Classification of amines
NCERT Class 12 Chemistry Chapter 9 Section 9.2
Quick answer
Amines are classified as primary (RNH), secondary (RNH) or tertiary (RN) by the number of hydrogen atoms of ammonia replaced by alkyl or aryl groups, that is, by the number of carbon groups on nitrogen. They are simple when all these groups are the same and mixed when they differ. Unlike an alcohol’s class, an amine’s class does not depend on the carbon that carries the nitrogen: (CH)CNH is a primary amine.
With your textbook
NCERT Chemistry Part II (Class 12), Chapter 9 “Amines”, Section 9.2 “Classification”, page 260.
Open the official NCERT chapter (PDF, 2.9 MB, ncert.nic.in) (opens in a new tab)
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How are amines classified?
Classification of amines
Amines are primary (1°), secondary (2°) or tertiary (3°) according to how many hydrogen atoms of ammonia have been replaced by alkyl or aryl groups, and simple or mixed according to whether those groups are the same. After the secondary amine, R–NHR′, NCERT says:
The second alkyl/aryl group may be same or different. Replacement of another hydrogen atom by alkyl/aryl group leads to the formation of tertiary amine.
NCERT Chemistry Part II (Class 12), Section 9.2, page 260
In plain words: look at the nitrogen and count the carbon groups on it. One carbon group (and two H) makes a primary amine, two make a secondary amine, three make a tertiary amine. What the carbon groups themselves look like does not matter.
Primary, secondary and tertiary amines
- Replace one hydrogen of NH3 by R or Ar and you get RNH2 or ArNH2, a primary amine (1°) (NCERT page 260).
- Replace two hydrogens of ammonia by alkyl or aryl groups, that is, one hydrogen of R–NH2 by another group R′, and you get R–NHR′, a secondary amine (2°). The second group may be the same as the first or different (NCERT page 260).
- Replacing the last hydrogen as well gives a tertiary amine (3°), RR′R″N (NCERT page 260).
- When the nitrogen’s unshared pair also forms a bond, as when an amine reacts further with an alkyl halide, the product is a quaternary ammonium salt, R4N+X−, which is not an amine (NCERT page 262; reasoned).
| Class | General formula | H left on N | Example |
|---|---|---|---|
| primary (1°) | RNH2 | 2 | CH3NH2 |
| secondary (2°) | R2NH | 1 | CH3NHCH3 |
| tertiary (3°) | R3N | 0 | (CH3)3N |
The examples are NCERT’s own (page 259); arylamines such as C6H5NH2 are classified in the same way.
Simple and mixed amines
- Amines are simple when all the alkyl or aryl groups on nitrogen are the same, and mixed when they are different (NCERT page 260).
- For example, (CH3)3N is a simple tertiary amine, and CH3NHCH2CH3 a mixed secondary amine (reasoned). A primary amine has only one group, so the distinction applies to secondary and tertiary amines (reasoned).
Not the same as alcohol classes
- For alcohols and haloalkanes, “primary, secondary, tertiary” describes the carbon that carries the –OH or the halogen. For amines it describes the nitrogen: how many carbon groups it carries (NCERT Unit 6, page 160, and Unit 7, page 194; reasoned from NCERT page 260).
- So (CH3)3C–NH2 is a primary amine, although its –NH2 sits on a tertiary carbon, while (CH3)3C–OH is a tertiary alcohol (reasoned).
Solved examples
Example 1: NCERT’s four amines
Classify these amines as primary, secondary or tertiary: (i) naphthalen-1-amine (–NH2 on a naphthalene ring); (ii) the same ring carrying –N(CH3)2; (iii) (C2H5)2CHNH2; (iv) (C2H5)2NH. (NCERT Intext Question 9.1, page 262.)
Answer: (i) primary: one aryl group on N; (ii) tertiary: the aryl group and two methyl groups; (iii) primary: the nitrogen carries one carbon group, the pentan-3-yl group, although that carbon is itself secondary; (iv) secondary: two ethyl groups (reasoned from NCERT page 260).
Example 2: Simple or mixed?
Say whether each is simple or mixed: (C2H5)3N, C6H5NHCH3, (CH3)2NC2H5.
Answer: (C2H5)3N is a simple tertiary amine: three ethyl groups. C6H5NHCH3 is a mixed secondary amine: phenyl and methyl. (CH3)2NC2H5 is a mixed tertiary amine: two methyl groups and an ethyl group (reasoned from NCERT page 260).
Common mistakes to avoid
Common mistake
Classifying amines by the carbon, as for alcohols
Count the carbon groups on nitrogen. (CH3)3CNH2 is a primary amine, not a tertiary one.
Common mistake
Calling a quaternary ammonium salt a tertiary amine
R4N+X− has four groups on nitrogen and no unshared pair; it is a salt, not an amine.
Quick revision summary
- 1° RNH2 or ArNH2; 2° R–NHR′; 3° RR′R″N: classified by the number of H of ammonia replaced.
- Simple: all groups on N the same; mixed: different.
- Amine class counts groups on N, unlike alcohol class, which describes the carbon carrying –OH.
Formula sheet
- Primary: RNH (two H on N)
- Secondary: RNH (one H on N)
- Tertiary: RN (no H on N)
- Quaternary ammonium salt: RNX
Questions students ask
Is benzylamine an aromatic amine like aniline?
Benzylamine, C6H5CH2NH2, is not an aromatic amine (arylamine) like aniline, although it contains a benzene ring: an arylamine has nitrogen directly bonded to an aryl carbon; primary, secondary and tertiary examples exist (compare NCERT pages 260–261), and in benzylamine it sits on a CH2 carbon, so its nitrogen behaves as in an aliphatic amine (reasoned). Both are primary amines, since each nitrogen carries one carbon group and two hydrogens (NCERT page 260). Its isomer o-toluidine, CH3C6H4NH2, is a true arylamine, because there the –NH2 is on the ring (NCERT Table 9.1, page 261; reasoned).
Are amino acids primary amines?
In the neutral structural formulas of the twenty common protein amino acids, nineteen have a primary α-amino group. Proline instead has a secondary amino group because its side chain closes a ring through nitrogen. Amino acids commonly exist as zwitterions, with a protonated amino group and a carboxylate group. This classification describes the underlying amino group; additional nitrogen-containing side chains, as in lysine or arginine, must be considered separately.
Is there a quaternary amine after primary, secondary and tertiary amines?
There is no fourth class of amine: a so-called quaternary amine is a quaternary ammonium salt, R4N+X−, formed when a tertiary amine’s unshared pair bonds a fourth alkyl group, the last step of ammonolysis (NCERT pages 262–263). Beyond the NCERT text, “quaternary amine” is loose usage for these salts, in the oil industry for example, and their cation cannot lose its charge by ordinary N–H deprotonation, because it has no N–H bond, while an ordinary amine salt gives back its amine with a base (Wikipedia, “Quaternary ammonium cation”; NCERT page 267). With one long alkyl group such an ion has a charged head and an oily tail, so these salts act as surfactants (NCERT page 259; reasoned).
Practice questions
Try each question first, then open the answer to check your working.
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Q1. Classify CH3CH2NH2, (CH3)2NH and C6H5N(CH3)2.
Show answerHide answer
Primary (one ethyl group), secondary (two methyl groups) and tertiary (a phenyl and two methyl groups) (reasoned from NCERT page 260).
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Q2. What makes an amine “mixed”?
Show answerHide answer
Having different alkyl or aryl groups on the nitrogen; an amine with all its groups the same is “simple” (NCERT page 260).