Chapter 9: Amines | Summary

Amines are classified as primary (1°), secondary (2°), and tertiary (3°) based on the number of hydrogen atoms of ammonia replaced by alkyl or aryl groups. Replacement of one hydrogen gives a primary amine (RNH2), two replacements give a secondary amine (R2NH or RNHR'), and three replacements give a tertiary amine (R3N).

Amines are called 'simple' when all alkyl or aryl groups attached to nitrogen are identical, and 'mixed' when the groups are different. This classification is fundamental for predicting the chemical behaviour, boiling points, hydrogen bonding capacity, and reactivity patterns of amines.

In the common naming system, aliphatic amines are named by prefixing the alkyl group name to 'amine' as one word (e.g., methylamine). For secondary and tertiary amines with identical groups, prefixes di- or tri- are used before the alkyl group name. In the IUPAC system, primary amines are named as alkanamines by replacing the terminal 'e' of the parent alkane with 'amine'. For example, CH3NH2 is methanamine and H2N–CH2–CH2–NH2 is ethane-1,2-diamine.

For secondary and tertiary amines in IUPAC nomenclature, the locant N is used to designate substituents on the nitrogen atom. For example, CH3NHCH2CH3 is N-methylethanamine. In arylamines, the –NH2 group is directly attached to the benzene ring; C6H5NH2 is known as aniline (common) or benzenamine (IUPAC), formed by replacing the terminal 'e' of benzene with 'amine'.

Amines are prepared by six main methods. (1) Reduction of nitro compounds using H2/Ni, Pd, or Pt, or by metals in acidic medium (Fe/HCl preferred). (2) Ammonolysis of alkyl halides: alkyl halides react with ethanolic NH3 in a sealed tube at 373 K via nucleophilic substitution to give a mixture of primary, secondary, and tertiary amines plus quaternary ammonium salts; primary amine is the major product when excess ammonia is used. Reactivity order: RI > RBr > RCl. (3) Reduction of nitriles with LiAlH4 or catalytic hydrogenation gives primary amines, useful for ascending the amine series.

(4) Reduction of amides with LiAlH4 yields the corresponding amines. (5) Gabriel phthalimide synthesis: phthalimide is treated with ethanolic KOH to form its potassium salt, which is then alkylated and hydrolysed to give a pure primary amine; aromatic primary amines cannot be made this way because aryl halides do not undergo nucleophilic substitution with the phthalimide anion. (6) Hoffmann bromamide degradation: an amide is treated with Br2 and aqueous NaOH; an alkyl or aryl group migrates from carbonyl carbon to nitrogen, giving a primary amine that contains one carbon less than the starting amide.

Lower aliphatic amines are gases with a characteristic fishy odour. Primary amines with three or more carbon atoms are liquids, and still higher members are solids. Aniline and other arylamines are colourless liquids that turn brown on prolonged storage due to atmospheric oxidation. Lower aliphatic amines are soluble in water because they form hydrogen bonds with water molecules, but solubility decreases as molar mass increases due to the growing hydrophobic alkyl part.

Primary and secondary amines form intermolecular hydrogen bonds through the N–H bond, while tertiary amines cannot form such bonds due to the absence of N–H hydrogen. The order of boiling points among isomeric amines is therefore primary > secondary > tertiary. Amines have lower boiling points than alcohols of similar molar mass because the N–H bond is less polar than the O–H bond, resulting in weaker hydrogen bonds.

Amines display several key reactions. (1) Basic character: Amines react with acids to form salts and behave as Lewis bases due to the lone pair on nitrogen. Aliphatic amines are stronger bases than ammonia (pKb 3–4.22) due to the +I effect of alkyl groups, while aromatic amines are weaker bases than ammonia (aniline pKb 9.38) because the lone pair is delocalised into the benzene ring. In aqueous solution, the basic order of alkylamines is influenced by inductive effect, solvation, and steric hindrance. (2) Alkylation: Amines react with alkyl halides to form higher amines. (3) Acylation: Primary and secondary amines react with acid chlorides, anhydrides, or esters in the presence of a base (e.g., pyridine) to form amides.

(4) Carbylamine reaction (isocyanide test): Only primary amines (aliphatic and aromatic) react with CHCl3 and ethanolic KOH to form foul-smelling isocyanides (carbylamines); secondary and tertiary amines do not respond. (5) Reaction with nitrous acid: Primary aliphatic amines form unstable diazonium salts that decompose to give alcohols and N2 gas; primary aromatic amines form stable diazonium salts at 273–278 K. Secondary amines form N-nitrosamines. (6) Hinsberg's test (reaction with benzenesulphonyl chloride): Primary amines give sulphonamides soluble in alkali; secondary amines give sulphonamides insoluble in alkali; tertiary amines do not react. (7) Electrophilic substitution of aniline: The –NH2 group is ortho/para directing and strongly activating. Bromination of aniline with Br2/H2O gives 2,4,6-tribromoaniline. Nitration and sulphonation also occur, giving mainly para products when the –NH2 group is protected by acetylation. Aniline does not undergo Friedel-Crafts reaction due to salt formation with AlCl3.

