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Structure Classification Nomenclature

🎓 Class 12 Chemistry CBSE Theory Ch 9 – Amines ⏱ ~14 min
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Amines — Structure, Classification and Nomenclature

Replace one, two or all three hydrogen atoms of an ammonia molecule by alkyl or aryl groups and you obtain an amine. Amines are not an academic curiosity: they occur in proteins, vitamins, alkaloids and hormones. Adrenaline and ephedrine, both carrying a secondary amino group, raise blood pressure. Novocain is an anaesthetic used in dentistry, Benadryl is an antihistaminic drug with a tertiary amino group, and quaternary ammonium salts are the surfactants in your fabric conditioner.

Where this chapter is going. Part 1 establishes what an amine is — its shape, its classes and its name. Everything later in the chapter (basicity, reactions, diazonium chemistry) follows from one structural fact: the nitrogen keeps a lone pair.

9.1 Structure of Amines

Like ammonia, the nitrogen atom of an amine is trivalent and carries an unshared pair of electrons. The nitrogen orbitals are therefore sp3 hybridised and the geometry about nitrogen is pyramidal.

Each of the three sp3 hybrid orbitals of nitrogen overlaps with an orbital of hydrogen or of carbon, depending on the composition of the amine. The fourth sp3 orbital holds the unshared electron pair.

Why the angle shrinks. A lone pair occupies more space than a bond pair. Because of the unshared pair, the C–N–E angle (where E is C or H) is less than 109.5°. In trimethylamine it measures 108°.
lone pair N CH₃ CH₃ CH₃ 108° sp³ nitrogen · three bond pairs + one lone pair
Fig. 9.1: Pyramidal shape of trimethylamine. The lone pair compresses the C–N–C angle to 108°, below the ideal tetrahedral 109.5°.
Do not say "trigonal planar". A common exam error is to describe amines as planar because nitrogen forms three bonds. It forms three bonds and holds a lone pair — four electron domains, hence sp³ and pyramidal.

9.2 Classification of Amines

Amines are classified as primary (1°), secondary (2°) and tertiary (3°) according to how many hydrogen atoms of ammonia have been replaced by alkyl or aryl groups.

ClassGeneral formulaH atoms of NH₃ replacedExample
Primary (1°)R–NH₂ or Ar–NH₂oneCH₃NH₂ (methanamine), C₆H₅NH₂ (aniline)
Secondary (2°)R–NH–R′two(CH₃)₂NH (N-methylmethanamine)
Tertiary (3°)R₃Nthree(CH₃)₃N (N,N-dimethylmethanamine)

If one hydrogen atom of ammonia is replaced by R or Ar, we get R–NH₂ or Ar–NH₂, a primary amine. If two hydrogen atoms of ammonia — or one hydrogen atom of R–NH₂ — are replaced by another alkyl or aryl group R′, we get R–NH–R′, a secondary amine. The second alkyl or aryl group may be the same or different. Replacement of a further hydrogen atom by an alkyl or aryl group gives a tertiary amine.

Simple and mixed amines. Amines are said to be simple when all the alkyl or aryl groups on nitrogen are the same, and mixed when they are different. So (C₂H₅)₂NH is a simple secondary amine, while CH₃–NH–C₂H₅ is a mixed secondary amine.
NH₃ ammonia R–NH₂ primary (1°) R–NH–R′ secondary (2°) R₃N tertiary (3°) –H, +R –H, +R′ –H, +R″ Each step replaces one N–H bond; the lone pair survives every step.
Classification of amines by successive replacement of the hydrogen atoms of ammonia.

9.3 Nomenclature of Amines

Common system

In the common system an aliphatic amine is named by prefixing the alkyl group to the word amine, written as one word — for example methylamine. In secondary and tertiary amines, when two or more groups are the same, the prefix di or tri is appended before the name of the alkyl group, giving dimethylamine and trimethylamine.

IUPAC system

In the IUPAC system, primary amines are named as alkanamines. The name is derived by replacing the terminal ‘e’ of the alkane by the word amine. Thus CH₃NH₂ is methanamine.

When more than one amino group is present at different positions in the parent chain, their positions are specified by numbering the carbon atoms that bear the –NH₂ groups, and a suitable prefix such as di or tri is attached to amine. Here the letter ‘e’ of the hydrocarbon suffix is retained. For example, H₂N–CH₂–CH₂–NH₂ is named ethane-1,2-diamine.

