Organic Compounds Containing Nitrogen – NEET UG ChemistryPractice Questions & PYQs

The synthesis of foul-smelling isocyanides serves as a classic diagnostic test for identifying primary amines through condensation with chloroform in a strongly basic medium. When primary aromatic amines like aniline are heated with chloroform and alcoholic potassium hydroxide, they undergo a concerted $\alpha$-elimination reaction to generate reactive dichlorocarbene intermediates. These electrophilic species attack the amino nitrogen to ultimately form carbylamines, characteristic compounds featuring an isocyanide functional group.

Lithium aluminium hydride is a powerful reducing agent that fully reduces the cyano group to yield a primary amine. Catalytic hydrogenation using hydrogen over nickel achieves the same transformation efficiently, and sodium amalgam in ethanol provides the necessary electrons and protons to accomplish this reduction. In contrast, stannous chloride with hydrochloric acid reduces nitriles only as far as the corresponding aldehyde intermediate rather than an amine, while bromine in aqueous sodium hydroxide is specific for the Hofmann degradation of amides.

Grignard reagents function as powerful bases that readily abstract acidic protons from active hydrogen sources like the N-H bonds present in amines. During this acid-base interaction, the alkyl group of the Grignard reagent (\text{RMgX}$) captures the acidic proton from the amine to yield its corresponding alkane hydrocarbon (\text{R-H}$) along with a magnesium amide salt.

Basic strength in nitrogen-containing organic compounds depends directly on the availability of the lone pair of electrons on the nitrogen atom for protonation. In cyclohexylamine, the nitrogen lone pair is freely available since there is no resonance delocalization, whereas in aniline the lone pair participates in the aromatic $\pi$-system of the benzene ring, significantly reducing its basicity. Consequently, cyclohexylamine is actually more basic than aniline, making the provided order that ranks aniline ahead of cyclohexylamine incorrect.

Name reactions in organic chemistry often immortalize the pioneering scientists who discovered specific pathways for carbon skeleton degradation or functional group interconversion. The specific protocol that transforms acid amides into primary amines containing one fewer carbon atom using bromine and caustic alkali was discovered by August Wilhelm von Hoffmann. This degradation sequence proceeds through halogenation and rearrangement of the nitrogen atom, establishing a direct synthetic link between carboxylic acid derivatives and lower homologue amines.

Primary amines react with chloroform and ethanolic potassium hydroxide to form foul-smelling isocyanides through the carbylamine reaction. In this transformation, the base generates a reactive dichlorocarbene intermediate that attacks the ethylamine nitrogen, ultimately yielding ethyl isocyanide along with potassium chloride and water.

When aniline undergoes nitration, the incoming $\mathrm{NO}_2^+$ electrophile is directed by the amino group toward both the ortho and para positions of the aromatic ring. Because this electrophilic attack occurs at these two active sites, the reaction successfully yields a mixture of both corresponding products.

Electron-withdrawing substituents stabilize the carboxylate anion through $-I$ and $-M$ effects, thereby increasing the acid strength of benzoic acid derivatives. Since the nitro group acts as a strong electron-withdrawing group, p-nitrobenzoic acid is indeed a stronger acid than unsubstituted benzoic acid. Conversely, electron-donating groups like methyl increase the electron density on the nitrogen atom via $+I$ effect and hyperconjugation, enhancing basicity and making p-toluidine more basic than aniline.

Ortho and para isomers of a disubstituted benzene exhibit different boiling points, making them ideal candidates for separation via fractional distillation. The ortho isomer boils at 222 °C, whereas the para isomer boils at 238 °C, providing a sufficient thermal gap for this technique to work effectively. To prevent any thermal decomposition of these delicate aromatic compounds during the procedure, the fractional distillation is performed under reduced pressure.

When a primary aliphatic amine like ethylamine encounters nitrous acid, an unstable diazonium salt is immediately generated as a transient intermediate. Because this species readily expels nitrogen gas, it rapidly transforms into a stable alcohol through a substitution reaction with water molecules. Following the overall chemical pathway represented by $\mathrm{C_2H_5NH_2 + HNO_2 \rightarrow C_2H_5OH + N_2 + H_2O}$, the major product Z produced in this reaction sequence is ethanol.