The basicity of amines depends on the availability of the lone pair on nitrogen for protonation. Alkyl groups, with their +I (electron-releasing) inductive effect, increase electron density on nitrogen, making alkylamines more basic than ammonia. However, in aqueous solution, solvation and steric effects modify the expected trend based on inductive effect alone. For methylamines: (CH3)2NH > CH3NH2 > (CH3)3N > NH3; for ethylamines: (C2H5)2NH > (C2H5)3N > C2H5NH2 > NH3.

Aromatic amines are far weaker bases than ammonia because the lone pair on nitrogen is conjugated with the benzene pi system, making it less available for protonation. Aniline exists as a resonance hybrid of five structures, while the anilinium ion has only two Kekulé structures. Greater resonance stabilisation of aniline relative to the anilinium ion means aniline is less prone to accept a proton. Electron-releasing substituents (–OCH3, –CH3) on the ring increase basic strength of aniline derivatives, while electron-withdrawing groups (–NO2, –SO3H, –COOH, –X) decrease it.

Diazonium salts have the general formula RN2+X–, where R is an aryl group and X– is an anion such as Cl–, Br–, HSO4–, or BF4–. They are named by adding 'diazonium' to the parent hydrocarbon name followed by the anion name (e.g., benzenediazonium chloride). Primary aliphatic amines form highly unstable alkyldiazonium salts, whereas primary aromatic amines form relatively stable arenediazonium salts at 273–278 K. This stability is explained by resonance delocalisation of the positive charge.

Benzenediazonium chloride is prepared by diazotisation: aniline is treated with NaNO2 and HCl at 273–278 K. It is a colourless crystalline solid, readily soluble in water, stable in cold solution, but decomposes on warming or in the dry state. Benzenediazonium fluoroborate is an exception—it is water insoluble and stable at room temperature.

Reactions of diazonium salts fall into two categories. (A) Displacement of the diazonium group (N2 is released): (1) Sandmeyer reaction—replacement by Cl–, Br–, or CN– using Cu(I) salts; (2) Gattermann reaction—replacement by Cl– or Br– using copper powder and halogen acid; (3) replacement by I– using KI; (4) replacement by F– via fluoroboric acid treatment (Balz-Schiemann reaction); (5) replacement by H using hypophosphorous acid or ethanol as mild reducing agents; (6) replacement by –OH when the diazonium salt solution is warmed to 283 K; (7) replacement by –NO2 using diazonium fluoroborate with aqueous NaNO2/Cu.

(B) Retention of the diazo group—coupling reactions: Diazonium salts react with activated aromatic compounds such as phenols and arylamines at the para position to form azo compounds (–N=N–) that are brightly coloured and used as azo dyes. Benzenediazonium chloride couples with phenol in alkaline medium to give p-hydroxyazobenzene (orange dye) and with aniline in acidic medium to give p-aminoazobenzene (yellow dye). Coupling is an electrophilic substitution reaction.

Diazonium salts are versatile synthetic intermediates that allow introduction of –F, –Cl, –Br, –I, –CN, –OH, –NO2, and –H groups into the aromatic ring. Many of these transformations are impossible by direct substitution. For instance, aryl fluorides and iodides cannot be prepared by direct halogenation; fluorobenzene must be made via the Balz-Schiemann reaction and iodobenzene via treatment of the diazonium salt with KI.

Cyanobenzene cannot be obtained by nucleophilic substitution of chlorobenzene, but is easily prepared from benzenediazonium chloride via the Sandmeyer reaction. Thus, diazonium salts bridge the gap between arylamines and a wide variety of substituted aromatic compounds, making them indispensable in synthetic organic chemistry.

Amines are nitrogen-containing organic compounds structurally derived from ammonia, and their chemistry is governed by the lone pair on nitrogen, the nature of substituents, and the number of hydrogen atoms on nitrogen. Their preparation routes—reduction of nitro compounds, ammonolysis, nitrile reduction, amide reduction, Gabriel synthesis, and Hoffmann degradation—each offer strategic advantages for specific synthetic goals. Their physical and chemical properties, including basicity, hydrogen bonding, acylation, carbylamine reaction, and electrophilic substitution, provide a comprehensive framework for understanding nitrogen chemistry. Diazonium salts, derived from primary aromatic amines, serve as pivotal intermediates for introducing diverse functional groups into aromatic rings.

From the CBSE board examination perspective, this chapter carries significant weightage. Key areas that are frequently tested include: comparison of basicity of aliphatic, aromatic, and substituted amines with reasoning; mechanisms and examples of Gabriel synthesis, Hoffmann degradation, and carbylamine reaction; Sandmeyer and Gattermann reactions; coupling reactions and azo dye formation; Hinsberg's test for distinguishing amine classes; and multi-step synthesis conversions involving diazonium salts. Mastery of this chapter requires both conceptual clarity and the ability to write balanced chemical equations.