Amines – JEE Main ChemistryPractice Questions & PYQs

The reaction involves the acid-catalyzed hydration of a terminal alkyne in the presence of $\text{Hg}^{2+}$ ions, which facilitates the electrophilic addition of water across the triple bond following Markovnikov's rule. This process generates an unstable enol intermediate that rapidly tautomerizes to form the more stable methyl ketone product. Consequently, the reaction transforms the alkyne substrate into the corresponding ketone, which is represented by the chemical structure in option D.

Benzyl isocyanide is synthesized via the nucleophilic substitution of benzyl chloride with silver cyanide ($AgCN$), where the covalent nature of the silver-carbon bond promotes the attack of the lone pair on nitrogen to form the isocyanide linkage. Furthermore, the carbylamine reaction of benzyl amine with chloroform ($CHCl_3$) and alcoholic potassium hydroxide ($KOH$) generates an intermediate dichlorocarbene that reacts with the primary amine to yield benzyl isocyanide.

Electrophilic aromatic substitution relies on the electron density of the benzene ring, as higher electron density makes the ring more nucleophilic and prone to attack by electrophiles. Substituents attached to the ring modulate this density, where electron-donating groups (EDGs) activate the ring by increasing electron availability, while electron-withdrawing groups (EWGs) deactivate the ring by reducing electron density. Adhering to the relationship $\text{Reaction Rate} \propto \text{EDG} \propto \frac{1}{\text{EWG}}$, one determines the decreasing order of reactivity by evaluating how effectively each substituent donates or withdraws electrons from the aromatic system.

The reaction between tertiary aromatic amines and nitrous acid proceeds via electrophilic aromatic substitution, where the nitrosyl cation, $\text{NO}^{+}$, serves as the reactive species. Under low temperature conditions, this electrophile preferentially attacks the electron-rich para position of the aromatic ring rather than the nitrogen atom, which is already fully bonded. This mechanism results in the formation of a $p$-nitroso compound, making the characterization of the product as an N-nitroso ammonium compound chemically inconsistent with the observed reaction behavior.

Carbocation stability is primarily determined by the ability of neighboring groups to donate electron density and minimize the positive charge on the carbon center. Among the potential substituents, the $\mathrm{NH}_2$ group is highly effective due to its strong $+M$ effect, which enables the delocalization of its lone pair electrons directly into the carbocation's vacant orbital. This resonance-driven stabilization provides a more significant energetic advantage than simple inductive donation, confirming that the structure containing the $\mathrm{NH}_2$ group represents the most stable configuration.

The reaction involves an electrophilic aromatic substitution where the nitrosonium ion ($NO^+$), generated in situ from the reaction of sodium nitrite with hydrochloric acid, acts as the electrophile. The dimethylamino group ($-N(CH_3)_2$) is a strongly activating substituent that increases the electron density of the aromatic ring through resonance, directing electrophilic attack predominantly to the ortho and para positions. Due to the significant steric hindrance exerted by the bulky dimethylamino group, the electrophile preferentially attacks the para position, resulting in the formation of $p$-nitroso-$N,N$-dimethylaniline as the major product.

The periodic trends within Group 2 alkaline earth metals dictate that properties such as electronegativity and ionization energy decrease down the group due to the increased atomic radius and the effective shielding of the nucleus by inner electron shells. Mapping the elements in List I ($Be$, $Mg$, $Ca$, $Ra$) to their respective values in List II relies on identifying this systematic decrease, where each element corresponds to a specific parameter value that matches the observed physical behavior of these metals. This structured pairing confirms that the alignment of elements to their properties is consistent with the established periodic table patterns for atomic number and electronic structure.

