26.1 Amines – Structure and Naming

Learning Objectives

By the end of this section, you will be able to:

  • Identify the general structure for an amine.
  • Identify the functional group for amines.
  • Determine the structural feature that classifies amines as primary, secondary, or tertiary.
  • Use nomenclature systems to name amines.

Amines are molecules that contain carbon-nitrogen bonds. The nitrogen atom in an amine has a lone pair of electrons and three bonds to other atoms, either carbon or hydrogen. Various nomenclatures are used to derive names for amines, but all involve the class-identifying suffix –ine as illustrated here for a few simple examples:

Three structures are shown, each with a red, central N atom which has a pair of electron dots indicated in red above the N atoms. The first structure is labeled methyl amine. To the left of the N, a C H subscript 3 group is bonded. H atoms are bonded to the right and bottom of the central N atom. The second structure is labeled dimethyl amine. This structure has C H subscript 3 groups bonded to the left and right of the N atom and a single H atom is bonded below. The third structure is labeled trimethyl amine, which has C H subscript 3 groups bonded to the left, right, and below the central N atom.
Figure 26.1a. Examples of amines. (Credit: Chemistry 2e, OpenStax, CC BY 4.0)

In some amines, the nitrogen atom replaces a carbon atom in an aromatic hydrocarbon. Pyridine (Figure 26.1b.) is one such heterocyclic amine. A heterocyclic compound contains atoms of two or more different elements in its ring structure.

A molecular structure is shown. A ring of five C atoms and one N atom is shown with alternating double bonds. Single H atoms are bonded, appearing at the outside of the ring on each C atom. The N atom has an unshared electron pair shown on the N atom on the outer side of the ring. The N atom, electron dot pair, and bonds connected to it in the ring are shown in red.
Figure 26.1b. The illustration shows one of the resonance structures of pyridine. (Credit: Chemistry 2e, OpenStax, CC BY 4.0)

Classifying Amines

Amines can be either alkyl-substituted (alkylamines) or aryl-substituted (arylamines). Although much of the chemistry of the two classes is similar, there are also substantial differences. Amines are classified according to the number of carbon atoms bonded directly to the nitrogen atom. They are classified as primary (RNH2), secondary (R2NH), or tertiary (R3N), depending on the number of organic substituents attached to nitrogen (Figure 26.1c.). A primary (1°) amine has one alkyl (or aryl) group on the nitrogen atom, a secondary (2°) amine has two, and a tertiary (3°) amine has three (Figure 26.1c.). Thus, methylamine (CH3NH2) is a primary amine, dimethylamine [(CH3)2NH] is a secondary amine, and trimethylamine [(CH3)3N] is a tertiary amine. Note that this usage of the terms primary, secondary, and tertiary differs from our previous usage. When we speak of a tertiary alcohol or alkyl halide, we refer to the degree of substitution at the alkyl carbon atom, but when we speak of a tertiary amine, we refer to the degree of substitution at the nitrogen atom (Figure 26.1d.).
Each individual compound represents a different molecule or organic compound. The first one represents water (2 hydrogen, 1 oxygen). The second represents an alcohol with a hydroxyl group (OH). The third represents an ether with an O bonded to 2 carbons. The bottom row represents Ammonia and then a primary, secondary and tertiary amine. We can see how ammonia is nitrogen with 3 surrounding hydrogens. A primary amine is nitrogen with 1 carbon and 2 hydrogens. A secondary amine is a nitrogen with 2 carbons and 1 hydrogen. Lastly a tertiary amine is a nitrogen with 3 carbons.
Figure 26.1c. The Structure of Amines Compared to Water, an Alcohol, and an Ether. (Credit: Intro Chem: GOB (V. 1.0)., CC BY-NC-SA 3.0.)

 

Three structures show tertiary-butyl alcohol (a tertiary alcohol), trimethylamine (a tertiary amine) and tertiary butyl amine (a primary amine).
Figure 26.1d. Comparison of tertiary alcohol and tertiary amine. (Credit: Organic Chemistry, OpenStax, CC BY-NC-SA 4.0)

To classify alcohols, we look at the number of carbon atoms bonded to the carbon atom bearing the OH group, not the oxygen atom itself. Thus, although isopropylamine looks similar to isopropyl alcohol, the former is a primary amine, while the latter is a secondary alcohol (Figure 26.1e.).

