Amines – JEE Main ChemistryPractice Questions & PYQs

Hinsberg's reagent, benzenesulfonyl chloride ($C_6H_5SO_2Cl$), reacts specifically with primary ($1^\circ$) and secondary ($2^\circ$) amines to form sulfonamides through a nucleophilic substitution mechanism. Tertiary ($3^\circ$) amines are inert toward this reagent because they lack the necessary hydrogen atom on the nitrogen to eliminate hydrogen chloride and facilitate product formation. Five compounds within the given set possess the required $1^\circ$ or $2^\circ$ amine functional groups, resulting in their reactivity with the reagent.

The reaction of phthalimide with KOH followed by benzyl chloride follows the Gabriel synthesis mechanism, resulting in the formation of N-benzylphthalimide through the substitution of the hydrogen atom on the nitrogen with a benzyl group. This product contains a phthalimide core fused to a benzene ring and two carbonyl groups, along with an appended benzyl moiety.

The phthalimide core provides three $\pi$ bonds from its benzene ring and two $\pi$ bonds from its two carbonyl groups. The benzyl group contributes an additional three $\pi$ bonds from its own phenyl ring. Adding these individual contributions, the phthalimide portion contains five $\pi$ bonds, while the benzyl group adds three $\pi$ bonds, leading to a total of 8 $\pi$ bonds in the final molecule.

The confirmatory test for aromatic primary amines is the azo dye test, which involves the formation of a diazonium salt followed by coupling with a phenol. Reagent (A) consists of sodium nitrite ($NaNO_2$) and hydrochloric acid ($HCl$) at $0-5^\circ C$, which facilitates the diazotization of the aromatic amine into a stable diazonium chloride intermediate. Reagent (B) is $\beta$-naphthol in an alkaline solution (aqueous $NaOH$), which reacts with the diazonium salt through electrophilic aromatic substitution to produce a characteristic orange or red azo dye.

Aniline reacts with concentrated sulfuric acid at 453-473 K to produce sulfanilic acid ($p$-aminobenzenesulfonic acid), which contains both nitrogen and sulfur; during Lassaigne's test, these elements combine to form sodium thiocyanate, which yields a characteristic blood-red color upon the addition of $\mathrm{Fe}^{3+}$ ions. In the context of Friedel-Crafts reactions, the amino group of aniline acts as a Lewis base and reacts with the Lewis acid catalyst $\mathrm{AlCl}_3$ to form an adduct, creating a positively charged nitrogen atom that withdraws electrons from the ring and effectively deactivates it. Because both chemical descriptions accurately reflect the behavior of aniline under the specified conditions, both statements are true.

The coupling reaction between aniline and benzenediazonium chloride proceeds in a 1:1 molar ratio to form aniline yellow, with the molar mass of aniline being 93 g/mol and that of aniline yellow being 197 g/mol. Given that 279 g of aniline is used, the number of moles of aniline is 279 / 93 = 3 mol. Since one equivalent of the diazonium salt reacts according to the stoichiometry $C_6H_5NH_2 + C_6H_5N_2Cl \rightarrow C_{12}H_{11}N_3 + HCl$, 3 mol of aniline yellow is produced, leading to a maximum mass of 3 * 197 = 591 g.

Ammonolysis of benzyl chloride generates a mixture containing primary, secondary, and tertiary amines. Because tertiary amines do not react with p-toluenesulphonyl chloride, the reaction mixture remains clear, identifying (X) as the tertiary amine, tribenzylamine. The chemical formula for tribenzylamine is $(C_6H_5CH_2)_3N$. Its molar mass is determined by calculating the sum of the atomic masses for three benzyl groups and one nitrogen atom: $3 \times (7 \times 12 + 7 \times 1) + 14 = 287$ g mol^-1.

The condensation reaction between ethanal and semicarbazide proceeds via nucleophilic addition followed by the elimination of a water molecule to form a semicarbazone.

$CH_3CHO + NH_2NHCONH_2 \rightarrow CH_3CH=NNHCONH_2 + H_2O$

The structure of the resulting ethanal semicarbazone, $CH_3CH=NNHCONH_2$, contains one nitrogen atom in the imine linkage ($C=N$), one nitrogen atom in the secondary amine group ($-NH-$), and one nitrogen atom in the terminal amide group ($-CONH_2$). Summing these contributions results in a total of 3 nitrogen atoms.

