24.1 Naming Amines

Amines are organic compounds built around a nitrogen atom bonded to one or more carbon-containing groups. Their classification as primary, secondary, or tertiary depends on how many organic groups are attached to that nitrogen, and this distinction directly affects their physical properties and chemical reactivity. Naming amines correctly requires understanding both IUPAC rules and the common names you'll encounter constantly in organic chemistry.

Amine Classification and Nomenclature

Types of amines by substituents

The key question for classifying an amine is simple: how many organic groups (alkyl or aryl) are bonded to the nitrogen?

• Primary (1°) amines have one organic substituent on nitrogen. General formula: $\text{R-NH}_2$. Examples: methylamine ($\text{CH}_3\text{NH}_2$), aniline ($\text{C}_6\text{H}_5\text{NH}_2$).
• Secondary (2°) amines have two organic substituents on nitrogen. General formula: $\text{R}_1\text{-NH-R}_2$. Examples: dimethylamine, N-methylaniline.
• Tertiary (3°) amines have three organic substituents on nitrogen. General formula: $\text{R}_1\text{R}_2\text{R}_3\text{N}$. Examples: trimethylamine, N,N-dimethylaniline.

Don't confuse this with alcohol classification. For alcohols, you look at the carbon bearing the $\text{-OH}$. For amines, you look directly at the nitrogen. A tertiary amine has three organic groups on nitrogen, while a tertiary alcohol has its $\text{-OH}$ on a carbon bonded to three other carbons.

IUPAC nomenclature for amines

Naming amines follows a systematic process. Here's how to work through it:

For simple alkylamines (the $\text{-NH}_2$ group is the highest-priority functional group):

1. Identify the longest carbon chain that contains the carbon bonded to $\text{-NH}_2$.
2. Number the chain to give the nitrogen-bearing carbon the lowest possible locant.
3. Drop the "-e" from the parent alkane name and add "-amine."
4. Add locants as needed: e.g., propan-1-amine, butan-2-amine.

For secondary and tertiary amines:

1. Choose the longest alkyl chain attached to nitrogen as the parent name.
2. Name the other groups on nitrogen as substituents with the prefix N- to show they're on nitrogen rather than on the carbon chain.
3. If two identical groups are on nitrogen, use N,N-di; for example, N,N-dimethylbutan-1-amine.

For arylamines:

• The parent compound is aniline (aminobenzene). Substituents on the ring are indicated by position numbers relative to the amino group: 4-chloroaniline, 2,6-dimethylaniline.

When the amino group is NOT the highest-priority functional group:

• The $\text{-NH}_2$ group is named as an "amino-" prefix instead of using the "-amine" suffix. This happens when the molecule contains a carboxylic acid, ester, amide, aldehyde, ketone, or nitrile, all of which outrank the amino group in IUPAC priority.
• Examples: 2-aminoethanol (amino group + alcohol, where $\text{-OH}$ and $\text{-NH}_2$ are close in priority but $\text{-OH}$ gets the suffix), 4-aminobenzoic acid (the carboxylic acid takes priority).

Alphabetical ordering: When listing substituents, arrange them alphabetically and ignore multiplying prefixes like "di-" or "tri-." So N-ethyl comes before N-methyl, and N,N-dimethyl is alphabetized under "m."

Structure of heterocyclic amines

Heterocyclic amines are cyclic compounds where at least one nitrogen atom is part of the ring itself. These have their own parent names that you need to memorize rather than derive from rules.

Common five-membered rings:

• Pyrrole: unsaturated five-membered ring with one nitrogen. Numbering starts at nitrogen (N = position 1). Example: 2-methylpyrrole.
• Pyrrolidine: the fully saturated version of pyrrole. Example: 2-ethylpyrrolidine.

Common six-membered rings:

• Pyridine: unsaturated six-membered ring with one nitrogen (think of it as benzene with one $\text{CH}$ replaced by $\text{N}$). Numbering starts at nitrogen. Example: 3-ethylpyridine.
• Piperidine: the fully saturated version of pyridine. Example: N-methylpiperidine.

Fused heterocyclic systems:

• Quinoline is a benzene ring fused to a pyridine ring. Indole is a benzene ring fused to a pyrrole ring. These fused systems have their own numbering conventions that you'll find in reference tables.

Additional Naming Considerations

• Common names are widespread and often preferred in practice. Trimethylamine, putrescine (1,4-diaminobutane), and cadaverine (1,5-diaminopentane) are examples you should recognize alongside their IUPAC names.
• Quaternary ammonium salts ($\text{R}_4\text{N}^+$) are related compounds with four organic groups on nitrogen and a positive charge. They aren't technically amines (no lone pair available), but they come up in this unit. They're named with an "-ammonium" suffix: tetramethylammonium chloride.