The formation of an enamine is a classic nucleophilic addition-elimination process where a secondary amine acts as the nucleophile attacking the carbonyl carbon of a ketone. When a ketone contains extremely bulky substituents such as the tert-butyl group, $\text{C}(\text{CH}_3)_3$, the intense steric congestion around the carbonyl carbon physically blocks the nucleophile from approaching and bonding with it. Because this severe crowding prohibits the initial nucleophilic attack required to begin the mechanism, the reaction cannot proceed to form an enamine.

p-Toluenesulphonyl chloride reacts with primary amines ($1^\circ$) to form N-alkyl-p-toluenesulphonamides containing an acidic hydrogen atom, making them soluble in aqueous alkali, whereas the reaction with secondary amines ($2^\circ$) produces N,N-dialkyl-p-toluenesulphonamides that lack this acidic hydrogen and are insoluble in alkali. Because the product formed from secondary amines does not possess an acidic hydrogen atom attached to the nitrogen, the assertion that this product is soluble in alkali is incorrect.

The reaction sequence described follows the Gabriel Phthalimide Synthesis, a standard pathway used for the preparation of pure primary amines. Compound C, with the molecular formula $\mathrm{C}_{8}\mathrm{H}_{5}\mathrm{NO}_{2}$, corresponds to phthalimide, which functions as the critical intermediate that reacts with ethanolic $\mathrm{KOH}$ to form a nucleophilic potassium salt, which is then alkylated and hydrolyzed to yield the primary amine.

The formation of phthalimide requires the thermal dehydration of ammonium salts or amides derived from phthalic acid. Specifically, the starting organic compound A is phthalic acid, which reacts with $\mathrm{NH}_{3}$ to produce ammonium phthalate. Upon heating, this intermediate dehydrates to form phthalamide, and further strong heating results in the loss of ammonia to yield the cyclic structure of phthalimide. Consequently, the entire chemical sequence confirms that the starting reactant is phthalic acid.

The conversion of $CH_3-CH_2-CH_2-OH$ to $CH_3-CH_2-CH_2-Br$ proceeds via a nucleophilic substitution reaction mediated by $PBr_3$, which replaces the hydroxyl group with a bromine atom to yield $1-bromopropane$ as the intermediate $A$. Subsequent treatment of this alkyl halide with alcoholic $KOH$ initiates an $E2$ elimination reaction, where the base abstracts a proton from the $\beta$-carbon and removes the halide to form the pi bond, resulting in the alkene propene as the final product $B$.

In a mixture of concentrated nitric and sulfuric acids, sulfuric acid acts as the stronger Brønsted-Lowry acid, while nitric acid accepts a proton, thereby functioning as a base to generate the nitronium ion ($\mathrm{NO}_2^{+}$). This active electrophile is necessary for the subsequent aromatic substitution reaction. Because $\mathrm{H}_2\mathrm{SO}_4$ actively donates a proton to $\mathrm{HNO}_3$, the statement identifying $\mathrm{H}_2\mathrm{SO}_4$ as an acid is correct, whereas the statement characterizing $\mathrm{HNO}_3$ as an acid is incorrect.

When aniline is subjected to this strongly acidic medium, the amino group is protonated to form the anilinium ion. This species is strongly electron-withdrawing and typically directs incoming electrophiles to the meta position. However, in the nitration of aniline, the reaction conditions lead to a mixture where m-nitroaniline and p-nitroaniline are the predominant products. The presence of the meta-directing anilinium ion accounts for the m-nitroaniline, while the p-nitroaniline forms in significant quantities due to the specific reactive pathways in the mixture.

The degree of unsaturation for the molecular formula $\mathrm{C}_4\mathrm{H}_5\mathrm{N}$ is calculated as $DU = 4 + 1 - (\frac{5 - 1}{2}) = 3$. This value signifies that the structure contains three pi bonds, and because the compound is acyclic, it must possess three degrees of unsaturation from these multiple bonds. Considering the stable structure but-2-enenitrile, represented as $\mathrm{CH}_3-\mathrm{CH}=\mathrm{CH}-\mathrm{C}\equiv\mathrm{N}$, the terminal methyl carbon bonded exclusively through single bonds is the only atom with $\mathrm{sp}^3$ hybridization, while the remaining three carbons are part of multiple bonds and thus exhibit $\mathrm{sp}^2$ or $\mathrm{sp}$ hybridization.

