Classification And Naming Of Amines

Q: Why is benzylamine not an aromatic amine?

In benzylamine, C6H5CH2NH2, the nitrogen is attached to a CH2 group, not to the benzene ring. Its lone pair is not delocalised into the ring, so it behaves like an aliphatic amine and is almost as basic (pKb 4.70). Only amines with nitrogen directly on the ring, like aniline, are aromatic.

Q: Is aniline an accepted IUPAC name?

Yes. IUPAC keeps aniline as the name of C6H5NH2, and benzenamine is also accepted. Substituted anilines are numbered from the ring carbon that carries the NH2 group, which is always C1. So p-bromoaniline is 4-bromoaniline or 4-bromobenzenamine, o-toluidine is 2-methylaniline, and a group on nitrogen takes the prefix N-.

Q: How many isomeric amines have the formula C4H11N?

C4H11N has eight amine isomers. Four are primary: butan-1-amine, butan-2-amine, 2-methylpropan-1-amine and 2-methylpropan-2-amine. Three are secondary: N-methylpropan-1-amine, N-methylpropan-2-amine and N-ethylethanamine. One is tertiary: N,N-dimethylethanamine. Only butan-2-amine has a chiral carbon, so it is the only one that can be optically active.

ChemistryAminesFor JEE aspirants

Classification and naming of amines rests on one idea: amines are ammonia with one, two or three hydrogen atoms replaced by alkyl or aryl groups. The number of carbon groups on nitrogen makes an amine primary (1°), secondary (2°) or tertiary (3°), and its IUPAC name ends in "amine" with N-prefixes for the other groups on nitrogen. This page covers the classification and naming of amines, their pyramidal shape, nitrogen inversion and isomer counting, with the NCERT examples asked in JEE Main and NEET.

Key Formulas - Quick Reference

General formula of open-chain saturated amines: $C_{n} H_{2n + 3} N$ (same for 1°, 2° and 3°)
Class = number of carbon groups on N: 1° $RN H_{2}$ , 2° $R_{2} NH$ , 3° $R_{3} N$ , quaternary salt $R_{4} N^{+} X^{-}$
IUPAC, 1° amine: alkane minus final 'e' plus 'amine': $C H_{3} C H_{2} N H_{2}$ = ethanamine
Locant of the C-N carbon goes before 'amine': $C H_{3} CH (N H_{2}) C H_{3}$ = propan-2-amine
2° and 3° amines: largest group on N is the parent, others are N-prefixes: $(C H_{3})_{2} NC H_{2} C H_{3}$ = N,N-dimethylethanamine
Diamines keep the 'e': $H_{2} N (C H_{2})_{6} N H_{2}$ = hexane-1,6-diamine
Aryl amines: $C_{6} H_{5} N H_{2}$ = aniline (benzenamine); ring C1 carries $N H_{2}$ : 4-bromoaniline
Senior group present: $N H_{2}$ becomes the prefix 'amino': $H_{2} NC H_{2} C H_{2} OH$ = 2-aminoethan-1-ol
Shape: $sp^{3}$ N, pyramidal, C-N-C angle $10 8^{\circ}$ in $(C H_{3})_{3} N$
Isomer counts: $C_{2} H_{7} N$ = 2, $C_{3} H_{9} N$ = 4, $C_{4} H_{11} N$ = 8, $C_{5} H_{13} N$ = 17

1. What Are Amines?

Amines are organic derivatives of ammonia. Replace one, two or three hydrogen atoms of $N H_{3}$ with alkyl or aryl groups and you get a primary, secondary or tertiary amine. Nitrogen keeps its lone pair, which is why amines behave as bases and nucleophiles.

Amine: a compound in which nitrogen is bonded to one, two or three carbon (alkyl or aryl) groups, the remaining bonds on nitrogen being to hydrogen, and nitrogen still carries one lone pair.

