Amines – JEE Main ChemistryPractice Questions & PYQs

Gabriel phthalimide synthesis is a specialized method used exclusively for the preparation of $1^{\circ}$ aliphatic amines. This reaction relies on the nucleophilic substitution of the phthalimide anion with an alkyl halide, a pathway that is limited to aliphatic substrates because aromatic halides are unreactive under these conditions. Since the mechanism incorporates a single alkyl group onto the nitrogen atom, it cannot be used to synthesize secondary or tertiary amines, making it the ideal pathway for the compound presented.

The basicity of an amine depends on the availability of the lone pair on the nitrogen atom, as a more accessible pair allows for stronger proton acceptance. Compound (III) acts as the strongest base because its lone pair is not involved in resonance and is further intensified by the $+I$ effect from both sides, while compound (II) ranks next since its lone pair is similarly free from resonance. Compound (I) sits further down the order because its lone pair engages in resonance, and compound (IV) ends as the least basic due to its lone pair being actively involved in the $6\pi e^{-}$ aromatic system.

The preparation of benzylamine from cyanobenzene requires the complete reduction of the nitrile group ($\mathrm{-CN}$) to a primary amine group ($\mathrm{-CH_2NH_2}$). Acid hydrolysis of cyanobenzene using $\mathrm{HCl} / \mathrm{H_2O}$ converts the nitrile group into benzoic acid ($\mathrm{C_6H_5COOH}$), which is a stable carboxylic acid derivative. Since $\mathrm{NaBH_4}$ is a mild reducing agent that is unable to reduce carboxylic acids to amines, this reaction pathway fails to synthesize the target molecule.

The reactivity of carbonyl compounds towards nucleophilic reagents like $LiAlH_4$ is primarily governed by the electrophilicity of the carbonyl carbon, where the rate of attack is directly proportional to the positive charge density at this site. Substituents that possess strong electron-donating resonance effects, such as oxygen or nitrogen atoms directly attached to the carbonyl, significantly decrease this electrophilicity and consequently slow down the reaction rate. In contrast, compounds lacking such strong donor groups maintain a higher positive charge on the carbonyl carbon, resulting in a faster nucleophilic attack and a higher overall reactivity.

Basicity in amines is primarily dictated by the availability of the nitrogen atom's lone pair of electrons to accept a proton, where greater accessibility results in a stronger base. Compound III acts as the strongest base because its nitrogen possesses two lone pairs, which are readily available for donation. In contrast, compound II is the least basic due to the delocalization of its nitrogen lone pair into the aromatic ring, which significantly restricts its participation in acid-base reactions. Given that compound I maintains an intermediate electron density, the decreasing order of basicity is $III > I > II$.

The Gabriel phthalimide synthesis relies on an $S_N2$ mechanism that favors unhindered primary alkyl halides for the efficient nucleophilic displacement of the phthalimide anion. Because $n$-butylamine is derived from a straight-chain primary alkyl halide, it is the only compound among the choices that undergoes this reaction readily, whereas more sterically crowded substrates like neo-pentylamine and $t$-butylamine effectively block the reaction, and triethylamine cannot be synthesized by this method as it is a tertiary amine.

This reaction proceeds via a nucleophilic acyl substitution where the primary amine functions as a nucleophile to attack the carbonyl carbon of the ethyl formate ester. Because the amino group possesses higher nucleophilicity compared to the hydroxyl group, the reaction selectively targets the amine to form an amide, with triethylamine acting as a base to neutralize the byproduct. This process yields the N-formylated product $\mathrm{CH}_3-\text{CH(OH)}\mathrm{CH}_2\mathrm{CH}_2\mathrm{NHCHO}$ while leaving the alcohol functionality unaffected.

The reaction sequence follows the Hofmann bromamide degradation, a specific pathway where an amide is treated with bromine and potassium hydroxide to yield a primary amine with the loss of a carbon atom. Since the final product 'C' is identified as aniline ($C_6H_7N$), the intermediate 'B' must be benzamide ($C_6H_5CONH_2$), formed by the reaction of benzoic acid with ammonia under heating. Therefore, compound 'A' is benzoic acid ($C_6H_5COOH$), which correctly corresponds to the molecular formula $C_7H_6O_2$ and completes the described transformation.

N-ethylphthalimide undergoes hydrazinolysis upon treatment with hydrazine ($NH_2NH_2$), which acts as a nucleophile to break the imide bonds. This chemical process results in the cleavage of the phthalimide ring, thereby liberating the primary amine, ethylamine ($C_2H_5NH_2$), and producing phthalhydrazide as a stable byproduct.

Hinsberg's reagent is chemically known as benzenesulfonyl chloride, which serves as a critical diagnostic tool used to distinguish between primary, secondary, and tertiary amines based on their unique sulfonamide formation. This reagent is defined by the chemical structure $\mathrm{C_6H_5SO_2Cl}$, characterized by a benzene ring attached to the reactive sulfonyl chloride functional group. Because it reacts selectively with nitrogenous bases to produce products with varying solubility properties, it remains the standard choice for amine identification in organic chemistry.

