Amines – JEE Main ChemistryPractice Questions & PYQs

Heating benzamide ($C_6H_5\mathrm{CONH}_2$) with bromine and sodium hydroxide initiates the Hofmann bromamide degradation, a reaction that removes the carbonyl carbon to form a primary amine with one fewer carbon atom, resulting in aniline ($C_6H_5\mathrm{NH}_2$) instead of benzylamine, which renders the first statement false. Conversely, nitration of aniline in a concentrated sulfuric acid medium at 288 K protonates the amine to form the anilinium ion ($C_6H_5\mathrm{NH}_3^+$), which is strongly electron-withdrawing and meta-directing, leading to m-nitroaniline as a significantly more abundant product than o-nitroaniline, making the second statement true.

The chemical sequence proceeds through successive electrophilic substitution steps, where initial reaction conditions incorporate two bromine atoms onto the aromatic ring. Subsequent reaction phases facilitate the addition of three more bromine atoms at available reactive sites. Summing these individual contributions throughout the transformation pathway confirms that the final product $P$ contains a total of 5 bromine atoms.

The synthesis of benzyl isocyanide, $C_6H_5CH_2NC$, depends on specific reaction conditions that favor the nitrogen atom as the nucleophile. The carbylamine reaction is a definitive pathway where benzylamine reacts with chloroform in the presence of an alcoholic base to generate the isocyanide linkage. Furthermore, treating a benzyl halide with silver cyanide utilizes the covalent character of the silver-cyanide bond, which directs the nucleophilic attack through the nitrogen atom rather than the carbon, thereby producing benzyl isocyanide effectively.

The sequence of reactions described represents the Hofmann Bromamide degradation of a primary amide, where compound 'A' ($\mathrm{C}_8\mathrm{H}_9\mathrm{ON}$) is transformed into a primary amine 'B', confirming that 'A' is $p$-methylbenzamide ($\mathrm{CH}_3\mathrm{C}_6\mathrm{H}_4\mathrm{CONH}_2$). This amide undergoes the reaction with aqueous $\mathrm{Br}_2/\mathrm{HO}^{(-)}$ to yield $p$-toluidine ($\mathrm{CH}_3\mathrm{C}_6\mathrm{H}_4\mathrm{NH}_2$), which is basic and thus soluble in dilute acid.

Treatment of $p$-toluidine with aqueous $\mathrm{NaNO}_2/\mathrm{HCl}$ at $0-5^{\circ}\mathrm{C}$ performs diazotization, producing $p$-toluenediazonium chloride (compound 'C'), which further reacts with $\mathrm{CuCN}/\mathrm{NaCN}$ via the Sandmeyer reaction to form $p$-tolunitrile ($\mathrm{CH}_3\mathrm{C}_6\mathrm{H}_4\mathrm{CN}$, compound 'D'). Subsequent hydrolysis of $p$-tolunitrile yields $p$-toluic acid ($\mathrm{CH}_3\mathrm{C}_6\mathrm{H}_4\mathrm{COOH}$, compound 'E'), a product also obtained from the hydrolysis of the initial amide 'A'.

Finally, the treatment of $p$-toluic acid with acidified $\mathrm{KMnO}_4$ results in the oxidation of the methyl group to a second carboxylic acid group, forming terephthalic acid ($\mathrm{HOOCC}_6\mathrm{H}_4\mathrm{COOH}$, compound 'F'). This final structure contains exactly two distinct types of hydrogen atoms, namely the four equivalent aromatic protons on the benzene ring and the two equivalent acidic protons belonging to the carboxyl groups.

Carius analysis allows for the quantification of halogens in an organic compound by measuring the mass of the precipitate formed upon reaction with silver nitrate. With $3.36\text{ g}$ of $\text{AgBr}$ produced from $2.0\text{ g}$ of the hydrocarbon, and knowing the molar mass of $\text{AgBr}$ is $188\text{ g/mol}$, we equate the moles of bromine atoms to the stoichiometric bromine content of the original substance, expressed as $x \cdot \frac{2.0}{M}$, where $x$ is the number of bromine atoms and $M$ is the molar mass of the compound. This calculation reveals $M \approx 112x\text{ g/mol}$.

Given the carbon content is $26.7\%$, the mass of carbon in the compound is determined by $0.267 \times 112x$, which equals approximately $30x\text{ g}$. Dividing this mass by the atomic mass of carbon, $12\text{ g/mol}$, results in $2.5x$ moles of carbon atoms. For the empirical formula to contain whole numbers of atoms, we must choose the smallest integer $x$ that results in an integer number of carbon atoms, which occurs when $x = 2$, yielding $5$ carbon atoms.