The reaction follows the principle of stoichiometry, where one mole of aniline reacts to yield one mole of the orange dye. First, determine the amount of aniline by dividing the 9.3 g mass by the 93 g/mol molar mass, which results in 0.1 mol of aniline. Since the reaction stoichiometry is $1:1$, this same molar amount of product is formed. Multiplying 0.1 mol by the 199 g/mol molar mass of the orange dye gives a total mass of 19.9 g, which rounds to 20 g when considering the nearest integer.

Percentage yield represents the efficiency of a chemical reaction, defined as the ratio of the experimentally obtained product to the theoretical mass predicted by stoichiometry. Because the reaction between aniline ($C_6H_5NH_2$) and bromine water produces 2,4,6-tribromoaniline in a 1:1 molar ratio, the mass relationship is determined by the molar masses of the reactants and products. Specifically, 93 g of aniline theoretically produces 330 g of 2,4,6-tribromoaniline; therefore, a starting mass of 9.3 g of aniline should yield 33 g of the product under ideal conditions.

To find the percentage yield, we compare the actual output to this theoretical value using the expression:
$\text{Percentage Yield} = \frac{\text{Actual mass}}{\text{Theoretical mass}} \times 100$
Substituting the actual yield of 26.4 g and the calculated theoretical mass of 33 g gives $\frac{26.4}{33} \times 100$, which results in a percentage yield of 80%.

Friedel-Crafts reactions function through electrophilic aromatic substitution using a Lewis acid catalyst like $AlCl_3$, a process that demands an adequately electron-rich ring to generate the required sigma complex. Strongly electron-withdrawing groups, particularly nitro groups, deplete the electron density of the benzene ring too severely to allow electrophilic attack, whereas the presence of amine groups leads to the formation of a Lewis acid-base adduct that binds the catalyst and effectively prevents catalytic activity. Consequently, nitrobenzene, aniline, m-nitroaniline, and m-dinitrobenzene are fundamentally unreactive under these conditions, bringing the total number of compounds incapable of undergoing Friedel-Crafts reactions to four.

The reaction proceeds via an electrophilic addition mechanism governed by Markovnikov's rule, where the electrophilic hydrogen adds to the carbon atom of the double bond that already possesses a greater number of hydrogen substituents. This sequence facilitates the formation of the more stable carbocation intermediate, such as a secondary carbocation ($R-CH^+-R$), which is energetically favored over the primary carbocation ($R-CH_2-CH_2^+$). Subsequent nucleophilic attack by the halide ion at the site of this more stable carbocation results in the substituted alkane product represented in the structure.

Fluoride ion acceptors such as Antimony pentafluoride ($SbF_5$) react with Xenon fluorides by abstracting a fluoride ion to generate stable ionic compounds. This process occurs because $SbF_5$ acts as a strong Lewis acid, accepting a fluoride ion from $XeF_6$ to produce the $[XeF_5]^+$ cation and $[SbF_6]^-$ anion according to the equation $XeF_6 + SbF_5 \rightarrow [XeF_5]^+[SbF_6]^-$

The fundamental principle behind Lassaigne's test involves fusing an organic compound with metallic sodium to convert covalently bonded nitrogen, sulfur, or halogens into ionic salts that can be detected chemically. For nitrogen detection, the nitrogen from the sample must react with sodium and carbon atoms present in the organic molecule to generate sodium cyanide. Because hydrazine ($\mathrm{NH}_2-\mathrm{NH}_2$) does not contain any carbon atoms, this required carbon-nitrogen fusion reaction cannot take place, preventing the formation of the cyanide species necessary to yield a positive test result.

Picric acid is chemically identified as 2,4,6-trinitrophenol, therefore the assertion that it is 2,4,6-trinitrotoluene is incorrect as that structure represents TNT. The preparation of picric acid involves the sulphonation of phenol to form phenol-2,4-disulphonic acid, which is then subjected to nitration using concentrated $\mathrm{HNO}_3$ to replace the sulphonic acid groups with nitro groups, confirming the validity of the second statement.

