Amines

Amines are organic derivatives of ammonia in which one, two or three hydrogens of $\text{NH}_3$ are replaced by alkyl or aryl groups. The nitrogen keeps a lone pair, which is responsible for the two themes of the chapter — basicity and nucleophilic reactions. A frequent NEET trap lies in the classification: amines are called primary, secondary or tertiary by the number of carbon groups attached to the nitrogen ($1^\circ$ = one, $2^\circ$ = two, $3^\circ$ = three), which is different from the way alcohols and halides are classified (by the carbon bearing the functional group). A fourth class, quaternary ammonium salts ($\text{R}_4\text{N}^+$), has four groups on a positively charged nitrogen.

Amines are also divided into aliphatic (e.g. methylamine) and aromatic (e.g. aniline, where $-\text{NH}_2$ is on a ring) — a distinction that matters greatly for basicity. In IUPAC naming they are alkanamines (methanamine, ethanamine), with $N$-prefixes for groups on the nitrogen of secondary and tertiary amines.

The main preparations add or keep carbons in characteristic ways, which NEET loves to test. Reduction of nitro compounds (nitrobenzene with Sn/HCl, Fe/HCl or $\text{H}_2$/Ni) is the standard route to aromatic amines such as aniline. Ammonolysis of an alkyl halide with excess $\text{NH}_3$ gives amines but is not selective — it produces a mixture of $1^\circ$, $2^\circ$, $3^\circ$ amines and the quaternary salt, which is its main drawback. Reduction of nitriles (R–CN with $\text{H}_2$/Ni or $\text{LiAlH}_4$) gives a primary amine with one more carbon than R, while reduction of amides with $\text{LiAlH}_4$ gives an amine with the same number of carbons.

Two named methods give clean products. The Gabriel phthalimide synthesis makes pure primary aliphatic amines (free of $2^\circ$/$3^\circ$ contamination) from an alkyl halide and potassium phthalimide; importantly it fails for aromatic amines, because aryl halides do not react with the phthalimide ion. The Hofmann bromamide degradation converts an amide to a primary amine using $\text{Br}_2$ and $\text{NaOH}$, and the product has one fewer carbon than the amide. Remembering 'nitrile $+1$ C, Hofmann $-1$ C, Gabriel = pure $1^\circ$ (aliphatic only)' answers many synthesis questions instantly.

Amines are bases because the nitrogen lone pair can accept a proton (or donate to an electrophile). Their strength is measured by $K_b$ (or $pK_b$): a larger $K_b$ / smaller $pK_b$ means a stronger base. Four factors decide how basic a particular amine is, and balancing them is one of the highest-frequency NEET topics.

First, the inductive ($+\text{I}$) effect of alkyl groups: they push electron density onto nitrogen, making the lone pair more available, so aliphatic amines are more basic than ammonia. Second, resonance: in aromatic amines like aniline, the nitrogen lone pair is delocalised into the benzene ring, so it is much less available — aniline is therefore a far weaker base than ammonia or any aliphatic amine. Third, steric hindrance: bulky groups around nitrogen hinder protonation and lower basicity, which is why tertiary amines can be weaker than expected. Fourth, solvation: the more N–H bonds the protonated amine (ammonium ion) has, the more hydrogen bonds it can form with water, stabilising it and boosting basicity in aqueous solution.

These factors explain the difference between gas-phase and aqueous orders — a classic NEET subtlety. In the gas phase, only the inductive effect operates, so basicity rises with more alkyl groups: $3^\circ \gt 2^\circ \gt 1^\circ \gt \text{NH}_3$. In aqueous solution, solvation and steric effects intervene, and for the methylamines the observed order is $(\text{CH}_3)_2\text{NH} \gt \text{CH}_3\text{NH}_2 \gt (\text{CH}_3)_3\text{N} \gt \text{NH}_3$ — the secondary amine wins because it balances good electron donation against reasonable solvation, while trimethylamine suffers from poor solvation and crowding.

Substituents on aromatic amines shift basicity predictably. Electron-donating groups on the ring (such as $-\text{CH}_3$, $-\text{OCH}_3$) increase the lone-pair density and make the amine more basic, while electron-withdrawing groups (such as $-\text{NO}_2$) pull electron density away and make it less basic — so $p$-nitroaniline is a much weaker base than aniline. NEET commonly asks you to rank a set of amines: the safe strategy is to compare aliphatic (strong) versus aromatic (weak) first, then apply substituent effects within each group.