The one rule students lose marks on. Drop the ‘e’ for a single amino group (ethane → ethanamine) but keep the ‘e’ before di-, tri- (ethane → ethane-1,2-diamine). The reason is pronounceability: "ethandiamine" is awkward.

To name secondary and tertiary amines we use the locant N to designate a substituent attached to the nitrogen atom. For example CH₃NHCH₂CH₃ is named N-methylethanamine, and (CH₃CH₂)₃N is named N,N-diethylethanamine.

Choosing the parent in a 2° or 3° amine. The largest alkyl group becomes the parent alkanamine; the remaining groups on nitrogen become N-substituents. In (CH₃CH₂)₃N one ethyl is the parent (ethanamine) and the other two are N,N-diethyl.

Arylamines

In arylamines the –NH₂ group is attached directly to the benzene ring. C₆H₅NH₂ is the simplest arylamine; in the common system it is known as aniline, which is also an accepted IUPAC name. While naming arylamines by the IUPAC system, the suffix ‘e’ of the arene is replaced by amine, so C₆H₅–NH₂ is named benzenamine.

Table 9.1 — Nomenclature of some alkylamines and arylamines

AmineCommon nameIUPAC name
CH₃–CH₂–NH₂EthylamineEthanamine
CH₃–CH₂–CH₂–NH₂n-PropylaminePropan-1-amine
(CH₃)₂CH–NH₂IsopropylaminePropan-2-amine
CH₃–NH–CH₂CH₃EthylmethylamineN-Methylethanamine
(CH₃)₃NTrimethylamineN,N-Dimethylmethanamine
(C₂H₅)₂N–C₄H₉N,N-DiethylbutylamineN,N-Diethylbutan-1-amine
CH₂=CH–CH₂–NH₂AllylamineProp-2-en-1-amine
H₂N–(CH₂)₆–NH₂HexamethylenediamineHexane-1,6-diamine
C₆H₅–NH₂AnilineAniline or Benzenamine
2-CH₃–C₆H₄–NH₂o-Toluidine2-Methylaniline
4-Br–C₆H₄–NH₂p-Bromoaniline4-Bromobenzenamine (or 4-Bromoaniline)
C₆H₅–N(CH₃)₂N,N-DimethylanilineN,N-Dimethylbenzenamine
Worked Example 9.1 — Classify and name

Question. Classify each amine and give its IUPAC name: (i) (CH₃)₂CHNH₂   (ii) CH₃NHCH(CH₃)₂   (iii) (CH₃CH₂)₂NCH₃

(i) Nitrogen bears one carbon group and two hydrogens → primary. Parent chain is propane with –NH₂ at C-2 → propan-2-amine.

(ii) Nitrogen bears two carbon groups and one hydrogen → secondary. The larger group is isopropyl, so the parent is propan-2-amine and the methyl becomes an N-substituent → N-methylpropan-2-amine.

(iii) Nitrogen bears three carbon groups → tertiary. Parent is ethanamine; the remaining ethyl and methyl are N-substituents, cited alphabetically → N-ethyl-N-methylethanamine.

Worked Example 9.2 — Isomeric amines of C₄H₁₁N

Question. Write the structures of the isomeric amines of molecular formula C₄H₁₁N and state the type of isomerism shown between different pairs.

Primary (4): CH₃CH₂CH₂CH₂NH₂ (butan-1-amine); CH₃CH₂CH(NH₂)CH₃ (butan-2-amine); (CH₃)₂CHCH₂NH₂ (2-methylpropan-1-amine); (CH₃)₃CNH₂ (2-methylpropan-2-amine).

Secondary (3): CH₃CH₂CH₂–NH–CH₃ (N-methylpropan-1-amine); (CH₃)₂CH–NH–CH₃ (N-methylpropan-2-amine); CH₃CH₂–NH–CH₂CH₃ (N-ethylethanamine).

Tertiary (1): (CH₃)₂N–CH₂CH₃ (N,N-dimethylethanamine).

Type of isomerism. Within the same class (e.g. the four primary amines) the isomers differ in carbon skeleton or in the position of –NH₂, so they are chain and position isomers. Across classes — a primary versus a secondary versus a tertiary amine — the functional group itself differs in class, so these pairs are metamers (a form of structural isomerism arising from unequal distribution of alkyl groups about the nitrogen).

🧪 Activity 9.1 — Counting the hydrogens on nitrogenL4 Analyse

Amines look alike on paper. This activity trains the one habit that makes classification automatic: count what is bonded to nitrogen, not what is bonded to carbon.

