Amines – JEE Main ChemistryPractice Questions & PYQs

Dehydrohalogenation of alkyl halides using a strong base such as alcoholic $KOH$ follows an $E2$ elimination mechanism, where the major product is determined by Saytzeff's rule. This rule dictates that the more substituted alkene is the major product due to greater stability, which arises from increased hyperconjugation and inductive effects associated with alkyl groups attached to the double-bonded carbons. Consequently, the transition state leading to the most stable alkene has the lowest activation energy, making it the dominant pathway.

The liberation of $\mathrm{N}_2$ gas upon treatment with $\mathrm{HNO}_2$ is a definitive characteristic of primary aliphatic amines, which form unstable diazonium salts that readily decompose. Furthermore, the reaction with $\mathrm{PhSO}_2\text{Cl}$ yielding a sulfonamide soluble in $\mathrm{KOH}$ confirms the presence of an acidic nitrogen-bound hydrogen atom, which is a feature specific to primary amines. For the molecular formula $\mathrm{C}_4\mathrm{H}_{11}\mathrm{N}$, the structure possessing a chiral center to exhibit optical activity while maintaining these properties is butan-2-amine, where the chiral carbon at the second position creates the necessary asymmetry.

Basicity in amines is inversely proportional to $pK_b$, so higher values indicate lower basic strength. Aniline exhibits the highest $pK_b$ value of 9.38 because resonance delocalization of the lone pair into the aromatic ring significantly hinders protonation. Among the remaining aliphatic amines, N-Ethylethanamine achieves the lowest $pK_b$ of 3.00 through a favorable balance of inductive effects and solvation. The primary amine Ethanamine and the tertiary amine N, N-Diethylethanamine display intermediate $pK_b$ values of 3.29 and 3.25 respectively, where steric factors and solvation dynamics play a crucial role in determining their basic nature.

The reaction proceeds via an intramolecular nucleophilic attack where the nitrogen atom acts as the initial nucleophile. The lone pair of electrons on the $-\mathrm{NH}_2$ group targets the electrophilic carbon, establishing a bond that bridges the nitrogen to the reaction center. Subsequently, the oxygen atom utilizes its lone pair of electrons to attack within the intermediate, completing the closure of the ring to yield the final cyclic structure.

The conversion of chlorobenzene to aniline occurs via the ammonolysis of aryl halides, a nucleophilic substitution reaction where the halogen atom is replaced by an amino group ($-NH_2$). When chlorobenzene is treated with ammonia in the presence of copper(I) oxide ($Cu_2O$) at elevated temperatures and pressures, the reaction proceeds as follows:
$C_6H_5Cl + 2NH_3 \xrightarrow{Cu_2O, \Delta} C_6H_5NH_2 + NH_4Cl$
This reaction successfully facilitates the nucleophilic substitution on the stable aromatic ring to yield aniline as the product.

Pure aniline exists as a colorless liquid, a property shared by many other arylamines in their pristine state. Upon exposure to air and light, these compounds undergo an oxidation reaction that turns them dark brown, which demonstrates that the claim attributing this color change to atmospheric reduction is chemically inaccurate.

The reduction of propanamide with lithium aluminum hydride ($LiAlH_4$) serves as the first step, yielding the primary amine, propan-1-amine ($A$), by converting the carbonyl functional group into a methylene group without altering the carbon chain length. This primary amine subsequently undergoes the carbylamine reaction upon treatment with chloroform ($CHCl_3$) and alcoholic potassium hydroxide ($KOH$), leading to the formation of propyl isocyanide ($B$), which is characterized by the distinct functional group ($-NC$).

This reaction sequence employs the amino group as a directing group to enable the controlled electrophilic substitution of the benzene ring before its eventual removal. The nitro group is initially reduced to an amine using $\mathrm{Fe}$ and $\mathrm{H}^{+}$, followed by electrophilic bromination with aqueous $\mathrm{Br}_2$, which activates the ortho and para positions for bromine substitution. Subsequently, the amino group is converted into a diazonium salt using nitrous acid ($\mathrm{HNO}_2$), allowing for its complete removal via reduction with $\mathrm{H}_3\mathrm{PO}_2$ to yield the final product.

