Amines – JEE Main ChemistryPractice Questions & PYQs

Electrophilic addition to the asymmetric alkene follows Markovnikov's rule, which dictates that the electrophile ($H^+$) binds to the carbon atom with the greater number of hydrogen substituents to generate the more stable carbocation intermediate. The greater stability of this intermediate, reinforced by inductive effects and hyperconjugation, directs the subsequent nucleophilic attack to the more substituted carbon, confirming that the structure in option C represents the favored major product.

Friedel-Crafts acylation of benzene with acetyl chloride in the presence of anhydrous $AlCl_3$ results in the electrophilic substitution of an acetyl group onto the benzene ring to yield acetophenone ($C_6H_5COCH_3$) as product A. Subsequent treatment of acetophenone with $Zn-Hg$ and concentrated $HCl$ facilitates a Clemmensen reduction, which converts the carbonyl group into a methylene group, producing ethylbenzene ($C_6H_5CH_2CH_3$) as product B.

The rate of diazotisation of primary aromatic amines is primarily determined by the electron density on the nitrogen atom, as the rate-determining step involves the nucleophilic attack of the lone pair of the amine on the nitrosyl cation ($NO^+$). Electron-withdrawing groups decrease the electron density at the nitrogen, thereby reducing the rate of reaction, while electron-donating groups increase the electron density, accelerating the process. Thus, the increasing order from (d) to (a) reflects the transition from compounds substituted with strong electron-withdrawing groups to those with electron-donating groups, which increase the nucleophilicity of the amino group and facilitate the reaction.

Basicity is primarily determined by the availability of the lone pair of electrons on the nitrogen atom, where a higher electron density on the nitrogen atom makes it a stronger base. Electron-donating groups enhance this availability through the inductive effect, whereas resonance delocalization or electron-withdrawing groups decrease basicity by stabilizing the lone pair and making it less accessible for protonation. The specific electronic influence of the substituents in these compounds dictates their relative basicity, resulting in the increasing sequence of $b < a < d < c$.

A molecule forms a zwitterion only when it possesses both an acidic group to donate a proton and a basic group to accept that proton, enabling the formation of a stable dipolar ion at a neutral pH. The compounds in options A, B, and C contain both acidic and basic functional groups, allowing them to exist in equilibrium as zwitterions. However, the molecule represented in option D lacks the necessary basic amine group to accept a proton from its acidic moiety, preventing it from forming a zwitterion at $pH=7$.

The reactivity of aromatic compounds toward electrophilic substitution depends on the electron density of the benzene ring, which is enhanced by electron-donating groups and diminished by electron-withdrawing groups. The nitro group ($-NO_2$) is strongly deactivating, the chloro group ($-Cl$) is weakly deactivating, and the methyl group ($-CH_3$) is activating, resulting in the reactivity order of nitrobenzene < chlorobenzene < benzene < toluene. This sequence confirms that the arrangement provided in the correct option accurately reflects the increasing rate of nitration for the respective substituents.

During the nitration process, which uses a mixture of concentrated $HNO_3$ and $H_2SO_4$, aniline acts as a base and readily undergoes protonation to form the anilinium ion ($C_6H_5NH_3^+$). The resulting positively charged anilinium group acts as a strong deactivating group, which pulls electron density away from the benzene ring. Because of this electronic effect, the reaction is directed to the meta position, resulting in a significant yield of the meta product.

Basic strength in nitrogenous compounds is determined by the availability of the lone pair of electrons on the nitrogen atom, which is enhanced by the electron-donating $+I$ inductive effect of alkyl groups but decreased by resonance delocalization or steric hindrance. Aromatic amines and amides are generally the weakest bases because the lone pair on the nitrogen is delocalized into the aromatic ring or carbonyl group via resonance, significantly reducing its availability for protonation. In aqueous media, the basicity of aliphatic amines is governed by a balance between the $+I$ effect, which increases electron density, and the solvation of the conjugate acid, where steric bulk from alkyl groups creates hindrance; this typically results in secondary amines being the strongest bases, followed by primary and tertiary amines.

Electrophilic addition of $HBr$ to $CH_3-CH=C(CH_3)-CH_2-CH_3$ proceeds via the formation of the most stable carbocation intermediate in accordance with Markovnikov's rule. Protonation of the C2 carbon of the double bond results in a tertiary carbocation at the C3 position, which is significantly more stable than the secondary carbocation that would be formed by protonation at the C3 carbon. The bromide nucleophile subsequently attacks the tertiary carbocation at C3, leading to the formation of 3-bromo-3-methylpentane as the major product.

The basicity of nitrogen-containing compounds is dictated by the availability of a lone pair of electrons to act as a proton acceptor. When a nitrogen atom's lone pair is consumed by resonance to satisfy aromaticity, it becomes delocalized and is no longer free to interact with an acid. In this specific heterocyclic framework, the nitrogen at position 9 contributes its lone pair to the $\pi$-electron system to fulfill the $4n+2$ rule, which maintains the aromatic character of the ring. Consequently, because these electrons are inherently locked into the aromatic sextet, N9 fails to exhibit basic character, unlike the other nitrogen atoms in the structure whose lone pairs remain localized and available for donation.

The Balz-Schiemann reaction involves the synthesis of aryl fluorides by the thermal decomposition of aryldiazonium tetrafluoroborate intermediates. Upon heating, the salt undergoes a unimolecular decomposition process where the $C-N$ bond cleaves to release nitrogen gas ($N_2$) and boron trifluoride ($BF_3$), allowing the fluoride ion to bond directly to the aromatic ring without the need for copper catalysts.