The high electron density of the amino group makes the aniline benzene ring overly reactive, typically resulting in unwanted polybromination if reagents are added directly. To ensure a controlled reaction, the amino group is protected using $\mathrm{H}_2\mathrm{SO}_4$ and $\mathrm{Ac}_2\mathrm{O}$, effectively modulating the reactivity and directing the subsequent electrophilic substitution of $\mathrm{Br}_2$ to the para position. Finally, the protective acetamido group is removed through acidic hydrolysis with $\mathrm{H}_2\mathrm{O}(\Delta)$ followed by neutralization with $\mathrm{NaOH}$ to recover the amine functionality.

Azo dyes are synthesized through an electrophilic aromatic substitution process known as azo coupling, where a diazonium salt reacts with an electron-rich aromatic compound such as a phenol or an amine. This sequence is initiated by the diazotization of a primary aromatic amine using $NaNO_2$ and $HCl$ at temperatures maintained between $0^\circ C$ and $5^\circ C$ to generate the reactive electrophilic diazonium cation. Subsequent reaction of this cation with an activated aromatic ring results in the formation of the stable $-N=N-$ chromophore, which constitutes the final azo dye structure.

The reaction proceeds via an acid-catalyzed dehydration involving the formation of a stable carbocation intermediate, which is the rate-determining step. Following the protonation of the hydroxyl group and subsequent loss of water, the carbocation undergoes elimination to form the most stable alkene, governed by Saytzeff's rule. This preference for the most substituted alkene occurs because hyperconjugation and the minimization of steric strain lower the activation energy, making it the major product obtained in the sequence.

The Dumas method is a well-established analytical technique used to estimate the nitrogen content in an organic substance by heating the sample with copper oxide. The description provided in the second statement actually refers to a different experimental procedure rather than the Dumas method, as the Dumas technique does not involve treating the sample with concentrated $\mathrm{H}_2\mathrm{SO}_4$ to generate ammonium sulphate. Thus, the first statement is true because it correctly identifies the objective of the method, whereas the second statement is false because it misrepresents the underlying chemical process.

The conversion of aniline into biphenyl begins with diazotization, where aniline reacts with $NaNO_2$ and $HCl$ at low temperatures to produce benzenediazonium chloride as the $[A]$ component. This diazonium species proceeds through the Sandmeyer reaction using $Cu_2Cl_2$ and $HCl$ to replace the diazo group, yielding chlorobenzene as intermediate $[B]$. The sequence concludes with the Wurtz-Fittig reaction, where chlorobenzene couples with metallic sodium in the presence of dry ether to form the final product biphenyl, denoted as $[C]$ or $Ph-Ph$.

The carbylamine test serves as a diagnostic chemical reaction specifically for primary amines, which react with chloroform ($CHCl_3$) and alcoholic potassium hydroxide ($KOH$) to yield a foul-smelling isocyanide ($R-NC$). Secondary amines like $(CH_3)_2NH$ and tertiary amines such as $(CH_3)_3N$ fail to undergo this reaction because they lack the necessary hydrogen atoms on the nitrogen atom to form the isocyanide linkage. Consequently, only the primary amines, corresponding to option A and the primary amine $CH_3NH_2$ in option C, produce a positive result.

The molar mass of a complex chemical product is determined by the summation of the atomic masses of all constituent elements present within its molecular structure. Once the molecular formula is derived from the reaction sequence, we apply this principle by multiplying the number of each specific atom by its respective mass and summing these values to obtain the total. This relationship is expressed as $M = \sum (n_i \times m_i)$, where $n_i$ denotes the count of each atom and $m_i$ is its atomic weight. Given the atomic masses of C = 12, H = 1, O = 16, N = 14, Br = 80, and P = 31 g/mol, the systematic addition of these contributions for product A yields a total molar mass of 171 g/mol.

The selective reduction of nitrogen-containing functional groups requires specific reagents tailored to the substrate's oxidation state and structural properties. Strong reducing agents like $LiAlH_4$ are necessary to reduce amides ($RCONH_2$) to primary amines ($RCH_2NH_2$), whereas the Hofmann bromamide degradation, utilizing $Br_2/KOH$, selectively converts amides into amines ($RNH_2$) with the loss of a carbon atom. Conversely, nitro compounds ($RNO_2$) are reduced to primary amines using acidic metal catalysts like $Sn/HCl$ or $Fe/HCl$, and nitriles ($RCN$) are typically reduced to primary amines via catalytic hydrogenation or hydride transfer, perfectly aligning these standard synthetic methods with their respective substrates.