Acetylation involves replacing a hydrogen atom on an amino group with an acetyl group, which increases the molecular mass because the acetyl group is heavier than the hydrogen atom by 42 g mol⁻¹. The number of reactive amino sites can be determined by analyzing the total change in molar mass from the original compound to the acetylated product.

The total mass increases from 108 to 192, a difference of:
$192 - 108 = 84$
Dividing this total gain by the increase associated with a single amino group reveals the quantity of amino groups:
$\frac{84}{42} = 2$

The reaction of an amide with bromine and aqueous potassium hydroxide is known as the Hofmann bromamide degradation, which converts a primary amide into a primary amine containing one fewer carbon atom. Propanamide ($\mathrm{CH}_3\mathrm{CH}_2\mathrm{CONH}_2$) undergoes this degradation, where the carbonyl carbon is removed as a carbonate salt, yielding ethylamine ($\mathrm{CH}_3\mathrm{CH}_2\mathrm{NH}_2$) as the major organic product. The transformation proceeds according to the equation: $\mathrm{CH}_3\mathrm{CH}_2\mathrm{CONH}_2 + \mathrm{Br}_2 + 4\mathrm{KOH} \rightarrow \mathrm{CH}_3\mathrm{CH}_2\mathrm{NH}_2 + \mathrm{K}_2\mathrm{CO}_3 + 2\mathrm{KBr} + 2\mathrm{H}_2\mathrm{O}$.

Aromatic amine synthesis generally relies on specific reduction or substitution mechanisms capable of introducing an amino group onto an aromatic ring. In the provided set of transformations, the second reaction fails to yield an amine because Gabriel phthalimide synthesis requires an $S_N2$ mechanism, which is fundamentally ineffective on aryl halides due to their inability to act as suitable substrates for such nucleophilic attacks. Since the first, third, and fourth reactions successfully proceed through valid pathways to form aromatic amines, a total of 3 transformations lead to the desired products.

The Gabriel phthalimide synthesis is exclusively used for preparing primary aliphatic amines and cannot be used for the preparation of aromatic primary amines, or anilines, because aryl halides do not undergo nucleophilic substitution with the phthalimide ion. Given the molecular formula $\mathrm{C}_8\mathrm{H}_{11}\mathrm{N}$, the degree of unsaturation is 4, which corresponds to a benzene ring. To be synthesized by this method, the nitrogen must be attached to an aliphatic side chain, meaning we are looking for isomeric primary aralkyl amines.

The possible structural isomers of this type are 2-phenylethanamine, 1-phenylethanamine, and the three positional isomers of methylbenzylamine, which are ortho-methylbenzylamine, meta-methylbenzylamine, and para-methylbenzylamine. Analyzing these structures, 2-phenylethanamine ($\mathrm{C_6H_5-CH_2-CH_2-NH_2}$) is an achiral isomer. The structure 1-phenylethanamine ($\mathrm{C_6H_5-CH(NH_2)-CH_3}$) contains a chiral carbon atom, meaning it exists as two distinct enantiomers, (R)-1-phenylethanamine and (S)-1-phenylethanamine. The three methylbenzylamines ($\mathrm{CH_3-C_6H_4-CH_2-NH_2}$) are all achiral. Summing these isomers, there is 1 for 2-phenylethanamine, 2 for the enantiomers of 1-phenylethanamine, and 3 for the methylbenzylamine structural isomers, yielding a total of 6 isomeric primary amines.

Clemmensen reduction employs an acidic medium of zinc amalgam and hydrochloric acid, which effectively reduces a keto group to a methylene group without affecting the carboxylic acid functional group, making the statement true. In contrast, Wolff-Kishner reduction proceeds in a strongly basic medium that deprotonates the carboxylic acid to form a carboxylate salt, meaning the free carboxylic acid is not produced directly under these conditions. Consequently, Statement I is valid as the acidic environment tolerates the carboxylic acid moiety, whereas Statement II is invalid because the basic conditions necessitate the formation of a carboxylate salt rather than the carboxylic acid.

The stability of resonance structures is governed by a fundamental hierarchy starting with the octet rule, where structures with complete octets are significantly more stable than those with incomplete octets. Further stabilization occurs with a greater number of covalent bonds and by minimizing formal charge separation, while the placement of negative charges on more electronegative atoms provides additional energetic favorability. By applying these criteria, the relative order of stability is determined to be $C < A < B < D$.