Organic Compounds Containing Nitrogen – NEET UG ChemistryPractice Questions & PYQs

Hydrolysis of alkyl isocyanides under acidic conditions yields a primary amine and formic acid. The resulting primary amine reacts with nitrous acid to form an alcohol, which in this case is propan-1-ol, a terminal alcohol that lacks the structural methyl-carbinol grouping required to test positive for the iodoform reaction. Meanwhile, the formic acid co-product contains an aldehyde-like hydrogen atom, enabling it to successfully reduce both Tollens' reagent and Fehling's solution.

Aromatic tertiary amines do not react at the nitrogen center when treated with $\mathrm{HNO}_2$, but instead undergo electrophilic aromatic substitution where the nitrosonium ion attacks the $p$-position of the benzene ring to yield green-colored $p$-nitrosoamines. Consequently, any reaction pathway depicting an aromatic tertiary amine forming an $N$-nitroso derivative or a diazonium salt is chemically incorrect.

Primary aliphatic amines uniquely react with nitrous acid to liberate nitrogen gas and yield an alcohol, which identifies compound $(A)$ as a primary amine with the molecular formula $\mathrm{C}_3\mathrm{H}_9\mathrm{N}$. Treating this amine with chloroform and caustic potash initiates the carbylamine reaction, producing an intermediate that upon reduction gives isopropylmethylamine. Because the reduction product features a branched three-carbon alkyl group, the carbon skeleton of the original reactant must be structured as isopropylamine rather than the straight-chain isomer.

The basicity of an amine relies heavily on the availability of the lone pair of electrons on its nitrogen atom for protonation. When this lone pair is delocalized through resonance with an aromatic ring, electron density decreases, which significantly reduces the compound's basic strength. However, in benzylamine, a methylene group separates the amino group from the benzene ring, preventing any resonance delocalization of the lone pair. Because this electron pair remains fully localized and readily available, the compound exhibits the highest basicity.

The reaction proceeds via the electrophilic addition of a proton to the double bond, generating the most stable carbocation intermediate in accordance with electronic effects. Subsequent attack by the nucleophile on this stable intermediate yields the corresponding substituted derivative as the major product. This regioselective pathway strictly follows Markovnikov's rule, ensuring the preferential formation of the thermodynamically favored isomer.

Primary aromatic amines react with nitrous acid at cold temperatures around 0-5°C to form stable diazonium salts rather than directly yielding phenols. Aliphatic primary amines react with nitrous acid to yield unstable aliphatic diazonium intermediates that rapidly decompose to release nitrogen gas and form alcohols. Meanwhile, alkyl amines possess electron-releasing alkyl groups that increase electron density on the nitrogen atom, making them stronger bases than both ammonia and aryl amines.

In the nitrating mixture composed of concentrated sulfuric and nitric acids, sulfuric acid is the stronger acid and therefore protonates nitric acid. Because nitric acid accepts a proton from sulfuric acid, it functions chemically as a Brønsted-Lowry base in this specific proton-transfer equilibrium. This protonation leads to the subsequent loss of water to generate the electrophilic nitronium ion required for aromatic substitution.

Secondary amines, regardless of whether they have aliphatic or aromatic substituents, readily react with nitrous acid to yield N-nitrosoamines as their characteristic products. During this transformation, the electrophilic nitrosonium ion from the acid attacks the amine nitrogen, displacing a proton to create the distinctive N-nitrosated functional group. This reliable reaction path provides a fundamental way to predict the chemical behavior of secondary amines in structural identification problems.

Aniline first undergoes diazotization to form a diazonium salt intermediate, which subsequently reacts with $N,N$-dimethylaniline via electrophilic aromatic substitution. Because the $-N(CH_{3})_{2}$ group strongly activates the aromatic ring toward ortho-para substitution, the reaction proceeds selectively at the para position to yield the brightly coloured product $Y$.

The reduction of isocyanides involves the addition of hydrogen across the carbon-nitrogen multiple bond, keeping both alkyl groups attached to the nitrogen atom. When methyl isocyanide reacts with lithium aluminium hydride, the carbon-nitrogen bond is reduced in a manner that preserves the linkage between the nitrogen and both carbon units, resulting in the formation of dimethylamine, a secondary amine. Other functional groups like cyanides or amides yield primary amines upon complete reduction with this reagent.