Open-chain saturated amines of every class share the general formula $C_{n} H_{2n + 3} N$ . For example, ethanamine $C H_{3} C H_{2} N H_{2}$ and N-methylmethanamine $(C H_{3})_{2} NH$ are both $C_{2} H_{7} N$ .

Two look-alikes that are not amines. In an amide such as ethanamide

C H_{3} CON H_{2}

, nitrogen is bonded to a C=O group, so it is named as an amide. In a quaternary ammonium salt such as

(C H_{3})_{4} N^{+} C l^{-}

, nitrogen carries four carbon groups and no lone pair; it is named with the amines but is a salt, not a base.

2. Classification of Amines

2.1 Primary, Secondary and Tertiary Amines

Count the carbon groups attached directly to nitrogen. One group gives a primary (1°) amine, two give a secondary (2°) amine and three give a tertiary (3°) amine. A fourth group makes nitrogen positive and gives a quaternary ammonium salt.

Figure 1: The class of an amine is the number of carbon groups on nitrogen, so tert-butylamine is a

1^{\circ}

amine even though its carbon is tertiary.

Class	General formula	N-H bonds	Example	IUPAC name
1° amine	$RN H_{2}$	2	$C H_{3} C H_{2} N H_{2}$	ethanamine
2° amine	$R_{2} NH$	1	$(C H_{3})_{2} NH$	N-methylmethanamine
3° amine	$R_{3} N$	0	$(C H_{3})_{3} N$	N,N-dimethylmethanamine
Quaternary salt	$R_{4} N^{+} X^{-}$	0 (no lone pair)	$(C H_{3})_{4} N^{+} C l^{-}$	tetramethylammonium chloride

Exam Trick

Amines count carbons on N; alcohols count carbons on C. $(C H_{3})_{3} C - N H_{2}$ (tert-butylamine, 2-methylpropan-2-amine) has only one carbon on nitrogen, so it is a 1° amine, while $(C H_{3})_{3} C - OH$ is a 3° alcohol. Check the atom that carries the functional group, not the carbon skeleton.

2.2 Aliphatic, Aromatic, Aralkyl and Heterocyclic Amines

The second way to classify an amine looks at the kind of carbon joined to nitrogen.

Aliphatic amines (alkylamines): nitrogen is on an $sp^{3}$ carbon of an alkyl or cycloalkyl group, e.g. methylamine $C H_{3} N H_{2}$ and cyclohexylamine $C_{6} H_{11} N H_{2}$ .
Aromatic amines (arylamines): nitrogen is directly on a benzene ring, e.g. aniline $C_{6} H_{5} N H_{2}$ , N-methylaniline $C_{6} H_{5} NHC H_{3}$ and the toluidines $C H_{3} C_{6} H_{4} N H_{2}$ . One aryl group on N is enough to call the amine aromatic.
Aralkyl amines: nitrogen is on a side-chain carbon of an aromatic compound, e.g. benzylamine $C_{6} H_{5} C H_{2} N H_{2}$ . Chemically they behave like aliphatic amines.
Heterocyclic amines: nitrogen is a ring atom, e.g. piperidine $C_{5} H_{10} NH$ and pyrrolidine $C_{4} H_{8} NH$ , both 2° amines. Pyridine and pyrrole also have ring nitrogen, but they are aromatic heterocycles, not simple amines.

Figure 2: Only an amine whose nitrogen sits directly on the benzene ring is aromatic; benzylamine is aralkyl and almost as basic as aliphatic amines.

2.3 Simple, Mixed and Polyamines

Simple amines carry identical groups on nitrogen: diethylamine $(C_{2} H_{5})_{2} NH$ , trimethylamine $(C H_{3})_{3} N$ .
Mixed amines carry different groups: ethylmethylamine $C H_{3} NH C_{2} H_{5}$ , N-ethyl-N-methylaniline $C_{6} H_{5} N (C H_{3}) C_{2} H_{5}$ .
Diamines and polyamines have two or more amino groups: ethane-1,2-diamine $H_{2} NC H_{2} C H_{2} N H_{2}$ and hexane-1,6-diamine $H_{2} N (C H_{2})_{6} N H_{2}$ , a monomer of nylon-6,6.