The basicity of nitrogen-containing compounds is primarily determined by the availability of the lone pair on the nitrogen atom, which is enhanced by electron-donating alkyl groups and reduced by resonance delocalization. After identifying the most basic compound among the options, which has a molar mass of 107 g mol⁻¹, the neutralization reaction with hydrochloric acid follows the equation $\text{Base} + \text{HCl} \rightarrow \text{Salt}$. With a 1.0 g sample of the compound, the number of moles present is calculated as $\frac{1.0}{107}$. Since the reaction proceeds with a 1:1 stoichiometry, the same number of moles of HCl is consumed. Given the molar mass of HCl is $1 + 35.5 = 36.5$ g mol⁻¹, the mass of HCl consumed is $\frac{1.0}{107} \times 36.5$ grams. This calculation yields 0.341 grams, which is equal to 341 mg of HCl.

The reaction presented in option A is chemically invalid because it violates the regioselectivity governed by Markovnikov's rule, which dictates that the nucleophile must bond to the more substituted carbon atom to favor the formation of a more stable carbocation intermediate. Under the specified reaction conditions, the electrophilic addition proceeds through a transition state that strictly precludes the formation of the anti-Markovnikov product shown. Therefore, this reaction pathway is energetically unfavorable and cannot occur as depicted.

The characterization of amines relies on specific reactivity patterns, particularly the carbylamine test and interactions with Hinsberg's reagent. Primary amines, whether aliphatic or aromatic, react with $\mathrm{CHCl}_3$ and $\mathrm{KOH}$ to produce isocyanides, while secondary and tertiary amines are incapable of this reaction. Benzenesulfonyl chloride is correctly identified as Hinsberg's reagent, and because tertiary amines cannot react with it due to the lack of an acidic hydrogen, they remain unreactive. Finally, since primary aromatic amines successfully form diazonium salts when treated with $\mathrm{NaNO}_2$ in an acidic medium, the statement that primary amines generally fail to perform this reaction is inaccurate.

Benzamide, which possesses a molar mass of 121 $\text{g/mol}$ and returns a positive Lassaigne's test for nitrogen, serves as the starting compound $A$. The hydrolysis of this amide yields benzoic acid, $B$, which is a carboxylic acid that reacts with aqueous $\mathrm{NaHCO_3}$ to release carbon dioxide gas, producing effervescence. Finally, reacting benzoic acid with ethanol in the presence of $\mathrm{H_2SO_4}$ undergoes Fischer esterification to produce ethyl benzoate, $C$, a compound readily identified by its characteristic fruity fragrance.

The $-NH_2$ group is a strongly activating ortho/para-directing substituent that typically drives electrophilic substitution to the 2, 4, and 6 positions, preventing the formation of the 3, 4, 5-substituted isomer through standard direct bromination. Sequence C utilizes the strategic acetylation of the amine to acetanilide, which moderates the electronic and steric profile of the nitrogen, thereby allowing for controlled bromination and subsequent hydrolysis to effectively yield 3, 4, 5-tribromoaniline.

The reactivity of chlorobenzene toward nucleophilic substitution is primarily governed by the presence of electron-withdrawing groups on the aromatic ring, which stabilize the Meisenheimer intermediate and significantly lower the activation energy. As the number of these activating groups increases, the electron density on the ring is reduced, making the electrophilic carbon atom more susceptible to nucleophilic attack and allowing the reaction to proceed at progressively milder temperatures. Consequently, unsubstituted chlorobenzene necessitates the most extreme conditions of 623 K and 300 atm pressure, while the addition of nitro groups at ortho and para positions facilitates the reaction at increasingly moderate conditions of 443 K and 368 K, ultimately enabling the transformation to occur in warm $\mathrm{H}_2\mathrm{O}$ for the most highly activated substrates.

Determining the molar mass of the final product in a reaction sequence relies on identifying the exact molecular formula of the compound and performing a summation of its elemental atomic weights. By using the general expression $M = \sum n_i A_i$, where $n_i$ represents the number of atoms of element $i$ and $A_i$ represents the standard atomic mass of that element, one can aggregate the contributions of every atom within the structure. Applying this systematic approach to the identity of the final product yields a total value of 154 g mol⁻¹, representing the combined mass of all atoms present in the molecule.