Isopropylamine is a 3 carbon chain with a nitrogen group bonded to the second/middle carbon. Isopropyl alcohol is a 3 carbon chain with a hydroxyl group bonded to the second/middle carbon.
Figure 26.1e. Comparison of an amine and an alcohol. (Credit: Intro Chem: GOB (V. 1.0)., CC BY-NC-SA 3.0.)

Compounds containing a nitrogen atom with four attached groups also exist, but the nitrogen atom must carry a formal positive charge. Such compounds are called quaternary ammonium salts (Figure 26.1f.).

The structure of a quaternary ammonium salt, which is a nitrogen positive linked to four R groups and an X negative ion.
Figure 26.1f. Example of a quaternary salt. (Credit: Organic Chemistry, OpenStax, CC BY-NC-SA 4.0)

Naming Amines

Primary amines are named in two main ways using the IUPAC system. They can either be named as alkylamines or as alkanamines. Most 1o amines which are attached to linear alkanes, cycloalkanes, and alkyl groups with common names tend to be named as alkylamines.

Steps:

  1. Identify the longest carbon chain bonded to the amine nitogen.
  2. The alkyl group is named as a substituent (prefix + alkyl).
  3. The suffix amine is added to the end.

Primary also have several common names. You might recall that the aromatic phenylamine, H2N–C6H5, has the common name aniline (Figure 26.1g.).

The structures of tertiary-butyl amine, cyclohexylamine, and aniline. Aniline is enclosed in square parentheses.
Figure 26.1g. Linear and ring structure examples of amines. (Credit: Organic Chemistry, OpenStax, CC BY-NC-SA 4.0)

Alternatively, primary amines tend to be named as alkanamines (Figure 26.1h.).

Steps:

  1. Identify the longest carbon chain bonded to the amine nitrogen.
  2. Identify the substituents.
  3. Number the parent chain, giving the amine the lowest number.
  4. Put all details together and ensure the substituents are in alphabetical order.
The structures of 4,4-dimethylcyclohexanamine and 1,4-butanediamine. 4,4-dimethylcyclohexanamine has cyclohexane ring with two methyl groups on C4 and amine on C1. 1,4-butanediamine is four-carbon chain with amines on C1 and C4.
Figure 26.1h. Examples of naming amines. (Credit: Organic Chemistry, OpenStax, CC BY-NC-SA 4.0)

Amines with more than one functional group are named by considering the −NH2 as an amino substituent on the parent molecule (Figure 26.1i.).

The structures of 2-aminobutanoic acid, 2,4-diaminobenzoic acid and 4-amino-2-butanone. The carbon atoms of 2-aminobutanoic acid and 4-amino-2-butanone are numbered explicitly.
Figure 26.1i. Examples of naming amines with more than one functional group. (Credit: Organic Chemistry, OpenStax, CC BY-NC-SA 4.0)

Symmetrical secondary and tertiary amines are named by adding the prefix di– or tri– to the alkyl group (Figure 26.1j.).

The structures of diphenylamine and triethylamine. Diphenylamine has two benzene rings linked to an N H group. Triethylamine has a nitrogen atom linked to three ethyl groups.
Figure 26.1j. Examples of symmetrical secondary and tertiary amines. (Credit: Organic Chemistry, OpenStax, CC BY-NC-SA 4.0)

Unsymmetrically substituted secondary and tertiary amines are referred to as N-substituted primary amines. The largest alkyl group takes the parent name, and the other alkyl groups are considered N-substituents on the parent (N because they’re attached to nitrogen) (Figure 26.1k.).

N,N-dimethylpropylamine has a nitrogen linked to two methyl and a propyl group. N-ethyl-N-methylcyclohexylamine has cyclohexane linked to a nitrogen. This is connected to methyl and ethyl group.
Figure 26.1k. Examples of unsymmetrical secondary and tertiary amines. (Credit: Organic Chemistry, OpenStax, CC BY-NC-SA 4.0)

The common names for simple aliphatic amines consist of an alphabetic list of alkyl groups attached to the nitrogen atom, followed by the suffix –amine. (Systematic names are often used by some chemists.) The amino group (NH2) is named as a substituent in more complicated amines, such as those that incorporate other functional groups or in which the alkyl groups cannot be simply named.