3. Structure of Amines

3.1 Pyramidal Nitrogen

Nitrogen in an amine is $sp^{3}$ hybridised, just as in ammonia. Three $sp^{3}$ orbitals form sigma bonds to carbon or hydrogen and the fourth holds the lone pair. The molecule is therefore pyramidal. The lone pair repels the bonding pairs, so the bond angle is a little less than the tetrahedral $109. 5^{\circ}$ : the C-N-C angle in trimethylamine is $10 8^{\circ}$ .

Figure 3: Amines are pyramidal like ammonia; a chiral tertiary amine turns inside out like an umbrella, so its two enantiomers cannot be separated.

3.2 Nitrogen Inversion and Optical Activity

A tertiary amine with three different groups, such as N-ethyl-N-methylpropan-1-amine, has no plane of symmetry, so its nitrogen looks like a stereocentre. Yet the compound is never optically active. The pyramid turns inside out like an umbrella in a gust of wind, passing through a flat $sp^{2}$ transition state. The energy barrier is so small that the two mirror-image forms interconvert rapidly at room temperature. This is called nitrogen inversion (the umbrella effect), and it keeps every sample racemic.

Not resolvable: 3° amines whose only "stereocentre" is nitrogen; 1° and 2° amines also invert (and exchange their N-H protons).
Resolvable: amines with a chiral carbon, such as butan-2-amine $C H_{3} CH (N H_{2}) C H_{2} C H_{3}$ ; quaternary ammonium salts with four different groups, because with no lone pair the nitrogen cannot invert.

JEE Advanced

The inversion barrier of simple amines is only about $25 - 30 kJ mol^{- 1}$ , so each molecule flips many millions of times a second at $298 K$ . Inversion is stopped, and enantiomers become separable, when nitrogen has no lone pair to flip (quaternary ammonium ions and amine oxides $R_{3} N^{+} - O^{-}$ with three different R groups) or when a rigid cage holds nitrogen in place: Tröger's base, with two bridgehead nitrogens, was resolved in 1944. Small rings such as aziridines also invert much more slowly because the flat transition state is badly strained.

3.3 Structure of Aniline

In aniline the nitrogen lone pair overlaps with the $π$ system of the ring. Nitrogen becomes flatter than in alkylamines, and the C-N bond gains some double-bond character: it is about $140 pm$ long, compared with about $147 pm$ in methylamine. Because the lone pair is shared with the ring, aniline is a much weaker base than aliphatic amines, which is taken up in Properties of Amines.

4. Common Names of Amines

Primary amines: name the alkyl group and add "amine" as one word: methylamine $C H_{3} N H_{2}$ , isopropylamine $(C H_{3})_{2} CHN H_{2}$ .
Secondary and tertiary amines: list the alkyl groups in alphabetical order before "amine"; use di- or tri- for identical groups: dimethylamine $(C H_{3})_{2} NH$ , ethylmethylamine $C H_{3} NH C_{2} H_{5}$ , triethylamine $(C_{2} H_{5})_{3} N$ .
Aryl amines: use the parent name aniline and its family names: the methylanilines are o-, m- and p-toluidine, the methoxyanilines are the anisidines, and N-alkyl derivatives are named as N-methylaniline and N,N-dimethylaniline.

Figure 4: The common name lists every group on N; the IUPAC name picks the largest group as the parent chain and turns the rest into N-prefixes.

5. IUPAC Naming of Amines

5.1 Primary Amines

Choose the longest carbon chain that contains the carbon bonded to nitrogen.
Number the chain from the end nearer to that carbon.
Replace the final "e" of the alkane with "amine": ethane becomes ethanamine.
Write the locant of the carbon carrying $N H_{2}$ just before "amine": $C H_{3} C H_{2} C H_{2} N H_{2}$ is propan-1-amine and $C H_{3} CH (N H_{2}) C H_{3}$ is propan-2-amine.

