An amine's lone pair makes it a good nucleophile as well as a good base, and that combination is both its usefulness and its problem. The reactions in this section are largely about controlling a nucleophile that does not know when to stop.
SN2 alkylation: a messy way to build amines
A free amine attacks an alkyl halide directly in an SN2 reaction, forming a new C–N bond. That sounds like a clean route to a substituted amine, and it is not.
The problem is that the product is a more substituted amine, which is at least as nucleophilic as the starting material. It reacts with a second equivalent of alkyl halide, and a third, and ends as a quaternary ammonium salt. Direct alkylation of ammonia or a primary amine therefore gives a statistical mixture of mono-, di-, tri- and tetra-alkylated products that is difficult to separate and wasteful of material.
This is a genuine practical limitation, not a theoretical caveat. Two strategies work around it. Exhaustive alkylation — deliberately using excess alkyl halide to push everything to the quaternary salt — is fine when the salt is what you want. And the Gabriel synthesis uses phthalimide, whose single N–H is acidic enough to deprotonate and whose nitrogen after one alkylation is an amide and therefore unreactive; hydrolysis then releases a clean primary amine. The trick is to make the first product non-nucleophilic.
Acylation: the clean, self-terminating alternative
React an amine with an acid chloride or anhydride and you get an amide, by the nucleophilic acyl substitution of Module 10. The crucial point is what happens to the nitrogen: its lone pair is now delocalized into the new carbonyl, so it is no longer nucleophilic and no longer basic.
The reaction therefore self-terminates. One acylation, and the nitrogen is switched off. No mixture, no over-reaction. This is the same resonance-quenches-reactivity logic from the previous section, now put to practical use.
One practical detail: acylation releases HCl, which would protonate the remaining amine and stop the reaction at 50% conversion. So a second equivalent of amine, or a non-nucleophilic base such as pyridine or triethylamine, is included to mop it up.
Reductive amination: the reliable route
Combining amine chemistry with carbonyl chemistry solves the over-alkylation problem completely. An amine condenses with an aldehyde or ketone to give an imine (Module 9), and that imine is then reduced to the amine.
The reason it does not over-react is that the new C–N bond is built through a controlled two-step sequence rather than a runaway chain: only one carbonyl can condense with a given nitrogen at a time, and the reduction step commits the product before it can do anything else.
The reducing agent matters. NaBH₃CN (sodium cyanoborohydride) is the standard choice because the electron-withdrawing cyano group makes it mild enough to reduce the protonated imine selectively while leaving the ketone starting material alone — which means the condensation and the reduction can be run in the same flask at the same time. Plain NaBH₄ would reduce the ketone before it had a chance to form the imine.
Route A, alkylation: cyclohexylamine + ethyl bromide. Gives a mixture of the secondary amine, the tertiary amine, the quaternary salt, and unreacted starting material. Poor yield, hard separation.
Route B, reductive amination: cyclohexylamine + acetaldehyde, with NaBH₃CN. The imine forms, is reduced, and the reaction stops — the product is a secondary amine that cannot condense again with acetaldehyde in the presence of the reducing agent. Clean.
Same bond formed, same fragments, and only one route is practical. This is why reductive amination is one of the most-used reactions in medicinal chemistry.
Hofmann elimination
Amines cannot undergo elimination directly — R₂N⁻ is a hopeless leaving group. But exhaustive methylation with excess CH₃I converts the nitrogen into a quaternary ammonium group, which is neutral trialkylamine once it leaves and therefore a perfectly good leaving group. Heating with hydroxide then gives an E2 elimination.
Because the leaving group is enormous, the base takes the most accessible beta hydrogen, which gives the less substituted alkene — the Hofmann product rather than the Zaitsev one. This is the same steric logic as a bulky base from Module 6, operating from the substrate's side instead of the reagent's.
Diazonium salts and the aromatic connection
Primary aromatic amines react with nitrous acid, generated in situ from NaNO₂ and HCl at 0 °C, to give diazonium salts, ArN₂⁺. These are remarkable because N₂ is the best leaving group in organic chemistry — nitrogen gas is so stable and so entropically favoured on leaving that its departure is essentially irreversible.
That opens the door to replacing the nitrogen with almost anything: CuCl, CuBr or CuCN give the aryl chloride, bromide or nitrile (the Sandmeyer reactions); KI gives the iodide; water gives the phenol; H₃PO₂ removes it entirely, leaving just H.
The synthetic significance is that an amino group becomes a placeholder. You install a nitro group on a ring where you want it, reduce it to an amine, use the amine's powerful directing effect to place the next substituent, then convert the amine to a diazonium salt and swap it for whatever the target actually needs. Aliphatic diazonium salts, by contrast, decompose immediately to carbocations and are not synthetically useful.
What carries forward
Reductive amination and acylation are standard moves in any synthesis involving nitrogen. Diazonium chemistry is the direct bridge into Module 13, where the substituent patterns achievable on an aromatic ring depend heavily on this placeholder strategy. And the recurring theme — control a reaction by making its product unreactive — is one of the more transferable ideas in the course.