The reactions of amines flow from the nitrogen lone pair acting as a nucleophile/base. With acids they form salts (and so dissolve in dilute acid — a handy purification). With acyl chlorides or anhydrides they undergo acylation, the N–H being replaced to give an amide; only $1^\circ$ and $2^\circ$ amines can do this, because a $3^\circ$ amine has no N–H. These simple facts underpin several NEET identification questions.

Two named tests identify the class of amine. The carbylamine (isocyanide) test — warming the amine with chloroform and alcoholic KOH — gives an isocyanide with a foul, penetrating smell, but only primary amines (aliphatic or aromatic) respond, so the bad odour is a specific test for $1^\circ$ amines. The Hinsberg test uses benzenesulphonyl chloride: $1^\circ$ amines give a product soluble in alkali, $2^\circ$ amines give an alkali-insoluble solid, and $3^\circ$ amines do not react — distinguishing all three classes at once.

The reaction with nitrous acid ($\text{HNO}_2$, made in situ from $\text{NaNO}_2 + \text{HCl}$) is especially important and class-specific. A $1^\circ$ aliphatic amine gives an unstable diazonium ion that immediately decomposes to an alcohol with brisk effervescence of $\text{N}_2$. A $1^\circ$ aromatic amine (aniline) gives a relatively stable arenediazonium salt if kept cold (273–278 K) — the gateway to the next topic. A $2^\circ$ amine gives a yellow oily $N$-nitrosamine, and a $3^\circ$ amine merely forms a salt. This behaviour is a NEET staple.

Aromatic amines also react on the ring. The $-\text{NH}_2$ group is strongly activating and ortho/para-directing, so aniline reacts very readily in electrophilic substitution — for instance with bromine water it gives a white precipitate of 2,4,6-tribromoaniline even without a catalyst. Because the group is so activating (and the amine is easily oxidised or protonated by the strong acids used), reactions like nitration are usually carried out after protecting the $-\text{NH}_2$ by acetylation, and aniline does not undergo Friedel–Crafts reactions (the Lewis-acid catalyst bonds to the lone pair). These practical points are exactly the kind of detail NEET examines.

Arenediazonium salts have the formula $\text{Ar–N}_2^+\,\text{X}^-$ (for example benzenediazonium chloride, $\text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^-$). They are made by diazotisation: a primary aromatic amine is treated with nitrous acid (from $\text{NaNO}_2 + \text{HCl}$) at low temperature, 273–278 K (0–5 °C). The cold is essential because these salts are unstable and decompose if warmed. Their value is that the $-\text{N}_2^+$ group is an excellent leaving group, making them versatile intermediates for building substituted benzenes that are otherwise hard to prepare.

Their reactions divide neatly into two types. In replacement reactions, the $-\text{N}_2^+$ group leaves (as $\text{N}_2$ gas) and is replaced by another group. Warm water gives phenol; the Sandmeyer reaction introduces $-\text{Cl}$ or $-\text{Br}$ using $\text{CuCl}$/$\text{CuBr}$, and the Gattermann reaction does the same with copper powder and HX; potassium iodide gives the iodoarene directly; the Balz–Schiemann reaction (via the fluoroborate, $\text{HBF}_4$) gives the fluoroarene; $\text{CuCN}$ gives the nitrile; and hypophosphorous acid ($\text{H}_3\text{PO}_2$) replaces $-\text{N}_2^+$ by $-\text{H}$, effectively removing the amino group. These routes let chemists place $-\text{I}$ or $-\text{F}$ on a ring, which direct halogenation cannot easily do.

In coupling reactions, the diazonium group is retained: the salt reacts with an electron-rich aromatic compound such as phenol (in mildly alkaline solution) or aniline to form an azo compound containing the $-\text{N}=\text{N}-$ linkage. These azo compounds are intensely coloured (yellow, orange, red) and form the basis of the huge family of azo dyes used in textiles and indicators. Coupling normally occurs at the para position of the partner ring.

For NEET, the practical messages are: diazotisation needs a $1^\circ$ aromatic amine and ice-cold conditions; replacement reactions lose $\text{N}_2$ (Sandmeyer/Gattermann for Cl/Br, KI for I, Balz–Schiemann for F, water for phenol, $\text{H}_3\text{PO}_2$ for H); and coupling reactions keep $\text{N}_2$ to give coloured azo dyes. Diazonium chemistry is the bridge that connects aniline to phenols, haloarenes and dyes — a recurring multi-step synthesis theme in the exam.