5.2 Secondary and Tertiary Amines

Take the largest carbon group on nitrogen as the parent and name it as an alkanamine.
Name each other group on nitrogen as a prefix with the locant N: $C H_{3} NHC H_{2} C H_{3}$ is N-methylethanamine.
Use N,N- for two identical groups: $(C H_{3})_{3} N$ is N,N-dimethylmethanamine.
Arrange different N-prefixes alphabetically: $C H_{3} C H_{2} C H_{2} N (C H_{3}) C H_{2} C H_{3}$ is N-ethyl-N-methylpropan-1-amine.

Figure 5: For

2^{\circ}

and

3^{\circ}

amines the largest group on N is the parent chain and every other group becomes an N-prefix: N-ethyl-N-methylpropan-1-amine.

5.3 Diamines and Unsaturated Amines

With two amino groups the suffix is "diamine" and the "e" of the alkane is kept, because the next letter is a consonant: $H_{2} NC H_{2} C H_{2} N H_{2}$ is ethane-1,2-diamine and $H_{2} N (C H_{2})_{6} N H_{2}$ is hexane-1,6-diamine. A double bond is shown by "en", and the carbon carrying nitrogen still gets the lowest locant: allylamine $C H_{2} = CHC H_{2} N H_{2}$ is prop-2-en-1-amine.

5.4 Aryl Amines

IUPAC keeps aniline for $C_{6} H_{5} N H_{2}$ ; benzenamine is also accepted. The ring carbon carrying $N H_{2}$ is always C1, and substituents are numbered from it: o-toluidine is 2-methylaniline, p-bromoaniline is 4-bromoaniline (4-bromobenzenamine) and $C_{6} H_{5} N (C H_{3})_{2}$ is N,N-dimethylaniline (N,N-dimethylbenzenamine). The prefix N- shows that a group sits on nitrogen and not on the ring.

Figure 6: Numbers place a group on the ring counted from the C-N carbon; the prefix N- places it on the nitrogen itself, so 2-methylaniline and N-methylaniline are different compounds.

5.5 When the Amino Group Is Only a Substituent

If the molecule also contains a group that ranks higher, that group decides the suffix and $N H_{2}$ is written as the prefix amino. The common groups rank as follows, highest first:

Rank	Group	As suffix	As prefix
1	carboxylic acid	-oic acid	carboxy
2	ester	-oate	alkoxycarbonyl
3	acid halide	-oyl halide	halocarbonyl
4	amide	-amide	carbamoyl
5	nitrile	-nitrile	cyano
6	aldehyde	-al	formyl or oxo
7	ketone	-one	oxo
8	alcohol	-ol	hydroxy
9	amine	-amine	amino

So $H_{2} NC H_{2} C H_{2} OH$ is 2-aminoethan-1-ol, glycine $H_{2} NC H_{2} COOH$ is 2-aminoethanoic acid and $H_{2} N C_{6} H_{4} COOH$ (para) is 4-aminobenzoic acid.

5.6 Ammonium Salts

Protonated amines and quaternary ions are named as substituted ammonium ions: $C H_{3} N H_{3}^{+} C l^{-}$ is methylammonium chloride (methanaminium chloride), $C_{6} H_{5} N H_{3}^{+} C l^{-}$ is anilinium chloride and $C_{6} H_{5} N (C H_{3})_{3}^{+} I^{-}$ is N,N,N-trimethylanilinium iodide.

The table collects the NCERT examples with both kinds of name.

