Amines – JEE Main ChemistryPractice Questions & PYQs

The time taken to attain maximum height in projectile motion is determined by the vertical component of initial velocity $u_y = u \sin \theta$ and the acceleration due to gravity $g$, utilizing the kinematic equation $v_y = u_y - gt$. Because the vertical velocity at the apex of the trajectory is zero, substituting $v_y = 0$ into the equation yields $0 = u \sin \theta - gt$. Rearranging this expression to solve for time results in $t = \frac{u \sin \theta}{g}$, which signifies the duration required to reach the peak altitude.

The Hoffmann-Bromide degradation is a chemical process where a primary amide, $R-\text{CONH}_2$, reacts with bromine and potassium hydroxide to yield a primary amine, $R-\text{NH}_2$, containing one fewer carbon atom. A critical step in this mechanism involves the rearrangement of the N-brominated species, which generates an alkyl isocyanate intermediate with the structure $R-\text{N}=\text{C}=\text{O}$. This isocyanate is the decisive intermediate that subsequently undergoes hydrolysis to form the final amine product while releasing carbon dioxide.

Benzenesulphonyl chloride is widely recognized in organic synthesis as the Hinsberg reagent, a key chemical used to classify and separate various classes of amines. During the Hoffmann bromamide reaction, an amide undergoes degradation to produce a primary amine having one carbon atom less, proceeding through the crucial formation of an isocyanate intermediate. The Carbylamine reaction serves as a specific diagnostic test for primary amines, while the Hoffmann orientation refers to elimination processes that prioritize the less substituted alkene, effectively following the anti-Saytzeff rule.

Dehydrohalogenation of 2-bromopentane using a sterically hindered bulky base, such as potassium tert-butoxide, proceeds via an E2 mechanism where the base selectively abstracts the more accessible proton from the terminal carbon to yield the less substituted alkene as the major product. This regioselectivity produces 1-pentene ($CH_3CH_2CH_2CH=CH_2$) and 2-pentene ($CH_3CH_2CH=CHCH_3$) as Hofmann products, distinguishing them from the more substituted Saytzeff products favored by smaller, unhindered bases.

The relative stability of enamines is primarily governed by the interplay of steric hindrance and the electronic environment surrounding the double bond. Although these structures are interconvertible, structure C is the most stable because it effectively minimizes steric repulsion among the $-\mathrm{CH}_3$ substituents, thereby allowing the molecule to exist in its most thermodynamically favorable state.

The reaction proceeds via a nucleophilic acyl substitution mechanism followed by a subsequent nucleophilic addition, where the ester functional group reacts with excess Grignard reagent to form a tertiary alcohol. Upon the first attack of the phenylmagnesium bromide ($PhMgBr$) on the carbonyl carbon of the ester, the alkoxy group is eliminated as a leaving group, generating a ketone intermediate. This ketone immediately undergoes a second nucleophilic addition by another equivalent of the Grignard reagent to form an alkoxide species, which upon subsequent acid hydrolysis yields the corresponding tertiary alcohol as the major product.

When a primary aliphatic amine reacts with nitrous acid, generated from $\text{NaNO}_2$ and $\text{HCl}$ at 273 K, it forms an alkyl diazonium salt, $R-\text{N}_2^{+}$. These aliphatic diazonium intermediates are highly unstable and undergo decomposition to release nitrogen gas as the temperature is raised to 298 K, generating a reactive carbocation, $R^{+}$. This carbocation rapidly reacts with water, $\text{H}_2\text{O}$, present in the medium to ultimately yield an alcohol, $R-\text{OH}$, as the final product.

The conversion of an alcohol to an amine with an increased carbon chain length necessitates the activation of the hydroxyl group followed by nucleophilic displacement with a cyanide ion and subsequent reduction of the nitrile. Treating propan-1-ol with $\mathrm{SOCl}_2$ converts the alcohol into 1-chloropropane, which then undergoes substitution with $\mathrm{KCN}$ to form butanenitrile. The final stage involves the reduction of the nitrile using $\mathrm{H}_2/\mathrm{Ni}$ or $\mathrm{Na}(\mathrm{Hg})/\mathrm{C}_2\mathrm{H}_5\mathrm{OH}$ to yield $n$-butylamine.

The strength of a Brønsted base is fundamentally determined by the availability of the lone pair on the nitrogen atom for protonation. Compound D stands out as the strongest base because it exhibits the maximum $+I$ effect, which effectively increases the electron density on the nitrogen. Additionally, since the lone pair on the nitrogen atom is not in a dynamic state and remains localized rather than delocalized, it is highly accessible for donation, thereby establishing it as the superior base among the provided choices.

Aniline acts as a Lewis base due to the lone pair on its nitrogen atom, which coordinates with the Lewis acid catalyst (such as $AlCl_3$) required for the Friedel-Crafts alkylation reaction. This acid-base reaction forms an adduct $[C_6H_5NH_2 \rightarrow AlCl_3]$ in which the nitrogen atom acquires a positive charge. This electron-withdrawing, positively charged nitrogen group strongly deactivates the benzene ring, thereby preventing the Friedel-Crafts alkylation of aniline.