Predict: Of the eight isomers of C₄H₁₁N, how many are primary? Write your guess before reading on.
  1. Draw the nitrogen atom alone. Give it three bonds and one lone pair.
  2. For each of the eight isomers of C₄H₁₁N, circle the nitrogen and count how many carbon atoms are directly attached to it.
  3. Tabulate: 1 carbon → 1°, 2 carbons → 2°, 3 carbons → 3°.
  4. Now try the trap: is (CH₃)₃CNH₂ a tertiary amine? Count carbons on nitrogen, not on the central carbon.

Four primary, three secondary, one tertiary.

The trap in step 4 is the classic one. In (CH₃)₃CNH₂ the carbon is tertiary — it carries three methyl groups — but the nitrogen carries only one carbon and two hydrogens, so the compound is a primary amine (2-methylpropan-2-amine, often called tert-butylamine). Amines are classified by substitution at nitrogen; alkyl halides and alcohols are classified by substitution at carbon. Mixing the two rules is the single most common source of lost marks in this chapter.

Intext questions

Intext 9.1 — Classify the following amines as primary, secondary or tertiary

Count the carbon atoms bonded to nitrogen in each case. (CH₃)₂CH–NH₂ is primary; C₆H₅–NH–CH₃ is secondary; (C₂H₅)₃N is tertiary; a cyclohexyl ring bearing –NH₂ is primary; and an N-methylated piperidine-type nitrogen carrying two ring carbons plus one methyl is tertiary.

Intext 9.2 — Isomeric amines of C₄H₁₁N

Structures, IUPAC names and the type of isomerism are worked out in full in Worked Example 9.2 above: eight isomers in total — four primary, three secondary and one tertiary.

Competency-Based Questions

A quality-control chemist at a dye intermediates plant receives three unlabelled drums, each containing a pure liquid of molecular formula C₇H₉N. The drums are known to contain, in some order, N-methylaniline, benzylamine and 2-methylaniline (o-toluidine). All three are colourless when fresh, but one drum has turned distinctly brown in storage.

1. Classify each of the three compounds as a primary, secondary or tertiary amine, and give the IUPAC name of benzylamine. L2 Understand

N-Methylaniline, C₆H₅–NH–CH₃: nitrogen carries two carbons (the ring and the methyl) and one hydrogen → secondary. Benzylamine, C₆H₅CH₂–NH₂: nitrogen carries one carbon → primary; IUPAC name phenylmethanamine. 2-Methylaniline, 2-CH₃–C₆H₄–NH₂: nitrogen carries one carbon → primary; IUPAC name 2-methylaniline (2-methylbenzenamine).

2. Which single structural feature distinguishes benzylamine from the other two, and why does it matter for the rest of this chapter? L4 Analyse

In benzylamine the –NH₂ group is attached to an sp³ CH₂ carbon, not directly to the benzene ring; it is therefore an aliphatic (alkyl) amine that merely happens to contain a ring. In N-methylaniline and 2-methylaniline the nitrogen is bonded directly to the ring, making them arylamines. This matters because only in arylamines can the nitrogen lone pair enter into conjugation with the ring — the fact that governs basicity (Part 3) and the whole of electrophilic substitution chemistry (Part 4).

3. Which drum is most likely the one that has turned brown? Justify your choice. L3 Apply

One of the two arylamines — N-methylaniline or 2-methylaniline. Aniline and other arylamines are usually colourless when pure but become coloured on storage because they are readily oxidised by atmospheric oxygen. Benzylamine, being an ordinary aliphatic amine, is far less prone to this aerial oxidation, so the brown drum is very unlikely to be the benzylamine.

4. State whether the following is true or false, and correct it if false: "Because the nitrogen atom of an amine forms three sigma bonds, the geometry around nitrogen is trigonal planar with bond angles of 120°." L2 Understand

False. Nitrogen has four electron domains — three bond pairs and one lone pair — so it is sp³ hybridised and the geometry is pyramidal, not trigonal planar. Moreover, because the lone pair repels the bond pairs more strongly than bond pairs repel each other, the C–N–E angle is less than the ideal tetrahedral 109.5°; in trimethylamine it is 108°.

5. A fourth drum contains a compound of formula C₉H₂₄N⁺Cl⁻ used as a fabric softener. Explain why this species cannot be classified as a primary, secondary or tertiary amine, and name the class to which it belongs. L5 Evaluate

The 1°/2°/3° classification counts how many hydrogens of ammonia have been replaced while nitrogen still retains its lone pair. Here all four nitrogen valencies are occupied by carbon groups, the lone pair has been used to form the fourth bond, and the nitrogen therefore carries a permanent positive charge balanced by a chloride counter-ion. Such a species is a quaternary ammonium salt. Because it has no lone pair available, it cannot act as a Lewis base at nitrogen — which is precisely why quaternary ammonium salts function as stable, permanently charged surfactants rather than as bases.