Example 26.1a

Name and classify each compound.

  1. CH3CH2CH2NH2
  2. A central nitrogen atom connected 3 ways: to a methyl group, an ethyl group and another methyl group
    (Credit: Intro Chem: GOB (V. 1.0)., CC BY-NC-SA 3.0.)
  3. CH3CH2CH2NHCH3
  4. CH3CH2NHCH2CH

Solution

  1. There is only one alkyl group attached to the nitrogen atom, so the amine is primary. A group of three carbon atoms (a propyl group) is attached to the NH2 group through an end carbon atom, so the name is propylamine.
  2. There are two methyl groups and one ethyl group on the nitrogen atom. The compound is ethyldimethylamine, a tertiary amine.
  3. There are two ethyl groups attached to the nitrogen atom; the amine is secondary, so the compound is diethylamine.
  4. The nitrogen atom has a methyl group and a propyl group, so the compound is methylpropylamine, a secondary amine.

Exercise 26.1a

Draw the structure for each compound and classify.

  1. isopropyldimethylamine
  2. dipropylamine

Check Your Answers:[1]

Exercise & solution image source: Intro Chem: GOB (V. 1.0)., CC BY-NC-SA 3.0.

Exercise 26.1b

Name this compound.

Provide the name of this heterocyclic compound with a 6 carbon ring, an NH2 group attached to the first carbon and a Bromine attached to the third carbon.
(Credit: Intro Chem: GOB (V. 1.0)., CC BY-NC-SA 3.0.)

Check Your Answer:[2]

Exercise 26.1c

Name and classify each compound.

  1. A central nitrogen atom with an isopropyl group attached, a methyl group and a lone hydrogen.
  2. A central nitrogen atom with 2 separate ethyl groups and a methyl group attached.
  3. CH3CH2CH2CH2NH2
  4. CH3CH2CH2NHCH2CH2 CH3

Check Your Answers:[3]

Exercise and image credits: Introduction to Chemistry: GOB (V. 1.0)., CC BY-NC-SA 3.0

Example 26.1b

Draw the structure for p-ethylaniline and classify.

Solution

The compound is a derivative of aniline. It is a primary amine having an ethyl group located para to the amino (NH2) group.

A heterocyclic compound with an NH2 attached to the first carbon in the ring and an ethyl group attached to the fourth carbon in the ring.
(credit: Intro Chem: GOB (V. 1.0)., CC BY-NC-SA 3.0).

Exercise 26.1d

Draw the structure for 2-amino-3-methylpentane.

Check Your Answer:[4]

Exercise & solution image source: Intro Chem: GOB (V. 1.0)., CC BY-NC-SA 3.0.

Attribution & References

Except where otherwise noted, this page is adapted by Caryn Fahey from:

    1. The name indicates that there are an isopropyl group (in red) and two methyl groups (in green) attached to the nitrogen atom; the amine is tertiary.A tertiary amine with nitrogen as the central atom. Nitrogen is surrounded by 2 methyl groups and an isopropyl group.
    2. The name indicates that there are two propyl groups attached to the nitrogen atom; the amine is secondary. (The third bond on the nitrogen atom goes to a hydrogen atom.) CH3CH2CH2NHCH2CH2CH3

    ↵

  2. The benzene ring with an amino (NH2) group is aniline. The compound is named as a derivative of aniline: 3-bromoaniline or m-bromoaniline.

    ↵

  3. 1) secondary amine, N-methylisopropylamine or methylisopropylamine 2) tertiary amine, N-ethyl-N-methylethylamine or diethylmethylamine 3) primary amine, butylamine, 4) secondary amine, n-propylpropylamine or dipropylamine

    ↵

  4. A structural diagram of a pentane parent chain with a methyl group attached to the third carbon in the chain and an amino group at the second carbon in the chain.

    Always start with the parent compound: draw the pentane chain. Then attach a methyl group at the third carbon atom and an amino group at the second carbon atom. ↵

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Organic and Biochemistry Supplement to Enhanced Introductory College Chemistry Copyright © 2024 by Gregory Anderson; Caryn Fahey; Adrienne Richards; Samantha Sullivan Sauer; David Wegman; and Jen Booth is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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