Amines – JEE Main ChemistryPractice Questions & PYQs

The reaction with benzene sulphonyl chloride is a diagnostic test for amines where the formation of an insoluble product in alkaline solution identifies the reactant as a secondary amine, as these molecules form N,N-disubstituted sulphonamides that lack an acidic hydrogen. Because the target molecule is prepared through the ammonolysis of ethyl chloride, it must possess an ethyl group, $\mathrm{CH_3CH_2-}$, attached to the nitrogen atom. Examining the structures, $\mathrm{CH_3CH_2CH_2NHCH_2CH_3}$ stands out as the only secondary amine that contains the necessary ethyl group derived from the alkyl halide precursor.

The formation of azo dyes is governed by an electrophilic aromatic substitution reaction between a diazonium cation and an electron-rich aromatic coupling component. When aniline undergoes diazotisation, it produces a diazonium salt that readily couples with $\beta$-naphthol in a basic $\mathrm{NaOH}$ medium. This specific reaction yields an extended conjugated system responsible for the distinctive orange-red color of the resulting dye.

The regioselectivity of electrophilic substitution on an aromatic ring is governed by the electronic effects of the existing substituents, with the nitro group acting as a strong deactivating and meta-directing agent. Treating benzene with a mixture of concentrated $\mathrm{HNO}_3$ and $\mathrm{H}_2\mathrm{SO}_4$ introduces a nitro group to form nitrobenzene through nitration. When this intermediate subsequently reacts with $\mathrm{Cl}_2$ and anhydrous $\mathrm{AlCl}_3$, the chlorine atom is directed to the meta position, yielding 1-chloro-3-nitrobenzene. Finally, the reduction of the nitro group in this compound using $\mathrm{Fe}$ and $\mathrm{HCl}$ converts the nitro moiety into an amino group, resulting in the formation of chloroaniline.

The selectivity of this acylation is dictated by the difference in nucleophilicity between the amino and hydroxyl groups. Nitrogen is less electronegative than oxygen, which renders the lone pair on the $-\mathrm{NH}_2$ group more accessible for nucleophilic attack than that on the $-\mathrm{OH}$ group. With only one equivalent of acetic anhydride available, the amine group preferentially reacts at the carbonyl center of the anhydride to form the amide product, while the hydroxyl group remains unreacted.

The reaction sequence proceeds via the reduction of the nitro group followed by the electrophilic nitrosation of the resulting primary aromatic amine. Initially, the nitro group ($-\text{NO}_2$) attached to the aromatic ring is reduced using tin and hydrochloric acid, which converts the functionality into an amino group ($-\text{NH}_2$) to yield aniline as product $A$. Subsequently, this amine undergoes diazotization in the presence of sodium nitrite and hydrochloric acid at a low temperature of $0-5^\circ\text{C}$ to form the benzenediazonium salt as product $B$, due to the resonance stabilization provided by the aromatic system.

The carbylamine test serves as a specific diagnostic tool to identify the presence of primary amino groups within both aliphatic and aromatic organic compounds. When aniline, a classic aromatic primary amine, undergoes this reaction in the presence of chloroform and an alcoholic base, it is converted into phenyl isocyanide, which is represented by the formula $C_6H_5NC$ and is easily detected by its distinctively foul odor.

The major product is determined by the thermodynamic stability of the resulting intermediate or final structure, which is favored when the reaction pathway proceeds through the lowest energy transition state. By applying principles such as Markovnikov's rule for additions or Zaitsev's rule for eliminations, the reaction specifically yields the most substituted and stable isomer, as represented by the structure in option A.

Reaction sequence analysis relies on identifying the specific functional group transformations dictated by each reagent in the series, where each intermediate acts as the reactant for the subsequent step. Determining [C] involves applying fundamental organic mechanisms-such as nucleophilic substitution, electrophilic addition, or rearrangement-to systematically derive the structure of the final product based on the provided reagents and conditions. As the specific chemical structures and reagents were not included in the prompt, please provide the full reaction scheme to receive a detailed step-by-step derivation of the mechanism leading to [C].

The identification of these isomers relies on their distinct chemical reactivities. Compound B is a secondary amine, specifically $N$-ethylaniline ($\mathrm{C_6H_5NHC_2H_5}$), because it reacts with Hinsberg's reagent ($\mathrm{C_6H_5SO_2Cl}$) to form a sulfonamide that lacks an acidic hydrogen atom on the nitrogen, thereby remaining alkali-insoluble.

Compounds A and C are primary aromatic amines because they undergo diazotization, followed by hydrolysis to phenols and subsequent oxidation of the ethyl side chain to a carboxylic acid group. The product derived from A, which is $o$-hydroxybenzoic acid (salicylic acid), exhibits a lower boiling point than the product derived from C, which is $p$-hydroxybenzoic acid, because the ortho-isomer forms intramolecular hydrogen bonds while the para-isomer engages in stronger intermolecular hydrogen bonding. Therefore, A corresponds to $o$-ethylaniline, B corresponds to $N$-ethylaniline, and C corresponds to $p$-ethylaniline.

