Amines as Derivatives of Ammonia
The cleanest way to define an amine is by descent. Begin with ammonia, $\ce{NH3}$, and replace one, two or all three of its hydrogen atoms with alkyl ($\ce{R}$) or aryl ($\ce{Ar}$) groups. Whatever survives that replacement is an amine. NCERT states this directly: amines are "derivatives of ammonia, obtained by replacement of one, two or all the three hydrogen atoms by alkyl and/or aryl groups."
This single sentence does a lot of work. It tells you the parent (ammonia), the variable part (how many H atoms are swapped), and the substituents (alkyl, aryl, or a mixture). The three formal replacements give the three families you will meet throughout this chapter:
| H atoms replaced | General formula | Family | Simplest example |
|---|---|---|---|
| One | $\ce{R-NH2}$ | Primary (1°) | $\ce{CH3NH2}$, methanamine |
| Two | $\ce{R-NHR'}$ | Secondary (2°) | $\ce{(CH3)2NH}$, N-methylmethanamine |
| Three | $\ce{R3N}$ | Tertiary (3°) | $\ce{(CH3)3N}$, trimethylamine |
Amines are not an exotic class. NCERT reminds us they occur naturally in proteins, vitamins, alkaloids and hormones; adrenaline and ephedrine, both bearing a secondary amino group, raise blood pressure, while quaternary ammonium salts serve as surfactants. The structural rules below explain why nitrogen behaves the way it does in all of these.
Structure of the Amino Group
In every amine the nitrogen atom is trivalent and carries one unshared pair of electrons, exactly as it does in ammonia. Nitrogen has five valence electrons; three of them form bonds to hydrogen or carbon, and the remaining two stay together as a lone pair. To accommodate four electron domains — three bonds plus the lone pair — nitrogen adopts sp3 hybridisation.
NCERT puts it precisely: "Nitrogen orbitals in amines are therefore sp3 hybridised and the geometry of amines is pyramidal. Each of the three sp3 hybridised orbitals of nitrogen overlaps with orbitals of hydrogen or carbon depending upon the composition of the amine. The fourth orbital of nitrogen in all amines contains an unshared pair of electrons." That fourth orbital, the lone pair, is the source of nearly every chemical property of amines: their basicity, their nucleophilicity, and their reactions with acids, acyl chlorides and nitrous acid.
Both species place three bonds and one lone pair around an sp³ nitrogen. The lone pair compresses the bond angle below the ideal 109.5°: about 108° in trimethylamine (NCERT Fig. 9.1) and 107° in ammonia.
Geometry: Amine vs Ammonia
Because four electron domains surround nitrogen, the electron-pair geometry is tetrahedral; but since one of those domains is the invisible lone pair, the molecular shape — the arrangement of atoms you can actually see — is trigonal pyramidal. The three bonded groups form the base of a tripod and the lone pair points up to the apex. NIOS describes the same picture: the three groups occupy three corners of a tetrahedron while the unshared pair is directed toward the fourth corner.
The lone pair is not just a spectator in this geometry. A lone pair is held closer to the nucleus and spreads out more than a bonding pair, so it presses harder on the three N–H or N–C bonds and squeezes them together. This is why the bond angle falls below the ideal tetrahedral value of 109.5°. NCERT records the angle in trimethylamine as 108°; in ammonia the smaller hydrogen substituents allow the angle to close further to about 107°.
| Feature | Ammonia, $\ce{NH3}$ | Amine (e.g. trimethylamine) |
|---|---|---|
| Hybridisation of N | sp3 | sp3 |
| Electron domains | 3 bonds + 1 lone pair | 3 bonds + 1 lone pair |
| Molecular shape | Trigonal pyramidal | Trigonal pyramidal |
| Bond angle | ~107° (H–N–H) | ~108° (C–N–C in $\ce{(CH3)3N}$) |
| Lone pair | One, on N | One, on N |
| Bonded groups | 3 × H | H and/or C, by class |
The structural payoff is that an amine is essentially "ammonia wearing carbon coats." Every reaction you study later — salt formation with acids, acylation, the carbylamine and Hinsberg tests — traces back to that one accessible lone pair on a pyramidal sp3 nitrogen.
