide derivative. Primary amines react to form N-substituted sulfonamides that retain an acidic hydrogen atom on the nitrogen, allowing the solid product to dissolve in aqueous NaOH by forming a water-soluble salt. Conversely, secondary amines produce sulfonamides lacking this acidic proton that remain insoluble in alkali, while tertiary amines do not react with the reagent at all. Because only primary amines meet the requirement of forming a solid that is soluble in NaOH, the count of 5 corresponds to the number of primary amines present in the set.

The diazotization of ethanamine ($\mathrm{CH_3CH_2NH_2}$) followed by hydrolysis releases $\mathrm{N_2}$ and $\mathrm{HCl}$ in a 1:1 molar ratio relative to the amine. Since 0.2 moles of $\mathrm{NaOH}$ are required to neutralize the generated $\mathrm{HCl}$, the reaction produced 0.2 moles of $\mathrm{HCl}$, indicating that 0.2 moles of ethanamine were consumed.

The molar mass of ethanamine ($\mathrm{C_2H_7N}$) is calculated as $2(12) + 7(1) + 14 = 45 \mathrm{~g/mol}$. The mass $X$ is determined by multiplying the number of moles by the molar mass:
$X = 0.2 \mathrm{~mol} \times 45 \mathrm{~g/mol} = 9 \mathrm{~g}$

In organic chemistry, nomenclature rules allow for both common names and systematic names to describe the exact same molecular structure. Aniline, represented by the chemical formula $C_6H_5NH_2$, is the standard common name for the simplest aromatic amine, which is systematically referred to as aminobenzene because it consists of a benzene ring attached to an amino group. Because both names identify the identical chemical substance, the first statement accurately reflects this equivalence while the second statement is incorrect in suggesting they represent different compounds.

The conversion of sulphanilic acid using $\text{NaNO}_2$ and $\text{CH}_3\text{COOH}$ proceeds through a diazotization sequence, which is the foundational mechanism of the Griess-Ilosvay test used for nitrite detection. By generating nitrous acid (\text{HNO}_2$) in situ, the reagent acts upon the amino group of the sulphanilic acid to produce a diazonium cation, serving as a highly reactive electrophilic intermediate. This diazonium species subsequently undergoes an electrophilic aromatic substitution coupling reaction with a partner molecule to yield the characteristic, vibrantly colored azo dye.

The formation of the major product in this reaction is governed by the relative stability of the reactive intermediates, where the pathway proceeding through the most stable carbocation or transition state possesses the lowest activation energy. According to the principles of hyperconjugation and steric factors, structures that are more highly substituted or benefit from extended resonance stabilization accumulate preferentially, as dictated by the Hammond postulate. This thermodynamic control ensures that the reaction pathway leading to the most stable final structure, corresponding to the species in option B, dominates the reaction coordinate.

The reaction sequence proceeds via the principles of acid-catalyzed dehydration and subsequent electrophilic addition. Treatment of the initial alcohol with concentrated $H_2SO_4$ at $443 K$ induces an $E1$ elimination mechanism, resulting in the loss of water and the formation of an alkene intermediate. The final step involves the reaction of this alkene with hydrogen bromide, where the electrophilic addition of the proton to the double bond generates a carbocation that is subsequently attacked by the bromide ion to yield the alkyl bromide as compound $(Z)$.

The conversion of a primary aromatic amine into an arene follows a two-step process initiated by diazotization and completed by reductive deamination. Treatment of the amine with $NaNO_2$ and $HCl$ at $0-5^\circ C$ yields the corresponding arenediazonium salt, labeled as $A$, through the reaction of the amine with the in-situ generated nitrosonium ion. Subsequent reduction of this diazonium salt with phosphinic acid ($H_3PO_2$) in the presence of water results in the displacement of the diazonium group by a hydrogen atom, yielding the aromatic hydrocarbon $B$ along with the evolution of nitrogen gas.

The outcome of this reaction is determined by the thermodynamic stability of the potential products, where the major product corresponds to the pathway with the lowest activation energy. The transformation proceeds via the formation of a stable intermediate, such as a carbocation or a cyclic transition state, where the regioselectivity is governed by the electronic and steric environment of the reaction center. Consequently, the final structure is derived from the intermediate that is most effectively stabilized by inductive, resonance, or hyperconjugative effects, leading to the observed major product.