The acidic nature of the compound and its molecular weight of approximately 129.5 g/mol, which falls within the given range of $131 \pm 2$, identify the compound as aniline hydrochloride ($C_6H_5NH_3Cl$). Treating this salt with aqueous $\mathrm{NaOH}$ liberates aniline as a free base, according to the reaction $C_6H_5NH_3Cl + \mathrm{NaOH} \rightarrow C_6H_5NH_2 + \mathrm{NaCl} + \mathrm{H}_2\mathrm{O}$. The resulting aniline then undergoes diazotization followed by azo coupling with phenol to form an orange dye, which is a classic chemical test used to confirm the presence of aromatic primary amines.

The reaction involves the diazotization of a primary aromatic amine using sodium nitrite and hydrochloric acid at $0-5^\circ \text{C}$ to generate a benzene diazonium chloride intermediate. This highly reactive species subsequently undergoes reduction by hypophosphorous acid ($H_3PO_2$) in the presence of water, which acts as a mild reducing agent, causing the displacement of the diazonium group by a hydrogen atom to yield benzene as the major product.

The reaction sequence utilizes the controlled conversion of aromatic functional groups, beginning with the diazotization of an amine using $\mathrm{HNO}_2$ followed by the nucleophilic substitution of the resulting diazonium salt with an iodine atom via $\mathrm{KI}$. The subsequent reduction of the remaining nitro group to an amine using $\mathrm{Fe} / \mathrm{H}^{+}$ permits a second round of diazotization with $\mathrm{HNO}_2$, which is then hydrolytically converted into a hydroxyl group upon treatment with warm $\mathrm{H}_2\mathrm{O}$. This strategic order allows for the selective transformation of specific substituents on the benzene ring by masking or activating different positions through intermediate functional group states.

The reaction sequence proceeds via an electrophilic aromatic substitution mechanism where the activating substituent directs the incoming electrophile, $E^+$, to the para position to minimize steric hindrance. Subsequent functional group transformation occurs through the specified reagents, maintaining the aromaticity of the ring while selectively modifying the side chain to arrive at the structure in option C. This regioselective control ensures that the major product aligns with the thermodynamic stability governed by the substituent effects at the transition state.

The Hofmann degradation reaction transforms an amide into a primary amine ($R-NH_2$) by creating an intermediate featuring an electron-deficient nitrogen atom, which acts as the target for the shifting group. Because the migration process is driven by the need to stabilize this nitrogen species, the limitation proposed in the first statement is inaccurate, as it incorrectly excludes the ability of aryl groups to undergo migration. The second statement correctly identifies the fundamental driving force of the rearrangement, as the group migrates specifically to satisfy the electron deficiency on the nitrogen atom.

Friedel-Crafts alkylation requires an aromatic ring to react with an electrophile, yet the amino group on aniline acts as a Lewis base. Since anhydrous $\mathrm{AlCl}_3$ is an electron-deficient Lewis acid, it coordinates with the nitrogen's lone pair to form a complex that deactivates the ring entirely and prevents the Friedel-Crafts reaction from occurring. Thus, while the assertion that $o$ and $p$ methylaniline are not formed is correct, the reason is false because the reaction does not proceed to produce $m$-methylaniline either.

The reaction sequence proceeds via nucleophilic addition to a nitrile followed by acid hydrolysis and a subsequent reduction of the carbonyl group. Initially, the nucleophilic methyl group from the Grignard reagent ($CH_3MgBr$) attacks the electrophilic carbon of ethyl cyanide ($CH_3CH_2CN$) to form an imine magnesium complex, which upon acid hydrolysis ($H_3O^+$) yields the ketone butan-2-one ($CH_3CH_2COCH_3$). The final step utilizes Clemmensen reduction with $Zn-Hg$ and $HCl$, which specifically reduces the ketone carbonyl group to a methylene group, converting butan-2-one into the final alkane product, n-butane ($CH_3CH_2CH_2CH_3$).