Amine	Common name	IUPAC name
$C H_{3} - C H_{2} - N H_{2}$	Ethylamine	Ethanamine
$C H_{3} - C H_{2} - C H_{2} - N H_{2}$	n-Propylamine	Propan-1-amine
$C H_{3} - CH (N H_{2}) - C H_{3}$	Isopropylamine	Propan-2-amine
$C H_{3} - NH - C H_{2} - C H_{3}$	Ethylmethylamine	N-Methylethanamine
$C H_{3} - N (C H_{3}) - C H_{3}$	Trimethylamine	N,N-Dimethylmethanamine
$C_{2} H_{5} - N (C_{2} H_{5}) - C H_{2} C H_{2} C H_{2} C H_{3}$	N,N-Diethylbutylamine	N,N-Diethylbutan-1-amine
$N H_{2} - C H_{2} - CH = C H_{2}$	Allylamine	Prop-2-en-1-amine
$N H_{2} - (C H_{2})_{6} - N H_{2}$	Hexamethylenediamine	Hexane-1,6-diamine
$C_{6} H_{5} - N H_{2}$	Aniline	Aniline or Benzenamine
$C H_{3} C_{6} H_{4} N H_{2}$ (ortho)	o-Toluidine	2-Methylaniline
$Br C_{6} H_{4} N H_{2}$ (para)	p-Bromoaniline	4-Bromobenzenamine or 4-Bromoaniline
$C_{6} H_{5} - N (C H_{3})_{2}$	N,N-Dimethylaniline	N,N-Dimethylbenzenamine

Figure 7: Three questions, asked in this order, name every amine on this page.

6. Isomerism in Amines

Amines show four kinds of structural isomerism. All of them appear in "how many isomers" questions.

Type	What changes	Example pair
Chain isomerism	the carbon skeleton	butan-1-amine and 2-methylpropan-1-amine
Position isomerism	the carbon that carries N	propan-1-amine and propan-2-amine
Functional (class) isomerism	1° vs 2° vs 3°	propan-1-amine, N-methylethanamine, N,N-dimethylmethanamine (all $C_{3} H_{9} N$ )
Metamerism	the groups on N of a 2° or 3° amine	N-ethylethanamine and N-methylpropan-1-amine

Figure 8:

C_{4} H_{11} N

has 8 amine isomers (4 primary, 3 secondary, 1 tertiary); only butan-2-amine has a carbon stereocentre.

Exam Trick

Split the carbons among the groups on N. For $C_{n} H_{2n + 3} N$ : primary isomers = number of different alkyl groups with n carbons; secondary = ways to share n carbons between two groups; tertiary = ways to share them among three. For $C_{4} H_{11} N$ : 4 butyl groups give 4 primary; methyl + propyl (2 kinds) and ethyl + ethyl give 3 secondary; methyl + methyl + ethyl gives 1 tertiary. Total 8. The same split gives 17 for $C_{5} H_{13} N$ (8 + 6 + 3).

Figure 9: Class, type, shape, name and isomers all come from one question: what is attached to the nitrogen?

7. Solved Examples

Solved Example 1

Which of the following exists as a racemic mixture that cannot be resolved?
(A)

C H_{3} CH (N H_{2}) C H_{2} C H_{3}

(B)

C H_{3} C H_{2} NHC H_{2} C H_{3}

(C)

C H_{3} C H_{2} C H_{2} NHC H_{3}

(D)

C H_{3} C H_{2} C H_{2} N (C H_{3}) C H_{2} C H_{3}

Solution:

Answer: (D). N-ethyl-N-methylpropan-1-amine is a 3° amine with three different groups, so each pyramidal form is chiral. Nitrogen inversion (Figure 3) turns one form into its mirror image many times a second, so the compound always exists as a racemic mixture that cannot be resolved.

(A) butan-2-amine has a chiral carbon; its enantiomers do not interconvert and can be resolved.
(B) N-ethylethanamine carries two identical ethyl groups, so it is achiral.
(C) N-methylpropan-1-amine inverts at nitrogen and also exchanges its N-H proton, so it cannot be resolved either; it is simply treated as achiral.

