Difficulty: Foundational | Prerequisites: Basic understanding of covalent bonding and carbon chemistry.
This is the naming system for organic molecules, and it underpins everything else in the course. If you cannot name a compound or read its name back into a structure, you will struggle with every reaction mechanism, every synthesis problem, and every exam question that presents a molecule by name. The system is rule-based and largely predictable once you learn the logic. If you are coming into organic chemistry cold, start here.
IUPAC nomenclature is a systematic set of rules for naming organic molecules. You identify the longest carbon chain (the parent chain), number it so that substituents get the lowest possible locants, and then assemble the name from prefixes (substituent names and positions), a root (chain length), and a suffix (functional group). Once you can do this reliably, you can convert any name to a structure and any structure to a name.
Parent chain
The longest continuous chain of carbon atoms in the molecule, which determines the root name. Think of it as the backbone that everything else hangs off.
Prefix (in naming)
The part of the IUPAC name that identifies substituents and their positions on the parent chain, placed before the root name. In simple terms, this is where you list what is attached to the backbone and where.
Root name
The portion of the name derived from the number of carbons in the parent chain (e.g. "eth" for 2, "prop" for 3, "but" for 4). Think of it as the size label for the molecule's backbone.
Suffix
The ending of the name that identifies the principal functional group: "-ane" for alkanes, "-ene" for alkenes, "-yne" for alkynes. In simple terms, this tells you what kind of bonds are in the chain.
Substituent
An atom or group of atoms that replaces a hydrogen on the parent chain (e.g. methyl, ethyl, isopropyl). Think of it as a branch or side group hanging off the main chain.
Locant
The number assigned to a carbon on the parent chain to indicate where a substituent or functional group is attached. In simple terms, it is the address of a group on the chain.
Stereocenter (chiral centre)
An sp3-hybridised carbon bearing four different substituents, which means the molecule can exist as non-superimposable mirror images. Think of it as the point in the molecule where left-handed and right-handed versions diverge.
Find the longest continuous chain of carbon atoms.
If an alkene or alkyne is present, the parent chain must include the double or triple bond, even if a longer all-single-bond chain exists.
Number from the end that gives substituents the lowest possible locants.
Exception: if the molecule contains a double or triple bond, number so the bond gets the lowest locant, even if that means substituents get higher numbers.
Carbons | Prefix |
|---|---|
2 | eth- |
3 | prop- |
4 | but- |
5 | pent- |
6 | hex- |
7 | hept- |
8 | oct- |
9 | non- |
10 | dec- |
Alkane (all single bonds): -ane
Alkene (one double bond): -ene
Two double bonds: -diene
Three double bonds: -triene
Four double bonds: -tetraene
Alkyne (one triple bond): -yne
Common substituents to memorise:
Methyl: CH₃
Ethyl: CH₂CH₃
Isopropyl: CH(CH₃)₂
List substituents alphabetically in the name. Use "di-", "tri-", "tetra-" for repeated identical substituents (these multiplying prefixes do not affect alphabetical order).
If the molecule contains a stereocenter (indicated by wedge and dash bonds), assign R or S configuration and place it in parentheses at the start of the name.
Example from the source: (R)-3,7-dimethyl-1,6-octadiene.
IUPAC names are the universal language of chemistry. When a pharmaceutical company files a patent or a regulatory submission, the active ingredient is identified by its IUPAC name. Databases such as SciFinder and PubChem index compounds by systematic names, so knowing the rules lets you search the literature accurately.
Students often think the parent chain is simply the horizontal chain drawn on paper. It is not. The parent chain is the longest continuous chain, and it may zigzag through the structure.
Students often forget that double/triple bonds take priority over substituent numbering. When an alkene or alkyne is present, the chain is numbered to give the unsaturation the lowest locant, not the substituents.
Students sometimes count multiplying prefixes (di-, tri-) when alphabetising substituents. These prefixes are ignored for alphabetical ordering. "Dimethyl" is still filed under M, not D.
Students confuse isopropyl with n-propyl. Isopropyl branches at the central carbon, CH(CH₃)₂, while n-propyl is a straight three-carbon chain, CH₂CH₂CH₃.
⚠️ Naming questions appear on virtually every organic chemistry exam. You will be asked to convert a structure to a name and a name to a structure.
⚠️ The double/triple bond priority rule is a favourite exam trap. Expect a molecule where the longest chain is not the one that contains the unsaturation, and you must choose correctly.
⚠️ Stereochemistry labels (R/S) in IUPAC names are tested heavily once stereocenters are introduced. Know when to include them and how to assign them.
⚠️ Alphabetical ordering of substituents with multiplying prefixes (e.g. "ethyl" before "dimethyl") is a common exam pitfall.
True or false: The parent chain is always the horizontal chain as drawn on paper. (False. It is the longest continuous chain, regardless of how the structure is drawn.)
Fill in the blank: A six-carbon parent chain uses the root name ______. (hex-)
True or false: When naming an alkene, substituent numbering takes priority over double-bond numbering. (False. The double bond gets the lowest possible locant.)
Fill in the blank: The suffix for a molecule with one triple bond is ______. (-yne)
True or false: The prefix "di-" counts when alphabetising substituents. (False. Multiplying prefixes are ignored for alphabetical order.)
Q: A molecule has a seven-carbon parent chain with all single bonds. What is its IUPAC root name and suffix?
A: Heptane.
Q: You are naming a molecule with an eight-carbon chain that contains two double bonds at positions 1 and 6, plus methyl groups at positions 3 and 7. The stereocenter at position 3 is R. Write the full IUPAC name.
A: (R)-3,7-dimethyl-1,6-octadiene.
Q: A student names a compound "2-dimethylhexane." What is wrong with this name?
A: "Dimethyl" requires two locants (one for each methyl group), not one. The name should specify both positions, e.g. "2,2-dimethylhexane" or "2,3-dimethylhexane" depending on the structure.
Q: When numbering the parent chain, under what circumstance does a double bond take priority over giving substituents the lowest numbers?
A: Always. When naming an alkene or alkyne, the chain is numbered so the double or triple bond receives the lowest possible locant, even if that gives substituents higher numbers.
Q: Place these substituents in the correct alphabetical order for an IUPAC name: ethyl, dimethyl, bromo.
A: Bromo, ethyl, dimethyl. (Alphabetical order: B before E before M. The "di-" prefix on dimethyl is ignored.)
This connects to stereochemistry (R/S configuration) because IUPAC names must include stereochemical descriptors whenever a stereocenter is present. You will revisit naming rules when you learn about alcohols, ethers, and carboxylic acids, each of which has additional suffix and priority rules layered on top of the basics here.
Functional group identification (covered in the next set of notes) feeds directly into Step 4 of the naming process, since the principal functional group determines the suffix.
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