Classification by N-Substitution
Amines are classified as primary (1°), secondary (2°) or tertiary (3°) according to how many hydrogen atoms of ammonia have been replaced by alkyl or aryl groups — equivalently, how many carbon groups are bonded directly to the nitrogen atom. This is the decisive rule, and it is worth memorising in the form: count the carbons on nitrogen.
| Class | Carbons on N | H on N | General formula | Example (IUPAC) |
|---|---|---|---|---|
| Primary (1°) | 1 | 2 | $\ce{R-NH2}$ | $\ce{CH3CH2NH2}$ — ethanamine |
| Secondary (2°) | 2 | 1 | $\ce{R-NHR'}$ | $\ce{CH3NHCH2CH3}$ — N-methylethanamine |
| Tertiary (3°) | 3 | 0 | $\ce{R3N}$ | $\ce{(C2H5)3N}$ — N,N-diethylethanamine |
NCERT walks through the logic step by step. Replace one hydrogen of ammonia by $\ce{R}$ or $\ce{Ar}$ and you get $\ce{R-NH2}$ or $\ce{Ar-NH2}$, a primary amine. Replace a second hydrogen — either of ammonia or of $\ce{R-NH2}$ — by another group $\ce{R'}$ and you reach $\ce{R-NHR'}$, a secondary amine; the two groups may be the same or different. Replace the third and you have a tertiary amine, $\ce{R3N}$. When all the groups on nitrogen are identical the amine is called simple; when they differ it is called mixed.
Successive replacement of N–H by N–C climbs the ladder 1° → 2° → 3°; a fourth carbon group quaternises nitrogen and yields a positively charged ammonium salt with no lone pair left.
Quaternary Ammonium Salts
Nitrogen has a fourth valency available even after the three positions of a tertiary amine are filled, because its lone pair can form a fourth bond. When all four of nitrogen's bonds go to carbon groups, the result is a quaternary ammonium ion, $\ce{R4N+}$, paired with a counter-anion $\ce{X-}$ to give a quaternary ammonium salt, $\ce{R4N+ X-}$.
Two structural changes follow from putting that fourth group on nitrogen. First, the lone pair is gone — it has been used to make the fourth bond — so a quaternary ammonium ion is no longer basic or nucleophilic in the way ordinary amines are. Second, the nitrogen now carries a formal positive charge and the species is ionic, existing as a salt rather than a neutral molecule. NCERT notes that quaternary ammonium salts find use as surfactants. They are best regarded as a separate category that sits beyond the 1°/2°/3° ladder rather than as a fourth "degree" of amine.
Once you can spot 1°/2°/3° amines, the next move is naming them. See Nomenclature of Amines for the alkanamine and N-locant rules.
The Alcohol Classification Trap
This is the single most examined confusion in the topic. Amines and alcohols are both classified as 1°, 2° and 3°, but the two systems count completely different things, and treating them alike produces wrong answers.
| What is counted | Amines | Alcohols |
|---|---|---|
| The thing classified | The nitrogen atom | The carbon bearing –OH |
| Counting rule | Number of C groups on N | Number of C groups on the C–OH carbon |
| 1° means | One carbon on N | The –OH carbon has one other carbon |
| Test molecule | $\ce{(CH3)3C-NH2}$ | $\ce{(CH3)3C-OH}$ |
| Verdict | Primary amine (N has 1 carbon) | Tertiary alcohol (C–OH carbon has 3 carbons) |
Same molecule, opposite degree
tert-butylamine, $\ce{(CH3)3C-NH2}$, is a primary amine: the nitrogen carries exactly one carbon group. But the analogous alcohol, $\ce{(CH3)3C-OH}$, is a tertiary alcohol because the carbon bearing –OH is bonded to three other carbons. Students who carry the alcohol habit into amines mislabel tert-butylamine as tertiary and lose the mark.
For amines: look only at the nitrogen and count carbons attached to it. Ignore how branched the carbon skeleton is.