The conversion of nitrobenzene to aniline requires strong reducing agents such as tin with hydrochloric acid or iron with hydrochloric acid to ensure the complete reduction of the nitro group. Milder reducing conditions, such as the use of zinc with ammonium chloride, halt the process prematurely and produce phenylhydroxylamine rather than the desired amine. Therefore, only the reaction pathways employing tin with hydrochloric acid and iron with hydrochloric acid successfully deliver aniline as the major product.

The carbylamine test serves as a specialized chemical fingerprint exclusively for primary amines, distinguishing them from secondary and tertiary counterparts. When a primary amine is heated alongside chloroform ($\mathrm{CHCl}_3$) and a base like alcoholic potassium hydroxide ($\mathrm{KOH}$), it undergoes a dehydration-condensation process to generate foul-smelling isocyanides. This transformation proceeds through the intermediate generation of dichlorocarbene, which attacks the amino group to yield the characteristic offensive-smelling product.

Aliphatic primary amines react with nitrous acid to form highly unstable aliphatic diazonium salts that rapidly decompose in aqueous media. This rapid decomposition expels nitrogen gas and converts the primary amino functionality directly into a hydroxyl group, producing an alcohol.

Electrophilic aromatic substitution of aniline with bromine water occurs readily because the $-\mathrm{NH}_2$ group strongly activates the benzene ring, directing substitution to the ortho and para positions to yield 2,4,6-tribromoaniline. Subsequent diazotization using sodium nitrite and hydrochloric acid converts the remaining amino group into a diazonium salt, which is then isolated as a tetrafluoroborate precipitate. Heating this tetrafluoroborate salt to dryness induces the Balz-Schiemann reaction, thermally decomposing the intermediate to replace the diazonium group with fluorine, resulting in 2,4,6-tribromofluorobenzene.

When alkyl halides interact with silver cyanide, the covalent character of the reagent directs the reaction to form isocyanides instead of standard cyanides. These resulting isocyanide compounds are uniquely characterized by an intensely foul smell. Because potassium cyanide yields odorless cyanides, the presence of this offensive odor points directly to silver cyanide as the required reagent. Consequently, the reactant $X$ is $\mathrm{AgCN}$ and the foul-smelling product $Z$ is $\mathrm{C}_2\mathrm{H}_5\mathrm{NC}$.

Basicity of amines depends on the availability of the lone pair on nitrogen, which is enhanced by electron-donating alkyl groups and diminished by electron-withdrawing aryl groups due to resonance delocalization. Secondary aliphatic amines possess two alkyl groups that intensify electron density on the nitrogen atom through inductive effects more effectively than primary aliphatic amines. Aromatic rings withdraw electron density through resonance, making aromatic amines significantly weaker bases than their aliphatic counterparts. Consequently, N-ethylethanamine stands as the strongest base, followed sequentially by ethanamine, N-methylaniline, and finally benzenamine.

Aniline features a basic nitrogen center that forms a stable salt with the aluminium chloride catalyst typically required for Friedel-Crafts alkylation, effectively halting the catalytic cycle. Furthermore, the preparation of primary aromatic amines via Gabriel synthesis is blocked because aryl halides resist the nucleophilic attack of the phthalimide anion. Both of these chemical limitations are well-established characteristics of aniline reactivity.

Deoxygenation of phenol using zinc (\text{Zn}$) dust yields benzene (\text{C}_6\text{H}_6$), which serves as the substrate for subsequent carbon-carbon bond formation. Treatment of this intermediate with acetyl chloride (\text{CH}_3\text{COCl}$) and anhydrous aluminium chloride (\text{AlCl}_3$) drives a Friedel-Crafts acylation reaction. This sequence successfully introduces an acyl group onto the aromatic ring, yielding acetophenone as the major product.

The amino group in aniline is a primary amine, which allows it to readily react in the carbylamine test to produce a characteristic foul-smelling isocyanide. However, the lone pair on the nitrogen atom coordinates with the Lewis acid catalyst aluminium chloride used in Friedel-Crafts alkylation, forming an inactive complex that deactivates the aromatic ring. This strong coordination prevents aniline from undergoing Friedel-Crafts alkylation altogether.

Reduction of alkyl cyanides, amides, and Hofmann bromamide degradation all yield primary amines containing the same or one fewer carbon atom. In contrast, alkyl isocyanides possess a carbon-nitrogen bond where the alkyl group is directly attached to the nitrogen atom rather than the carbon. Consequently, reducing an isocyanide with lithium aluminum hydride delivers a secondary amine containing an $\mathrm{N}$-methyl group instead of a primary amine.