Assertion–Reason Questions

For each pair choose: (A) Both A and R are true and R is the correct explanation of A. (B) Both A and R are true but R is not the correct explanation of A. (C) A is true but R is false. (D) A is false but R is true.

Assertion (A): The C–N–C bond angle in trimethylamine is 108° rather than 109.5°.

Reason (R): The unshared electron pair on nitrogen occupies more space than a bond pair and compresses the bond angles.

Answer: A. Both statements are true and the lone pair–bond pair repulsion is exactly the reason the angle falls below the ideal tetrahedral value.

Assertion (A): H₂N–CH₂–CH₂–NH₂ is named ethane-1,2-diamine and not ethan-1,2-diamine.

Reason (R): The terminal ‘e’ of the parent hydrocarbon is retained when the suffix amine is preceded by a multiplying prefix such as di or tri.

Answer: A. For a single amino group the ‘e’ is dropped (ethane → ethanamine), but before di- or tri- it is kept, giving ethane-1,2-diamine.

Assertion (A): (CH₃)₃C–NH₂ is a tertiary amine.

Reason (R): The carbon atom bearing the –NH₂ group is attached to three methyl groups.

Answer: D. The assertion is false — amines are classified by the number of carbon groups on nitrogen, and here nitrogen carries only one, so the compound is a primary amine. The reason is a true statement about the carbon skeleton (that carbon is indeed tertiary), but it is the wrong criterion for classifying an amine.
Coming next. Part 2 takes up Section 9.4 — the six standard routes to amines, including ammonolysis, Gabriel phthalimide synthesis and the Hoffmann bromamide degradation, together with the rules for ascending and descending the amine series.

Frequently Asked Questions

Why are amines pyramidal and not planar?
The nitrogen atom of an amine has four electron domains — three bond pairs and one unshared electron pair — so it is sp3 hybridised. Four domains give a tetrahedral arrangement of electron pairs, but since one domain is a lone pair, the shape described by the atoms alone is pyramidal. The lone pair also repels the bond pairs more strongly than bond pairs repel one another, so the C–N–E angle drops below the ideal 109.5° — it is 108° in trimethylamine.
How do I decide whether an amine is primary, secondary or tertiary?
Count the number of carbon atoms bonded directly to the nitrogen atom: one carbon means primary (1°), two means secondary (2°), three means tertiary (3°). Never count substituents on carbon. This is why (CH₃)₃CNH₂ is a primary amine even though its central carbon is tertiary — the nitrogen carries just one carbon and two hydrogens.
When do I drop the 'e' in IUPAC names of amines?
Drop the terminal 'e' of the alkane when a single amine suffix follows directly: ethane becomes ethanamine, propane becomes propan-1-amine. Retain the 'e' when a multiplying prefix such as di- or tri- comes between, as in ethane-1,2-diamine and hexane-1,6-diamine.
What is the difference between a simple amine and a mixed amine?
A secondary or tertiary amine is called simple when all the alkyl or aryl groups attached to nitrogen are identical, for example (C₂H₅)₂NH or (CH₃)₃N. It is called mixed when the groups differ, for example CH₃–NH–C₂H₅ or (CH₃)₂N–C₂H₅.
Why is benzylamine not an arylamine?
In an arylamine the –NH₂ group must be bonded directly to the aromatic ring, as in aniline (C₆H₅NH₂). In benzylamine (C₆H₅CH₂NH₂) the nitrogen is bonded to an sp³ CH₂ carbon that in turn joins the ring, so the nitrogen lone pair cannot conjugate with the ring. Benzylamine therefore behaves as an ordinary aliphatic amine and is a considerably stronger base than aniline.
Why is a quaternary ammonium salt not counted as a fourth class of amine?
In primary, secondary and tertiary amines the nitrogen still retains its unshared electron pair, which is what makes them Lewis bases and nucleophiles. In a quaternary ammonium salt all four valencies of nitrogen are taken by carbon groups, the lone pair has been used up in bonding, and the nitrogen carries a permanent positive charge with a counter-ion. Having no lone pair, it cannot act as a base at nitrogen, so it is classified separately as a quaternary ammonium salt.
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