Two ideas run the topic: how many carbons sit on nitrogen (the class), and the nitrogen lone pair (its basicity and reactivity). Reductive amination is the headline route.
The headline route — reductive amination: a carbonyl plus an amine, reduced to a new C–N bond. Structures drawn live.
1. Amines are classified 1°, 2°, or 3° by how many carbons sit on nitrogen
Count carbons on the nitrogen itself: one is 1°, two is 2°, three is 3°, and a fourth gives a charged quaternary ammonium ion with no lone pair.
2. The nitrogen lone pair makes every amine both a base and a nucleophile
The lone pair grabs protons (base) or attacks electron-poor carbon (nucleophile); aniline is ~million-fold weaker because its lone pair delocalizes into the ring.
3. Reductive amination is the most controllable general route to amines
Condense a carbonyl with an amine to an imine, then reduce with mild NaBH3CN, which hits the iminium but spares the carbonyl; the carbonyl and amine you pick set the class.
A ketone plus methylamine gives a secondary amine — the amine and carbonyl you pick set the class.
4. Reducing a nitrile, amide, or azide builds an amine from a nitrogen precursor
LiAlH4 reduces nitriles (adding a carbon) and amides to amines; an alkyl halide + NaN3, then reduction, gives a clean primary amine.
Nitrile → primary amine: LiAlH4 reduces the C≡N and adds a carbon to the chain.
Halide → azide → primary amine: azide is a single-charged nucleophile, so no over-alkylation.
5. Direct alkylation over-alkylates, so azide and Gabriel give clean primary amines
The primary amine formed is more nucleophilic than ammonia, so it keeps reacting into a 1°/2°/3°/quaternary mixture — which is why the azide route and the Gabriel synthesis (alkylate-once phthalimide, then hydrolyze) win.
The over-alkylation mixture from ethyl bromide + ammonia — every class at once, hard to separate.
6. Summary
Class = carbons on N · lone pair = base + nucleophile · reductive amination = flexible general route · LiAlH4 reduces nitrile/amide/azide · azide or Gabriel for clean 1° · never alkylate ammonia directly.
Quiz yourself
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Direct alkylation over-alkylates: the primary amine formed is more nucleophilic than ammonia, so it keeps reacting to give a mixture of 1°, 2°, 3°, and quaternary products. Reductive amination goes through an imine/iminium that is reduced in a controlled step, so you cleanly get the class of amine you designed.
A primary amine, R–CH2–NH2. The nitrile carbon ends up in the chain, so this route adds one carbon while installing the amine.
In aniline the nitrogen lone pair is delocalized into the benzene ring by resonance, so it is less available to accept a proton. In ethylamine the lone pair is localized on nitrogen (and alkyl groups even donate electron density), making it far more basic — about a million-fold.
The azide route (SN2 with NaN3, then reduce with LiAlH4 or H2) or the Gabriel synthesis (alkylate potassium phthalimide, then hydrolyze). Both introduce nitrogen as a single-charged or non-nucleophilic species that reacts only once, so there is no over-alkylation.
Draw this on the whiteboard
Open the OChem Board whiteboard — benzene rings, curved arrows, wedge/dash bonds and a clickable periodic table built in. No account needed.