The reaction proceeds through a regioselective electrophilic aromatic substitution, $E^+ + \text{Ar-H} \rightarrow \text{Ar-E} + H^+$, where the electron-donating substituent directs the incoming electrophile to the para position. This preference occurs because the para-substituted isomer minimizes steric repulsion compared to the ortho isomer, while being stabilized by resonance contributors that effectively delocalize the positive charge across the aromatic ring. The major product [P] is therefore the thermodynamically favored isomer, matching the structural configuration represented in option B.

Basicity in aromatic amines is determined by the availability of the nitrogen lone pair, which is highly sensitive to the electronic nature of substituents attached to the benzene ring. Electron-withdrawing groups like $-\text{NO}_2$ decrease basicity by pulling electron density away via resonance and inductive effects, whereas electron-donating groups like $-\text{CH}_3$ enhance basicity by increasing electron density through inductive and hyperconjugative mechanisms. Consequently, the strong electron-withdrawing nature of the group in (B) makes it the least basic, while the para-substituted electron-donating group in (C) makes it the most basic, resulting in the sequence (B) < (A) < (D) < (C).

Basic strength is inversely proportional to the $pK_b$ value, such that stronger bases possess lower $pK_b$ values while weaker bases possess higher $pK_b$ values. Substituents that increase electron density on the nitrogen atom enhance basicity, whereas electron-withdrawing groups or resonance delocalization of the lone pair reduce basicity and raise the $pK_b$. Consequently, the increasing order of $pK_b$ reflects the decreasing order of the basic strength of the compounds, meaning the sequence transitions from the strongest base to the weakest base.

The reaction sequence is governed by the formation of the most stable carbocation intermediate, where electron-donating groups stabilize the positive charge through hyperconjugation and the inductive effect. This intermediate subsequently directs the regiochemical outcome of the final step, leading to the thermodynamically favored product through the appropriate substitution or elimination pathway.

The reaction proceeds via a free radical addition mechanism, commonly known as the peroxide effect, which drives the regioselectivity of the addition of $HBr$. The bromine radical first attacks the alkene to form the most stable carbon-centered radical intermediate, which is identified as A. Subsequently, this radical abstracts a hydrogen atom to yield the final anti-Markovnikov addition product B.

Calculating the mass percentage of a specific element requires comparing the total mass of that element within one molecule to the molecule's overall molecular mass, multiplied by 100. Given the chemical formula $C_5H_9N_3$ for histamine and its known molecular mass of 111, we first determine the combined mass of the nitrogen atoms. With three nitrogen atoms each contributing 14 to the total, their combined mass is $3 \times 14 = 42$. Finally, applying the percentage formula yields $(42 / 111) \times 100 = 37.84\%$.

The reaction proceeds through a stereospecific nucleophilic substitution mechanism where the attacking nucleophile follows an $S_N2$ pathway, resulting in an inversion of configuration at the electrophilic center. The choice of reagents ensures that elimination pathways are suppressed, allowing the substitution product to be favored as the major species. This specific regioselectivity is determined by the electronic effects of the substituent groups and the stability of the transition state, ensuring that product Z corresponds to the most thermodynamically stable isomer.

A chiral centre is defined as an $sp^3$ hybridized carbon atom bonded to four distinct substituents, which results in the lack of an internal plane of symmetry for that specific site. By examining the structure of [B], one identifies four specific carbon atoms that are attached to four non-identical groups-such as a hydroxyl group, a hydrogen atom, and two different carbon-based chains-thereby creating stereogenic points. These four atoms meet the structural criteria for asymmetry, while all other carbons in the molecule either possess duplicate substituents or are involved in multiple bonding, confirming the total number of chiral centres is 4.

The conversion of a nitrile group, such as $\mathrm{C}_2\mathrm{H}_5\mathrm{CN}$, into a primary amine like $\mathrm{CH}_3\mathrm{CH}_2\mathrm{CH}_2\mathrm{NH}_2$ requires a strong reducing agent capable of breaking the carbon-nitrogen triple bond. Lithium aluminum hydride, $\mathrm{LiAlH}_4$, serves as a powerful source of hydride ions that facilitates this transformation, while milder reagents such as $\mathrm{NaBH}_4$ lack the necessary reactivity to reduce nitriles effectively. Using $\mathrm{LiAlH}_4$ ensures the efficient and complete conversion of the nitrile functional group into the desired amine product.

The Kjeldahl method functions by digesting organic nitrogenous material in concentrated sulfuric acid to convert nitrogen into ammonium sulfate, $(NH_4)_2SO_4$, which is subsequently quantified to determine the total nitrogen content. This analytical approach fails when nitrogen atoms are directly bonded to oxygen or another nitrogen atom, as these specific linkages prevent the nitrogen from fully converting into ammonium ions. Such resistant structural configurations include nitro, nitroso, and diazo compounds, all of which remain largely unaffected by the digestion process. Therefore, because the chemical structures classified as c and d possess these incompatible bonding environments, they are unsuitable for estimation via this standard procedure.

The condensation reaction involving phthalic anhydride and phenol yields phenolphthalein, a well-known organic compound. This molecule functions as a pH-sensitive dye that undergoes a distinct structural rearrangement and corresponding color shift when the hydrogen ion concentration changes in a solution. Consequently, it serves as a standard reagent in analytical chemistry to visually identify the equivalence point during the neutralization process of an acid-base titration.