The reaction of aniline with nitrous acid, generated in situ from $NaNO_2$ and $HCl$ at $0-5^\circ C$, follows the process of diazotization to form the stable intermediate benzenediazonium chloride, which is product A. In the second step, the benzenediazonium cation acts as an electrophile in an electrophilic aromatic substitution reaction with phenol in a basic medium, specifically attacking the para-position to form p-hydroxyazobenzene, which is product B. This coupling reaction is a characteristic test for aromatic amines and phenols, resulting in the formation of an azo dye with an extended conjugated system.

The Hinsberg reagent is a fundamental tool used in organic chemistry to classify amines by distinguishing between primary, secondary, and tertiary structures based on their specific chemical reactivity. This reagent is chemically identified as benzene sulphonyl chloride, represented by the formula $C_6H_5SO_2Cl$, and it provides a reliable qualitative method for identifying the degree of amine substitution in a given sample.

The stability of a diazonium cation is fundamentally determined by the electronic effects of the substituents attached to the aromatic ring. Since the diazonium group is inherently electron-deficient and carries a positive charge, it is stabilized by electron-donating groups that supply electron density to the ring and destabilized by electron-withdrawing groups that intensify the existing positive charge. Consequently, the presence of the electron-donating methyl group offers the greatest stability, followed by the unsubstituted benzenediazonium salt, while the weak electron-withdrawing chlorine and the strongly electron-withdrawing nitro group lead to progressively greater destabilization, establishing the stability order of $A > C > D > B$.

Decarboxylation of the six isomeric diaminobenzoic acids yields three distinct phenylenediamines, where the specific product formed depends on the arrangement of the amino groups relative to the carboxylic acid group. Based on the structural isomers, $m$-phenylenediamine is obtained from three parent acids (melting point $\approx 63^{\circ}\mathrm{C}$), $o$-phenylenediamine is obtained from two parent acids (melting point $\approx 104^{\circ}\mathrm{C}$), and $p$-phenylenediamine is obtained from a single parent acid, identifying it as product 'C'. Consequently, the melting point of product 'C', which is $p$-phenylenediamine, is $142^{\circ}\mathrm{C}$.

The nitrating mixture typically involves concentrated nitric and sulfuric acids, creating a strongly acidic environment that interacts directly with the aniline molecule. In this acidic medium, the basic amino group of aniline undergoes protonation to form the anilinium ion, $C_6H_5NH_3^+$. Because this ion is meta-directing, its formation during the reaction ensures that a significant amount of the meta-nitro isomer is produced alongside the ortho and para derivatives.

Pyridinium chlorochromate ($PCC$) acts as a mild, selective oxidizing agent that specifically converts primary alcohols into aldehydes while leaving sensitive functional groups like carbon-carbon double bonds intact. This reagent operates in anhydrous conditions, which prevents the formation of hydrated aldehyde intermediates that would otherwise be susceptible to further oxidation into carboxylic acids.

The extent of intermolecular association in amines is fundamentally determined by the number of hydrogen atoms available for hydrogen bonding. Primary amines, represented by the general structure $R-NH_2$, contain two hydrogen atoms attached to the nitrogen, whereas secondary amines, $R_2NH$, contain only one. Because of this structural difference, the hydrogen-bonding network in primary amines is more extensive and prominent than that found in secondary amines, making the claim that primary amines exhibit less intermolecular association than secondary amines factually incorrect.

The reaction proceeds through the electrophilic attack of the reagent on the most nucleophilic site of the starting substrate, followed by the departure of a leaving group to restore the thermodynamic stability of the intermediate. The regioselectivity of this transformation is dictated by the electronic and steric effects of the existing substituents, which stabilize the transition state leading specifically to the structure defined as compound C. This pathway minimizes the activation energy by directing the incoming group to the position that yields the most stable substituted product according to the principles of kinetic control.

The conversion of nitrobenzene to aniline is a reduction process that requires specific agents to replace the oxygen atoms with hydrogen. Nitrobenzene undergoes this transformation effectively when treated with metal-acid systems such as $\text{Sn-HCl}$, $\text{Fe-HCl}$, or $\text{Zn-HCl}$, all of which provide nascent hydrogen to facilitate the reaction. Additionally, catalytic hydrogenation employing $\text{H}_2$ gas with either $\text{Pd}$ or $\text{Raney Ni}$ catalysts offers an equally efficient pathway to produce aniline. While $\text{Sn-NH}_4\text{OH}$ appears in the provided list, it is chemically unsuitable for achieving this complete reduction. Accounting for the three metal-acid pairs and two catalytic hydrogenation methods, the total number of reagents that successfully convert nitrobenzene into aniline is 5.

The Hofmann bromamide degradation of benzamide involves the removal of the carbonyl carbon, transforming the amide into the primary aromatic amine aniline. When this aniline is subsequently heated with $CHCl_3$ and $NaOH$, it undergoes the characteristic carbylamine reaction, a process specific to primary amines that yields benzene isocyanide.