Solved Example 2

The statement NOT correct for piperidine is
(A) it is a heterocyclic compound
(B) it is a homocyclic compound
(C) it is more basic than pyridine
(D) it is more basic than aniline

Solution:

Answer: (B). The ring of piperidine $C_{5} H_{10} NH$ contains a nitrogen atom, so it is heterocyclic, not homocyclic. Its nitrogen is an $sp^{3}$ 2° amine nitrogen ( $p K_{b} = 2.88$ ), so it is far more basic than pyridine ( $sp^{2}$ N, $p K_{b} \approx 8.8$ ) and aniline ( $p K_{b} = 9.38$ ). Statements (A), (C) and (D) are correct.

Solved Example 3

N-Ethyl-N-methylpropan-1-amine has no plane of symmetry, yet it shows no optical activity. Explain.

Solution:

The molecule exists as two pyramidal forms, I and II, which are mirror images (Figure 3). They pass into each other through a planar transition state, and the energy difference to reach it is very small. The two forms therefore interconvert rapidly, so they cannot be separated and any sample contains equal amounts of both. The rotations cancel and no optical activity is seen. This is nitrogen inversion, also called the umbrella effect.

Solved Example 4

How many isomeric amines with the formula

C_{7} H_{9} N

contain a benzene ring?
(A) two
(B) three
(C) four
(D) five

Solution:

Answer: (D). $C_{7} H_{9} N$ has 4 degrees of unsaturation, which is exactly one benzene ring. The five amines are:

benzylamine $C_{6} H_{5} C H_{2} N H_{2}$ (1°, aralkyl)
N-methylaniline $C_{6} H_{5} NHC H_{3}$ (2°, aromatic)
2-, 3- and 4-methylaniline ( $C H_{3} C_{6} H_{4} N H_{2}$ , the o-, m- and p-toluidines; 1°, aromatic)

Solved Example 5

The IUPAC name of

C H_{3} C H_{2} C H_{2} N (C H_{3}) C H_{2} C H_{3}

is
(A) N-methyl-N-propylethanamine
(B) N-ethyl-N-methylpropan-1-amine
(C) N-ethyl-N-propylmethanamine
(D) 1-(ethylmethylamino)propane

Solution:

Answer: (B). The largest group on nitrogen is propyl, so the parent is propan-1-amine. The ethyl and methyl groups on nitrogen become N-prefixes in alphabetical order: N-ethyl-N-methylpropan-1-amine (Figure 5). (A) and (C) chose a smaller parent group; (D) treats the amine as a substituent although no senior group is present.

Solved Example 6

Classify each as a 1°, 2° or 3° amine or a quaternary salt, and as aliphatic, aromatic, aralkyl or heterocyclic:
(i)

(C H_{3})_{3} C - N H_{2}

(ii)

(C H_{3})_{2} CH - NH - C H_{3}

(iii)

C_{6} H_{5} N (C H_{3})_{2}

(iv) piperidine
(v)

(C H_{3})_{4} N^{+} C l^{-}

(vi)

C_{6} H_{5} C H_{2} N H_{2}

Solution:

Compound	Carbon groups on N	Class	Type
(i) 2-methylpropan-2-amine	1	1° amine	aliphatic
(ii) N-methylpropan-2-amine	2	2° amine	aliphatic
(iii) N,N-dimethylaniline	3	3° amine	aromatic
(iv) piperidine	2 (ring carbons)	2° amine	heterocyclic
(v) tetramethylammonium chloride	4	quaternary salt	aliphatic
(vi) benzylamine	1	1° amine	aralkyl

The trap is (i): its carbon is tertiary, but nitrogen carries only one carbon group, so it is a primary amine.

Solved Example 7

Draw and name all the amines with molecular formula

C_{4} H_{11} N

. How many are primary, and which one is optically active?

Solution:

There are 8 isomers (Figure 8).