Base strength in amines depends on the availability of the nitrogen lone pair for protonation, which is significantly enhanced when the lone pair is insulated from aromatic resonance. In aniline, the lone pair on nitrogen is partially delocalized into the benzene ring via resonance, reducing its electron density and making it a weaker base. Conversely, benzylamine features a methylene group that interrupts resonance between the amino group and the aromatic ring, keeping the lone pair fully localized and readily available. Thus, benzylamine exhibits greater basicity than aniline, whereas aryl substitutions like nitro groups or multiple phenyl rings further decrease basicity through electron withdrawal or extended delocalization.

Conversion of aniline to benzyl alcohol proceeds through a sequence of diazotization, cyanation, reduction, and deamination via nitrous acid. Aniline is first converted to benzene diazonium chloride using nitrous acid, which is then treated with cuprous cyanide to yield benzonitrile ($\mathrm{C}_6\mathrm{H}_5\mathrm{CN}$). Reduction of benzonitrile with lithium aluminum hydride produces benzylamine ($\mathrm{C}_6\mathrm{H}_5\mathrm{CH}_2\mathrm{NH}_2$), which upon reaction with nitrous acid undergoes diazotization followed by hydrolysis to yield benzyl alcohol ($\mathrm{C}_6\mathrm{H}_5\mathrm{CH}_2\mathrm{OH}$).

The reduction pathway of aromatic nitro compounds is heavily dictated by the reaction medium and the nature of the proton source available during electrolysis. When nitrobenzene undergoes electrolytic reduction under weakly acidic conditions, the stepwise transfer of electrons and protons proceeds efficiently all the way to the primary aromatic amine stage. This transformation yields aniline as the ultimate product, whereas strongly acidic environments typically induce acid-catalyzed rearrangements leading to hydroxylated derivatives instead.

Acid-catalyzed rearrangement of cumene hydroperoxide proceeds via the migration of a phenyl ring from the tertiary carbon to the adjacent oxygen atom, driven by the departure of a water molecule. Protonation of the terminal oxygen weakens the peroxide bond, facilitating this intramolecular aryl migration to form a resonance-stabilized oxonium ion intermediate. Subsequent attack by water breaks down this intermediate to yield phenol and acetone as the final products of the cleavage reaction.

The reaction involves the generation of a reactive intermediate that subsequently attacks the nucleophilic center to yield the substituted derivative. Electronic effects, such as resonance and inductive stabilization, determine the regiochemical outcome by favoring the more stable intermediate. Therefore, the structural arrangement of product P reflects the thermodynamically favored pathway under the given reaction conditions.

The Hoffmann degradation of amides involves a sequence of halogenation and rearrangement steps where an unsubstituted amide is converted into a primary amine with the loss of a carbonyl carbon. Initially, the base abstracts a proton from the nitrogen atom, which reacts with bromine to form an $N$-bromamide intermediate represented as $R\mathrm{CONHBr}$. Subsequent base treatment promotes a molecular rearrangement that eliminates a halide ion to yield an isocyanate intermediate denoted as $R\mathrm{NCO}$. Hydrolysis of this isocyanate species ultimately affords the primary amine and a carbonate salt.

The Hoffmann bromamide degradation is a name reaction specifically designed to convert unsubstituted carboxylic acid amides into primary amines with the loss of a carbonyl carbon atom. This degradation relies on treating the amide with bromine and a strong base like potassium hydroxide ($\mathrm{KOH}$), which triggers a rearrangement involving an acyl nitrene intermediate. Among the choices provided, only an unsubstituted amide containing the $-\mathrm{CONH}_2$ functional group possesses the precise structural requirements to undergo this halogenation and migration sequence.

Partial acid hydrolysis of nitriles selectively halts at the amide stage rather than progressing completely to carboxylic acids. Treating acetonitrile with cold concentrated hydrochloric acid hydrates the nitrile triple bond, successfully yielding acetamide as the stable intermediate product.

Working backward from N-methylaniline, the reduction of an isocyanide produces a secondary amine containing a methyl group, which indicates that intermediate C is phenyl isocyanide ($\mathrm{C_6H_5NC}$). This isocyanide is formed via the carbylamine reaction using chloroform and potassium hydroxide on a primary amine, identifying intermediate B as aniline ($\mathrm{C_6H_5NH_2}$). Because primary aromatic amines are synthesized through the reduction of nitro compounds, the starting material A must be nitrobenzene ($\mathrm{C_6H_5NO_2}$).