The evolution of gas upon reaction with $\mathrm{NaHCO}_3$ is a diagnostic test for carboxylic acids, which release $\mathrm{CO}_2$ gas, whereas the reaction of $R$ with Hinsberg's reagent ($\mathrm{PhSO}_2\mathrm{Cl}$) to yield a solid soluble in $\mathrm{NaOH}$ confirms it is a primary amine, as the resulting sulfonamide retains an acidic $N-H$ proton. Because compound $P$ is characterized as neutral, it must lack the acidic or basic functional groups that exhibit such reactivity, distinguishing it from compounds like acids or amines. Consequently, the structure represented in option B contains an amide functional group, which fulfills the requirement of being a neutral compound under these experimental conditions.

The interaction between $o$-phenylenediamine and $\text{HNO}_2$, usually prepared from $\text{NaNO}_2$ and $\text{HCl}$ under cold conditions, is governed by the mechanism of diazotization followed by rapid intramolecular cyclization. Because the two amino groups are located on adjacent carbon atoms of the benzene ring, the formation of the diazonium intermediate at one amino group immediately invites the neighboring amino group to perform a nucleophilic attack on the diazonium nitrogen. This swift reaction closes the five-membered heterocyclic ring, resulting in the formation of $1\text{H-benzotriazole}$ as the final stable product.

Reduction of an alkyl nitrile using $LiAlH_4$ produces a primary amine, which in this case yields propan-1-amine ($CH_3CH_2CH_2NH_2$). When this primary amine is treated with $NaNO_2$ and $HCl$, it undergoes diazotization to form a highly unstable alkyldiazonium salt that immediately loses nitrogen gas, generating a primary carbocation intermediate. This primary carbocation undergoes a rapid 1,2-hydride shift to form the more stable secondary carbocation ($CH_3CH^+CH_3$), which subsequently reacts with water to form a mixture of propan-1-ol and propan-2-ol as the final product.

This reaction sequence is an application of organometallic chemistry involving the synthesis of a primary alcohol via the addition of a Grignard reagent to formaldehyde. Toluene undergoes photo-chlorination with $Cl_2/h\nu$ to form benzyl chloride, which is subsequently hydrolyzed to benzyl alcohol and then converted to benzyl bromide using $PBr_3$. Treatment of benzyl bromide with magnesium in ether generates benzyl magnesium bromide, which performs a nucleophilic attack on the electrophilic carbonyl carbon of formaldehyde to yield a magnesium alkoxide intermediate, which subsequently undergoes protonation during acidic workup to produce 2-phenylethanol as the final product.

The reaction involving ferric ions ($Fe^{3+}$) and thiocyanate ions ($SCN^-$) results in the formation of the coordination complex $[Fe(SCN)(H_2O)_5]^{2+}$, which imparts an intense blood-red color to the solution. This characteristic coloration occurs due to the ligand-to-metal charge transfer within the complex, providing a standard qualitative test for the identification of iron (III).

The identification of isomeric amines relies on distinct chemical properties: Gabriel phthalimide synthesis is exclusive to primary alkyl amines, the Hinsberg test identifies secondary amines by the formation of an alkali-insoluble sulfonamide, and the azo dye test confirms the presence of a primary aromatic amine. For the molecular formula $\mathrm{C}_8 \mathrm{H}_{11} \mathrm{N}$, isomer $P$ is identified as a primary aliphatic amine, as this class is specifically prepared by the Gabriel phthalimide synthesis. Isomer $Q$ represents a secondary amine, producing a sulfonamide derivative that remains insoluble in $\mathrm{NaOH}$ due to the absence of an acidic hydrogen on the nitrogen atom. Finally, isomer $R$ acts as a primary aromatic amine, which undergoes diazotization and subsequent coupling with $\beta$-naphthol to yield the characteristic red azo dye.