Primary (4): butan-1-amine, butan-2-amine, 2-methylpropan-1-amine, 2-methylpropan-2-amine.
Secondary (3): N-methylpropan-1-amine, N-methylpropan-2-amine, N-ethylethanamine.
Tertiary (1): N,N-dimethylethanamine.

Only butan-2-amine $C H_{3} CH (N H_{2}) C H_{2} C H_{3}$ is optically active: C-2 carries four different groups (H, $N H_{2}$ , $C H_{3}$ , $C_{2} H_{5}$ ), and a carbon stereocentre does not invert.

Solved Example 8

Give IUPAC names:
(a)

H_{2} N - C H_{2} - C H_{2} - OH

(b)

H_{2} N - C H_{2} - COOH

(c)

H_{2} N C_{6} H_{4} COOH

, with the two groups para
(d)

N \equiv C - C H_{2} - C H_{2} - C H_{2} - N H_{2}

Solution:

In each molecule a group senior to the amine sets the suffix, so $N H_{2}$ becomes the prefix "amino".

(a) alcohol is senior: 2-aminoethan-1-ol
(b) carboxylic acid is senior: 2-aminoethanoic acid (glycine)
(c) carboxylic acid on C1 of the ring: 4-aminobenzoic acid
(d) nitrile is senior and its carbon is C1, so $N H_{2}$ is on C4: 4-aminobutanenitrile

Practice Questions

Give the IUPAC name of $C H_{3} C H_{2} CH (N H_{2}) C H_{2} C H_{3}$ .Answer: pentan-3-amine
Give the IUPAC name of $(C H_{3})_{2} CHNHC H_{3}$ .Answer: N-methylpropan-2-amine
Name $C H_{2} = CHC H_{2} N H_{2}$ by the IUPAC system.Answer: prop-2-en-1-amine (allylamine)
Name $H_{2} N (C H_{2})_{4} N H_{2}$ .Answer: butane-1,4-diamine
Write the structure of N-ethyl-N-methylaniline and state its class.Answer: $C_{6} H_{5} N (C H_{3}) C_{2} H_{5}$ , a 3° aromatic amine
What is the IUPAC name of m-toluidine?Answer: 3-methylaniline (3-methylbenzenamine)
How many isomeric amines have the formula $C_{3} H_{9} N$ ? Name them.Answer: 4: propan-1-amine, propan-2-amine, N-methylethanamine, N,N-dimethylmethanamine
Is 2-methylpropan-2-amine a 1°, 2° or 3° amine?Answer: 1° (nitrogen carries only one carbon group)
Which can be resolved into enantiomers: ethylmethylpropylamine or ethylmethylphenylpropylammonium iodide?Answer: the quaternary salt; with no lone pair its nitrogen cannot invert
How many isomeric amines have the formula $C_{5} H_{13} N$ ?Answer: 17 (8 primary, 6 secondary, 3 tertiary)

Common Mistakes to Avoid

Watch out

Calling tert-butylamine (2-methylpropan-2-amine) a 3° amine. The class depends on the number of carbon groups on nitrogen, and here there is only one.
Treating benzylamine $C_{6} H_{5} C H_{2} N H_{2}$ as an aromatic amine. Its nitrogen sits on a $C H_{2}$ group, so it is an aralkyl amine that behaves like an aliphatic one.
Choosing the parent of a 2° or 3° amine by alphabetical order. The parent is the largest group on nitrogen; the alphabet only orders the N-prefixes.
Leaving out the locant N: "dimethylmethanamine" should be N,N-dimethylmethanamine, and "methylethanamine" should be N-methylethanamine.
Dropping the 'e' in diamines: "ethan-1,2-diamine" is wrong; the name is ethane-1,2-diamine.
Using the suffix 'amine' when a senior group is present: $H_{2} NC H_{2} C H_{2} OH$ is 2-aminoethan-1-ol, not 2-hydroxyethanamine.
Counting amides such as $C H_{3} CON H_{2}$ as amines. A C=O group on nitrogen makes the compound an amide.
Expecting a 3° amine with three different groups to be optically active. Nitrogen inversion racemises it; only a carbon stereocentre or a quaternary nitrogen can be resolved.
Numbering a substituted aniline from any ring carbon. C1 is always the carbon that carries $N H_{2}$ .

