Chapter 9 — Amines
Replace the hydrogens of ammonia by carbon groups and you have the amines: the smell of fish, the dyes in your clothes, many medicines and the nitrogen of every protein. How they are named and made, why they are bases, and how diazonium salts turn aniline into almost anything.
Leave a piece of fish out of the refrigerator for a day and the kitchen soon tells you about it: the smell is trimethylamine, released as bacteria break down the fish. The odours of rotting flesh have names to match, putrescine and cadaverine, both diamines. Yet the same family of compounds keeps you alive. Adrenaline, which makes your heart race before an exam, is an amine; so is histamine, which makes your nose run in the pollen season and which antihistamine tablets block. Local anaesthetics used by dentists, many antidepressants and the caffeine and nicotine of everyday drugs all contain basic nitrogen atoms, and every amino acid in your proteins carries an $\mathrm{{-}NH_{2}}$ group.
Amines are ammonia, $\mathrm{NH_{3}}$, with one, two or three of its hydrogen atoms replaced by alkyl or aryl groups. Like ammonia they have a lone pair on nitrogen, and almost all of their chemistry comes from it: amines are bases and nucleophiles. In this chapter you will classify, name and prepare amines, explain why some are stronger bases than ammonia and why aniline is a far weaker one, and use test-tube reactions to tell primary, secondary and tertiary amines apart. Finally, cold nitrous acid turns aromatic amines into diazonium salts, the "switchboard" of aromatic synthesis: the diazonium group can be swapped for a halogen, a cyanide, a hydroxyl or a hydrogen atom, or coupled with phenols to give the brightly coloured azo dyes.
| Question | Key idea | Section |
|---|---|---|
| Is it a 1°, 2° or 3° amine? | count the carbons bonded to N | 9.1 |
| How do I make a pure primary amine? | Gabriel synthesis; Hoffmann bromamide degradation | 9.1 |
| Which amine is the stronger base? | smaller $\mathrm{p}K_{b}$; $+I$ and solvation help, resonance in aniline hurts | 9.2 |
| How can I tell 1°, 2° and 3° amines apart? | Hinsberg's reagent; carbylamine test | 9.2 |
| How do I put F, I or CN on a benzene ring? | via $\mathrm{C_{6}H_{5}N_{2}^{+}Cl^{-}}$ | 9.3 |
- describe the pyramidal structure of amines, classify them as primary, secondary or tertiary, aliphatic or aromatic, and give their common and IUPAC names;
- prepare amines by reducing nitro compounds, nitriles and amides, by ammonolysis of haloalkanes, by the Gabriel phthalimide synthesis and by the Hoffmann bromamide degradation, and say which routes give a pure primary amine and how the carbon count changes;
- explain the boiling points and solubilities of amines through hydrogen bonding;
- compare the basic strengths of ammonia, alkylamines and arylamines using $K_{b}$ and $\mathrm{p}K_{b}$, the inductive effect, solvation, steric hindrance and resonance, in water and in the gas phase;
- write the alkylation, acylation, carbylamine and nitrous acid reactions of amines, distinguish 1°, 2° and 3° amines with Hinsberg's reagent, and control the electrophilic substitution of aniline by acetylation;
- prepare benzenediazonium chloride, convert it into haloarenes, phenol, benzonitrile, nitrobenzene and benzene, couple it to make azo dyes, and plan multi-step aromatic syntheses through diazonium salts.
Sections in this chapter
Structure, Classification, Nomenclature and Preparation
The pyramidal nitrogen, primary, secondary and tertiary amines, common and IUPAC names, six ways to make amines, and boiling points and solubility.
Basicity and Chemical Reactions of Amines
$K_{b}$ and $\mathrm{p}K_{b}$, alkylamines against ammonia and aniline, alkylation, acylation, the carbylamine and Hinsberg tests, nitrous acid, and substitution in the ring of aniline.
Diazonium Salts
Diazotisation, stability, replacement of the diazonium group (Sandmeyer, Gattermann, iodide, fluoride, hydrogen, hydroxyl, nitro), coupling to azo dyes, and their use in synthesis.