Appendix E: IUPAC Nomenclature Reference
This appendix provides a complete reference for IUPAC organic nomenclature. It is designed to be used alongside Chapter 1 (Functional Groups) and returned to throughout the course as new compound classes are introduced.
The Logic of IUPAC Nomenclature
The IUPAC system assigns a unique, unambiguous name to every organic compound. The name encodes three things:
- The length of the parent chain (how many carbons)
- The principal functional group (what kind of compound it is)
- The location and identity of all substituents (what is attached, and where)
Given a name, a trained chemist can reconstruct the structure. Given a structure, a trained chemist can derive the name. The goal for students is to move comfortably in both directions for the compound classes covered in Organic Chemistry I and II.
Step-by-Step Naming Procedure
Step 1 — Identify the principal functional group
The suffix is determined by the highest-priority functional group present. Use the priority order in the table below. If the molecule contains only carbon and hydrogen single bonds, it is an alkane.
Step 2 — Find the parent chain
Identify the longest continuous carbon chain that includes the principal functional group. For aldehydes and carboxylic acids, the carbonyl carbon is always carbon 1 and must be included in the chain.
Step 3 — Number the chain
Number from the end that gives the principal functional group the lowest possible locant. If there is a tie, number from the end that gives substituents the lowest locants.
Step 4 — Name substituents
Identify all groups attached to the parent chain that are not the principal functional group. Name them as prefixes using the substituent table below.
Step 5 — Assemble the name
List substituents alphabetically (ignoring multiplying prefixes such as di-, tri-), include locants for all substituents and the principal functional group, and add the suffix.
Format: locant(s)-substituent(s) + parent chain prefix + locant-suffix
Parent Chain Prefixes
| Carbons | Prefix | Carbons | Prefix |
|---|---|---|---|
| 1 | meth- | 6 | hex- |
| 2 | eth- | 7 | hept- |
| 3 | prop- | 8 | oct- |
| 4 | but- | 9 | non- |
| 5 | pent- | 10 | dec- |
For chains longer than 10 carbons: undec- (11), dodec- (12), tridec- (13), tetradec- (14), pentadec- (15).
Functional Group Priority and Suffixes
When multiple functional groups are present, the one with the highest priority (lowest number in this table) determines the suffix. All lower-priority groups are expressed as prefixes.
| Priority | Functional Group | Suffix | Prefix (when not principal) |
|---|---|---|---|
| 1 | Carboxylic acid | -oic acid | carboxy- |
| 2 | Ester | -oate | (alkoxy-carbonyl-) |
| 3 | Amide | -amide | carbamoyl- |
| 4 | Aldehyde | -al | oxo- (on terminal carbon) |
| 5 | Ketone | -one | oxo- |
| 6 | Alcohol | -ol | hydroxy- |
| 7 | Amine | -amine | amino- |
| 8 | Alkyne | -yne | — |
| 9 | Alkene | -ene | — |
| 10 | Alkane | -ane | — |
Note on aldehydes: The carbonyl carbon of an aldehyde is always C-1, so no locant is needed in the name (e.g., propanal, not propan-1-al).
Note on carboxylic acids: Similarly, the carboxyl carbon is always C-1.
Common Substituent Names
| Substituent | Structure | Name |
|---|---|---|
| Methyl | –CH₃ | methyl– |
| Ethyl | –CH₂CH₃ | ethyl– |
| Propyl | –CH₂CH₂CH₃ | propyl– |
| Isopropyl | –CH(CH₃)₂ | isopropyl– (or 1-methylethyl–) |
| Butyl | –CH₂CH₂CH₂CH₃ | butyl– |
| tert-Butyl | –C(CH₃)₃ | tert-butyl– (or 1,1-dimethylethyl–) |
| Phenyl | –C₆H₅ | phenyl– |
| Benzyl | –CH₂C₆H₅ | benzyl– |
| Fluoro | –F | fluoro– |
| Chloro | –Cl | chloro– |
| Bromo | –Br | bromo– |
| Iodo | –I | iodo– |
| Hydroxy | –OH | hydroxy– |
| Methoxy | –OCH₃ | methoxy– |
| Amino | –NH₂ | amino– |
| Nitro | –NO₂ | nitro– |
| Oxo | =O | oxo– |
Multiplying Prefixes
When the same substituent appears more than once, use a multiplying prefix. These are not alphabetized — alphabetize by the substituent name itself.
| Count | Prefix |
|---|---|
| 2 | di– |
| 3 | tri– |
| 4 | tetra– |
| 5 | penta– |
| 6 | hexa– |
Example: 2,3-dimethylbutane (two methyl groups, not dimethyl for alphabetization purposes).