Frequently Asked Questions

What are primary, secondary and tertiary amines?

They are amines in which one, two or three hydrogen atoms of ammonia are replaced by carbon groups: $RN H_{2}$ is primary, $R_{2} NH$ is secondary and $R_{3} N$ is tertiary. The class counts the carbon groups on nitrogen, not the type of carbon, so tert-butylamine is primary. Four groups give a quaternary ammonium salt.

How do you write the IUPAC name of a secondary or tertiary amine?

Take the largest carbon group on nitrogen as the parent alkanamine. Name every other group on nitrogen as a prefix with the locant N, and list different prefixes in alphabetical order. So $C H_{3} NH C_{2} H_{5}$ is N-methylethanamine, $(C H_{3})_{3} N$ is N,N-dimethylmethanamine and $C H_{3} C H_{2} C H_{2} N (C H_{3}) C_{2} H_{5}$ is N-ethyl-N-methylpropan-1-amine, because propyl is the largest group.

Why is benzylamine not an aromatic amine?

In benzylamine, $C_{6} H_{5} C H_{2} N H_{2}$ , the nitrogen is attached to a $C H_{2}$ group, not to the benzene ring. Its lone pair is not delocalised into the ring, so it behaves like an aliphatic amine and is almost as basic (pKb 4.70). Only amines with nitrogen directly on the ring, like aniline, are aromatic.

Is aniline an accepted IUPAC name?

Yes. IUPAC keeps aniline as the name of $C_{6} H_{5} N H_{2}$ , and benzenamine is also accepted. Substituted anilines are numbered from the ring carbon that carries the $N H_{2}$ group, which is always C1. So p-bromoaniline is 4-bromoaniline or 4-bromobenzenamine, o-toluidine is 2-methylaniline, and a group on nitrogen takes the prefix N-.

What is the shape of an amine molecule?

The nitrogen of an amine is sp3 hybridised. Three orbitals form bonds and the fourth holds a lone pair, so the molecule is pyramidal like ammonia. Lone pair repulsion squeezes the bond angle slightly below the tetrahedral value: the C-N-C angle in trimethylamine is 108 degrees.

Why can a chiral tertiary amine not be resolved into enantiomers?

A tertiary amine with three different groups flips like an umbrella through a flat transition state. The energy barrier is only about 25 to 30 kJ per mole, so the two mirror-image forms interconvert rapidly at room temperature. This nitrogen inversion keeps the sample racemic, so it cannot be separated.

How many isomeric amines have the formula C4H11N?

$C_{4} H_{11} N$ has eight amine isomers. Four are primary: butan-1-amine, butan-2-amine, 2-methylpropan-1-amine and 2-methylpropan-2-amine. Three are secondary: N-methylpropan-1-amine, N-methylpropan-2-amine and N-ethylethanamine. One is tertiary: N,N-dimethylethanamine. Only butan-2-amine has a chiral carbon, so it is the only one that can be optically active.

What kind of naming questions on amines appear in NEET?

NEET usually asks for the IUPAC name of a given amine, the class of a structure (primary, secondary or tertiary), or a match between common and IUPAC names from the NCERT table, such as trimethylamine with N,N-dimethylmethanamine and hexamethylenediamine with hexane-1,6-diamine.

How does JEE Main test the classification of amines?

JEE Main often asks you to count the isomeric amines of a formula such as $C_{4} H_{11} N$ or $C_{7} H_{9} N$ , to find the only chiral isomer, or to explain why a tertiary amine with three different groups is optically inactive. Knowing the class rule and nitrogen inversion answers most of these.

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