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24.1: Naming Amines

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    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 as primary (RNH2), secondary (R2NH), or tertiary (R3N), depending on the number of organic substituents attached to nitrogen. 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.

    Three structures show tertiary-butyl alcohol (a tertiary alcohol), trimethylamine (a tertiary amine) and tertiary butyl amine (a primary amine).

    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.

    The structure of a quaternary ammonium salt, which is a nitrogen positive linked to four R groups and an X negative ion.

    Primary amines are named in the IUPAC system in several ways. For simple amines, the suffix -amine is added to the name of the alkyl substituent. You might also recall from the chapter on Benzene and Aromaticity that the aromatic phenylamine, H2N–C6H5, has the common name aniline.

    The structures of tertiary-butyl amine, cyclohexylamine, and aniline. Aniline is enclosed in square parentheses.

    Alternatively, the suffix -amine can be used in place of the final -e in the name of the parent compound.

    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.

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

    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.

    Symmetrical secondary and tertiary amines are named by adding the prefix di- or tri- to the alkyl group.

    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.

    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).

    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.

    Heterocyclic amines—compounds in which the nitrogen atom occurs as part of a ring—are also common, and each different heterocyclic ring system has its own parent name. The heterocyclic nitrogen atom is always numbered as position 1.

    The structures of pyridine, pyrrole, quinoline, imidazole, indole, pyrimidine, pyrrolidine, and piperidine. The compounds are numbered from the nitrogen atom.
    Exercise \(\PageIndex{1}\)

    Name the following compounds:

    1. The structure of triisopropyl amine. It comprises three isopropyl groups linked to a nitrogen atom.
    2. A central nitrogen bonded to three cyclohexane rings.
    3. A central nitrogen bonded to methyl, ethyl, and cyclohexyl groups.
    4. A five-membered ring, one member of which is nitrogen. There is a methyl group also attached to the nitrogen.
    5. A nitrogen bonded to one hydrogen and two isopropyl groups.
    6. A three-carbon chain with, counting from the left, amine groups on the first and third carbon. There is also a methyl group on the third carbon.
    Answer
    1. N-Methylethylamine
    2. Tricyclohexylamine
    3. N-Ethyl-N-methylcyclohexylamine
    4. N-Methylpyrrolidine
    5. Diisopropylamine
    6. 1,3-Butanediamine
    Exercise \(\PageIndex{2}\)

    Draw structures corresponding to the following IUPAC names:

    1. Triisopropylamine
    2. Triallylamine
    3. N-Methylaniline
    4. N-Ethyl-N-methylcyclopentylamine
    5. N-Isopropylcyclohexylamine
    6. N-Ethylpyrrole
    Answer
    1. The structure of triisopropyl amine. It comprises three isopropyl groups linked to a nitrogen atom.
    2. The structure of triallylamine. It has a nitrogen connected to three groups with structural formula C H 2 C H C H 2 (double bond between second and third carbons).
    3. The structure of N-methylaniline. It comprises a benzene ring with an N H group linked to a methyl group.
    4. The structure of N-ethyl-N-methylcyclopentylamine. It comprises nitrogen with ethyl, methyl, and cyclopentyl substituents.
    5. The structure of N-isopropylcyclohexylamine. It comprises cyclohexane linked to an N H linked to a C H group further connected to two methyl groups at C 1.
    6. A five-membered ring, one member of which is nitrogen. There are double bonds at C 2 and C 4, and nitrogen has an additional ethyl substituent.
    Exercise \(\PageIndex{3}\)

    Draw structures for the following heterocyclic amines:

    1. 5-Methoxyindole
    2. 1,3-Dimethylpyrrole
    3. 4-(N,N-Dimethylamino)pyridine
    4. 5-aminopyrimidine
    Answer
    1. A benzene ring fused to another five-membered ring, one member of which is nitrogen. There is a double bond between non-fusion carbon atoms. Benzene has a methoxy substituent.
    2. A five-membered ring, one member of which is nitrogen. There are double bonds at C 2 and C 4, methyl at C 3, and nitrogen has an additional methyl substituent.
    3. A six-membered ring, one member of which is nitrogen, with alternating single and double bonds. Opposite the nitrogen is the substituent N with two methyl groups.
    4. A six-membered ring, two members of which are nitrogen, with one carbon between them. There are alternating single and double bonds. C 5 has an amino substituent.

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