Reactivity towards alkyl halides, known as nucleophilicity, is primarily governed by the ability of a species to donate an electron pair to an electrophilic carbon, a process deeply influenced by the polarizability of the nucleophile and its interaction with the surrounding solvent. In polar protic solvents, smaller and more electronegative ions are effectively trapped by strong solvent stabilization, which significantly hinders their ability to attack the substrate in a substitution reaction represented by $Nu^- + R-X \rightarrow R-Nu + X^-$. Because larger ions possess more deformable electron clouds that are less susceptible to this solvent shielding, their nucleophilic strength increases as one moves down the group, resulting in an ascending order of reactivity where the least polarizable species is the least reactive.

Azo coupling is an electrophilic aromatic substitution reaction where the benzene diazonium cation ($C_6H_5N_2^+$) serves as a weak electrophile and the electron-rich naphthoxide ion functions as a strong nucleophile. In an alkaline medium, 1-naphthol is deprotonated to form a naphthoxide ion, which significantly increases the electron density on the naphthalene ring and directs the electrophile specifically to the 4-position (para to the hydroxyl group) due to electronic activation and steric considerations. The resulting product is a stable azo dye formed through this highly regioselective substitution process.

The reaction sequence initiates with the diazotization of a primary amine using $\mathrm{NaNO}_2/\mathrm{H}^+$, which forms a diazonium intermediate that is subsequently converted into an alcohol. The treatment with concentrated $\mathrm{H}_2\mathrm{SO}_4$ at high temperatures promotes acid-catalyzed dehydration, where a water molecule is eliminated from the alcohol to generate an alkene. This dehydration follows Zaitsev's rule, resulting in the most substituted and thermodynamically stable alkene as the major product.

The reaction proceeds according to Markovnikov's rule, which states that the electrophilic hydrogen atom from the reagent adds to the carbon atom of the alkene double bond bearing the greater number of hydrogen atoms. This addition generates the most stable carbocation intermediate, which is then rapidly attacked by the nucleophilic anion to yield the final substituted product. Because the stability of the intermediate carbocation determines the reaction path, the product resulting from the most substituted intermediate is formed as the major product.

The major product is dictated by the relative stability of the reactive intermediate, where the transition state leading to the most stable configuration-typically the most substituted carbocation or the least sterically hindered pathway-determines the regiochemical outcome. By minimizing steric hindrance and maximizing electronic stabilization through hyperconjugation and inductive effects, the reaction pathway selectively favors the formation of the structure shown in option D over other constitutional isomers. This control ensures that the activation energy barrier, represented by $\Delta G^\ddagger$, is significantly lower for the chosen pathway than for alternative routes, thereby yielding the specified product as the dominant species.

The reaction proceeds via an electrophilic addition mechanism where the initial attack of the electrophile generates the most stable carbocation intermediate, typically a secondary or tertiary carbocation, in accordance with Markovnikov's rule. Subsequent nucleophilic attack at this stable cationic center yields the thermodynamically favored product, as represented in option B, which is stabilized by hyperconjugation and the inductive effects of adjacent alkyl groups.

The regioselectivity of electrophilic substitution on aromatic rings is primarily dictated by the electronic nature of existing substituents, where electron-donating groups exert a $+M$ effect that increases electron density at ortho and para positions, thereby directing the incoming electrophile to these locations. While resonance effects significantly activate specific positions by stabilizing the arenium ion intermediate, inductive effects may create competing influences, yet the resonance contribution typically dominates the product distribution. The formation of product B is favored over other isomers because the transition state leading to this structure maximizes the stabilization of the carbocation intermediate while minimizing steric repulsion between the incoming electrophile and existing substituents.

The reaction with $\mathrm{Br_2}$ and $\mathrm{NaOH}$ is the Hofmann bromamide degradation, which converts a primary amide into a primary amine by removing the carbonyl carbon. Since the product $\mathrm{C_3H_9N}$ undergoes the carbylamine test with $\mathrm{CHCl_3}$ and $\mathrm{KOH}$, it must be a primary amine. Because the degradation process results in a three-carbon amine, the original amide must have had four carbon atoms, making $\mathrm{CH_3CH_2CH_2CONH_2}$ the necessary precursor.

Benzene diazonium chloride undergoes an electrophilic aromatic substitution reaction known as azo coupling when treated with aniline in a weakly acidic medium. The diazonium cation acts as a weak electrophile and attacks the electron-rich para position of the aniline ring to yield p-aminoazobenzene as the final product. The reaction is represented as follows:

$C_6H_5N_2^+Cl^- + C_6H_5NH_2 \xrightarrow{H^+} C_6H_5-N=N-C_6H_4NH_2 + HCl$

The major product is determined by the relative stability of the intermediate species formed during the reaction, where the pathway proceeding through the most stable intermediate-typically the most substituted carbocation-dominates the reaction kinetics. According to the principle of regioselectivity, the reagent attacks the electrophilic center in a manner that maximizes the electronic stabilization of the transition state, following the carbocation stability order of $3^{\circ} > 2^{\circ} > 1^{\circ}$. This preference ensures that the reaction path with the lowest activation energy yields the thermodynamic product, which corresponds to the structure depicted in option C.