Alkenes and Alkynes
Locants
The double or triple bond position is given the lowest possible locant. The locant refers to the lower-numbered carbon of the multiple bond.
Example: but-1-ene (double bond between C1 and C2), but-2-ene (double bond between C2 and C3).
E/Z Geometry
When a double bond has two different groups on each carbon, it can exist as E or Z isomers.
Z (zusammen) — higher-priority groups on the same side of the double bond.
E (entgegen) — higher-priority groups on opposite sides.
Priority is assigned by atomic number (Cahn–Ingold–Prelog rules): higher atomic number = higher priority. When the attached atoms are the same, move outward along the chain until a difference is found.
Cis/trans is an older convention still used for simple cases: cis = same side, trans = opposite side. E/Z is unambiguous and applies to all cases.
Ring Systems
Cycloalkanes
Add the prefix cyclo– before the parent chain name.
| Structure | Name |
|---|---|
| 3-carbon ring | cyclopropane |
| 4-carbon ring | cyclobutane |
| 5-carbon ring | cyclopentane |
| 6-carbon ring | cyclohexane |
For substituted cycloalkanes, number the ring to give substituents the lowest locants. If one substituent is present, it is at C-1 (no locant needed).
Benzene
Benzene is named as its own parent. Monosubstituted benzenes are named as benzene derivatives (e.g., chlorobenzene, nitrobenzene). For disubstituted benzenes, three arrangements are possible:
- ortho (o–) — substituents at positions 1 and 2
- meta (m–) — substituents at positions 1 and 3
- para (p–) — substituents at positions 1 and 4
The ortho/meta/para prefix system is widely used alongside IUPAC locants.
When benzene is a substituent rather than the parent, it is named phenyl–.
Worked Examples
Example 1 — Aldehyde
Structure: CH₃CH₂CH₂CHO (four-carbon chain, terminal aldehyde)
- Principal functional group: aldehyde → suffix -al
- Parent chain: 4 carbons → but-
- Aldehyde carbon is C-1; no other substituents
- Name: butanal
Example 2 — Alcohol with substituent
Structure: CH₃CH(OH)CH₂CH₃ (four-carbon chain, hydroxyl on C-2)
- Principal functional group: alcohol → suffix -ol
- Parent chain: 4 carbons → but-
- Number from the end closest to the OH: hydroxyl at C-2
- No other substituents
- Name: butan-2-ol
Example 3 — Ketone
Structure: CH₃COCH₂CH₂CH₃ (five-carbon chain, carbonyl at C-2)
- Principal functional group: ketone → suffix -one
- Parent chain: 5 carbons → pent-
- Number from the end closest to the carbonyl: C-2
- Name: pentan-2-one
Example 4 — Alkene with substituent
Structure: CH₃CH=C(CH₃)CH₂CH₃ (five-carbon chain with a double bond and a methyl branch)
- Principal functional group: alkene → suffix -ene
- Parent chain: 5 carbons → pent-
- Double bond at C-2 (lower locant from left end)
- Methyl substituent at C-3
- Name: 3-methylpent-2-ene
Example 5 — Carboxylic acid
Structure: CH₃CH₂COOH (three-carbon chain, carboxylic acid)
- Principal functional group: carboxylic acid → suffix -oic acid
- Parent chain: 3 carbons → prop-
- Carboxyl carbon is C-1; no locant needed
- Name: propanoic acid
Common Name ↔︎ IUPAC Name Table
| Common Name | IUPAC Name | Functional Group |
|---|---|---|
| Formaldehyde | methanal | aldehyde |
| Acetaldehyde | ethanal | aldehyde |
| Acetone | propan-2-one | ketone |
| Acetic acid | ethanoic acid | carboxylic acid |
| Formic acid | methanoic acid | carboxylic acid |
| Propionic acid | propanoic acid | carboxylic acid |
| Butyric acid | butanoic acid | carboxylic acid |
| Ethanol | ethanol | alcohol |
| Isopropanol | propan-2-ol | alcohol |
| Glycerol | propane-1,2,3-triol | polyol |
| Chloroform | trichloromethane | haloalkane |
| Ethylene | ethene | alkene |
| Acetylene | ethyne | alkyne |
| Aniline | benzenamine (or aminobenzene) | amine |
| Toluene | methylbenzene | aromatic |
| Phenol | phenol | aromatic alcohol |
A Note on Ambiguity
The IUPAC system is updated periodically, and two valid IUPAC names sometimes exist for the same compound (particularly for alcohols, where both propan-1-ol and 1-propanol appear in the literature). Either form is generally accepted in coursework unless an instructor specifies otherwise.
Common names remain in widespread use in industry, biochemistry, and pharmacology. Developing fluency with both systems is a practical skill that continues to matter well beyond the introductory course.