Aliphatic vs Aromatic Amines
A second, independent way of sorting amines asks what kind of group sits on nitrogen. If nitrogen is bonded only to alkyl groups, the amine is aliphatic; if the –NH2 (or substituted amino group) is attached directly to a benzene ring, it is aromatic. NCERT defines an arylamine as one in which "the –NH2 group is directly attached to the benzene ring," with $\ce{C6H5NH2}$ (aniline) as the simplest example.
The word "directly" is doing essential work here. In aniline the lone pair on nitrogen lies next to the ring and can be delocalised into the aromatic π system; this conjugation reshapes both the basicity and the substitution chemistry of the molecule. Contrast benzylamine, $\ce{C6H5CH2NH2}$: although it contains a benzene ring, the nitrogen sits on a $\ce{-CH2-}$ spacer and is insulated from the ring, so it behaves as an aliphatic primary amine. The distinction is geometric, not merely cosmetic.
| Property | Aliphatic amine | Aromatic amine |
|---|---|---|
| Group on N | Alkyl only | Aryl bonded directly to N |
| Example | $\ce{CH3NH2}$, $\ce{C2H5NH2}$ | $\ce{C6H5NH2}$ (aniline) |
| Lone pair | Localised on N | Delocalised into the ring |
| Relative basicity | Stronger base than $\ce{NH3}$ | Weaker base than $\ce{NH3}$ |
| Borderline case | Benzylamine $\ce{C6H5CH2NH2}$ (N off the ring) | — |
These two classification axes are orthogonal: a single amine carries one label from each. Aniline is an aromatic primary amine; N-methylaniline, $\ce{C6H5NHCH3}$, is an aromatic secondary amine; triethylamine is an aliphatic tertiary amine. The basicity consequences of being aliphatic versus aromatic are developed fully in the basicity subtopic.
The aliphatic-vs-aromatic split is the foundation of every basicity ranking question. Continue to Basicity of Amines to see how the lone pair's environment fixes pKb.
Worked Classification Set
The reliable procedure for any amine: locate the nitrogen, count the carbon groups bonded to it (1°/2°/3°), then ask whether any of those carbons is a ring attached directly to N (aromatic) or only alkyl (aliphatic).
Classify each amine as 1°/2°/3° and as aliphatic/aromatic. (NCERT Exercise 9.1, selected)
(i) $\ce{(CH3)2CHNH2}$, propan-2-amine. Nitrogen carries one carbon group → primary, aliphatic. The branching of the carbon skeleton is irrelevant.
(iii) $\ce{CH3NHCH(CH3)2}$, N-methylpropan-2-amine. Nitrogen carries two carbon groups → secondary, aliphatic.
(iv) $\ce{(CH3)3CNH2}$, 2-methylpropan-2-amine. Nitrogen carries one carbon group → primary, aliphatic. This is the classic trap molecule: primary amine, not tertiary.
(v) $\ce{C6H5NHCH3}$, N-methylaniline. Nitrogen carries two carbon groups, one of which is a ring bonded directly to N → secondary, aromatic.
(vi) $\ce{(CH3CH2)2NCH3}$, N-ethyl-N-methylethanamine. Nitrogen carries three carbon groups → tertiary, aliphatic.
(vii) $\ce{m\text{-}BrC6H4NH2}$, 3-bromoaniline. Nitrogen carries one carbon group, a ring bonded directly to N → primary, aromatic.
Structure & classification in one screen
- Amines = ammonia with 1, 2 or 3 H atoms replaced by alkyl/aryl groups; nitrogen stays trivalent with one lone pair.
- Nitrogen is sp3-hybridised; molecular shape is trigonal pyramidal; bond angle ~108° in $\ce{(CH3)3N}$ (~107° in $\ce{NH3}$), below 109.5° because the lone pair compresses the bonds.
- Classify by carbons on nitrogen: 1° ($\ce{RNH2}$), 2° ($\ce{R2NH}$), 3° ($\ce{R3N}$); a fourth carbon gives the charged quaternary salt $\ce{R4N+ X-}$ with no lone pair.
- Trap: amines count carbons on N; alcohols count carbons on the C–OH carbon. $\ce{(CH3)3CNH2}$ is a primary amine, not tertiary.
- Aliphatic = alkyl on N; aromatic = ring bonded directly to N (aniline). Benzylamine is aliphatic. The two axes combine independently.