The major product of this reaction is governed by the stability of the intermediate carbocation, which follows the order of tertiary $>$ secondary $>$ primary. The reaction proceeds through a rearrangement step, such as a hydride or alkyl shift, to convert a less stable carbocation into a more stable one before the final nucleophilic attack or elimination occurs. This thermodynamically controlled pathway ensures that the product corresponding to the most stable intermediate is formed in the highest yield, adhering to the principles of reaction energetics.

The catalytic hydrogenation of nitrobenzene using hydrogen gas in the presence of palladium ($\mathrm{Pd}$) reduces the nitro group ($\mathrm{-NO_2}$) to a primary amino group ($\mathrm{-NH_2}$), resulting in aniline as the intermediate product $[A]$. Treatment of the resulting aniline with acetic anhydride ($\mathrm{(CH_3CO)_2O}$) in a pyridine medium induces an acylation reaction where the amino group undergoes nucleophilic attack on the carbonyl carbon of the acetic anhydride to yield acetanilide ($\mathrm{C_6H_5NHCOCH_3}$) as the final product $[B]$. These consecutive transformations demonstrate the functional group conversion from an aromatic nitro compound to an amine and finally to an amide.

Organic named reactions are uniquely identified by the specific reagents required to facilitate their respective chemical transformations. The Hoffmann Degradation requires the application of $\mathrm{Br}_2$ and $\mathrm{NaOH}$, whereas the Clemenson reduction is characterized by the use of $\mathrm{Zn}-\mathrm{Hg}/\mathrm{HCl}$. Furthermore, the Cannizaro reaction proceeds with $\mathrm{Conc. \ KOH}, \Delta$, and the Reimer-Tiemann reaction relies on $\mathrm{CHCl}_3, \mathrm{NaOH}/\mathrm{H}_3\mathrm{O}^{+}$. Identifying these fundamental reaction types through their standard reagent counterparts clarifies the relationships between the two lists.

The reactivity of 1-bromopropane with these specific reagents depends on whether the reagent acts primarily as a base or a nucleophile. Treating 1-bromopropane with alcoholic $\mathrm{KOH}$ initiates an elimination reaction, specifically dehydrohalogenation, which removes hydrogen and bromine to yield an alkene (A-III). Substituting the bromide with a cyanide group occurs when using $\mathrm{KCN}$, as the carbon of the cyanide ion acts as a nucleophile to generate a nitrile (B-I). Meanwhile, the reaction with silver nitrite ($\mathrm{AgNO}_2$) directs the nitrogen atom to bond with the alkyl chain, producing a nitroalkane (C-IV). Finally, when 1-bromopropane is treated with silver acetate ($\mathrm{CH}_3\mathrm{COOAg}$), the silver ions assist in removing the halide, allowing the acetate group to bond with the alkyl chain to form an ester (D-II).

The reaction proceeds through the nucleophilic addition of a Grignard reagent to the carbonyl carbon of the ester, occurring in two successive steps to yield a tertiary alcohol. In the first step, the Grignard reagent attacks the ester to form a ketone intermediate after the elimination of the leaving alkoxy group. This ketone intermediate is highly reactive and undergoes a second nucleophilic attack by another equivalent of the Grignard reagent to form a magnesium alkoxide, which is finally converted into the tertiary alcohol, $R-C(OH)(R')_2$, upon acidic workup.

Propanamine ($CH_3CH_2CH_2NH_2$) and N-methylethanamine ($CH_3CH_2NHCH_3$) act as functional isomers due to the difference in their amine classification, while hexan-2-one and hexan-3-one are metamers because the carbonyl group is flanked by alkyl chains of different lengths. Ethanamide ($CH_3CONH_2$) and hydroxyethanimine ($CH_3C(OH)=NH$) represent a classic case of tautomerism, and o-nitrophenol and p-nitrophenol serve as positional isomers because the location of the nitro substituent on the aromatic ring varies.