The Hofmann bromamide degradation is a classical chemical transformation used to convert a primary amide into a primary amine with the loss of a single carbon atom. This reaction occurs in an alkaline medium where bromine reacts with the amide, leading to an intermediate isocyanate that subsequently hydrolyzes to yield the amine and a carbonate salt. Balancing the chemical equation for this process demonstrates that each mole of the amide requires 4 moles of $\mathrm{NaOH}$ and 1 mole of $\mathrm{Br}_2$ to complete the reaction and yield 1 mole of the corresponding amine, as represented by the balanced equation:

$\mathrm{RCONH_2} + 4\mathrm{NaOH} + \mathrm{Br}_2 \rightarrow \mathrm{RNH_2} + 2\mathrm{NaBr} + \mathrm{Na}_2\mathrm{CO}_3 + 2\mathrm{H}_2\mathrm{O}$

The distinction between primary, secondary, and tertiary amines is determined by their unique reactivity with benzenesulphonyl chloride, represented as $C_6H_5SO_2Cl$. Primary amines form an N-substituted sulfonamide that dissolves in alkali because of an acidic hydrogen on the nitrogen atom, while secondary amines yield an insoluble N,N-disubstituted sulfonamide because they lack that specific acidic hydrogen. Tertiary amines do not react at all with $C_6H_5SO_2Cl$ as they have no hydrogen attached to the nitrogen, providing a clear method to differentiate between all three classes.

The nucleophilic addition of the Grignard reagent $\text{CH}_3\text{MgBr}$ to the nitrile $\text{CH}_3\text{CH}_2\text{CH}_2\text{CN}$ generates an intermediate imine salt, which upon acidic hydrolysis yields the ketone pentan-2-one. The subsequent treatment with amalgamated zinc and hydrochloric acid initiates a Clemmensen reduction, which effectively reduces the carbonyl group of the pentan-2-one into a methylene group. This two-step transformation results in the formation of n-pentane, $\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{CH}_3$, which constitutes the product E.

This transformation is known as the carbylamine reaction, which serves as a definitive diagnostic test specifically for primary amines. When an aliphatic primary amine is heated with chloroform and alcoholic potassium hydroxide, it undergoes a chemical change that yields an alkyl isocyanide as the primary organic product, along with potassium chloride and water as byproducts. The balanced chemical equation for this reaction is $\mathrm{RNH}_2 + \mathrm{CHCl}_3 + 3\mathrm{KOH} \rightarrow \mathrm{RNC} + 3\mathrm{KCl} + 3\mathrm{H}_2\mathrm{O}$.

The basic strength of an amine is inversely related to its $pK_b$ value, meaning that the strongest base will exhibit the smallest $pK_b$. Aromatic amines such as $C_6H_5NH_2$ are significantly weaker than aliphatic amines because the lone pair of electrons on the nitrogen atom is delocalized into the benzene ring. Among the aliphatic amines in water, the order of basicity is $(CH_3)_2NH > CH_3NH_2 > (CH_3)_3N$ due to the interplay between the inductive effect of the alkyl groups and steric hindrance during solvation. Consequently, $(CH_3)_2NH$ acts as the strongest base and possesses the smallest $pK_b$ value among the provided choices.

The process of acylation involves replacing a hydrogen atom from an amino group with an acetyl group, $\mathrm{CH}_3\mathrm{CO}-$. This substitution increases the molecular mass of the compound because the mass of the added acetyl group exceeds the mass of the hydrogen atom it replaces, leading to a consistent mass increase per reaction site.

Given that the molecular mass changes from 180 to 390, the total mass added during the reaction is $390 - 180 = 210$. Because each acetyl group contributes to a net mass increase of 42, dividing the total mass gain by this value determines the number of amino groups: $\frac{210}{42} = 5$. Thus, the original molecule contains 5 amino groups.

The Bhopal gas tragedy was triggered by the massive industrial leakage of methyl isocyanate, an intermediate compound essential for pesticide manufacturing. This highly toxic substance, chemically denoted as $CH_3NCO$, was inadvertently released into the environment when water contamination initiated a rapid, exothermic reaction within the storage tank.

The reaction sequence proceeds via the Hofmann Bromamide Degradation, where an amide ($\mathrm{RCONH}_2$) reacts with $\mathrm{Br}_2$ and $\mathrm{KOH}$ to yield a primary amine with one fewer carbon atom, $\mathrm{RNH}_2$. Since the final product is $\mathrm{CH}_3\mathrm{CH}_2\mathrm{NH}_2$, the intermediate amide C must be propanamide ($\mathrm{CH}_3\mathrm{CH}_2\mathrm{CONH}_2$). Propanamide is produced by heating the ammonium salt of propanoic acid, which is formed when propanoic acid (A) reacts with $\mathrm{NH}_3$, confirming that compound A is $\mathrm{CH}_3\mathrm{CH}_2\mathrm{COOH}$.

The preparation of aryl fluorides from primary aromatic amines is best achieved through diazotization followed by the Balz-Schiemann reaction. When a primary aromatic amine is treated with nitrous acid generated from $\text{NaNO}_2$ and $\text{HX}$ at low temperatures, it is converted into the corresponding arene diazonium salt, such as benzene diazonium chloride. This intermediate then reacts with fluoroboric acid (\text{HBF}_4$) to form a diazonium fluoroborate precipitate, which undergoes controlled thermal decomposition to yield the final aryl fluoride, known as fluorobenzene.