Difficulty: Introductory | Prerequisites: Basic understanding of carbon bonding and structural formulas.
IUPAC nomenclature is the universal naming system for organic molecules. Getting the name right means you can draw the structure from a name and vice versa, which is tested on every exam and used throughout every subsequent topic. This section covers naming acyclic alkanes, cycloalkanes, and substituted cycloalkanes, including cis/trans designation. If you can name and draw structures confidently, you can communicate with any chemist about any molecule.
Find the longest carbon chain (or largest ring), name it, number the carbons to give substituents the lowest possible locants, list substituents alphabetically with their position numbers, and add cis/trans for ring stereochemistry when needed.
Parent chain
The longest continuous chain of carbon atoms in the molecule. This gives the base name (methane, ethane, propane, butane, pentane, hexane, etc.). In simple terms, it is the backbone you name first.
Substituent
A group branching off the parent chain. Named by replacing the "-ane" ending of the corresponding alkane with "-yl" (methyl, ethyl, propyl, etc.).
Locant
The number assigned to a carbon on the parent chain to specify where a substituent is attached.
Cycloalkane
An alkane whose carbon chain forms a ring. Named by adding the prefix "cyclo-" to the alkane name (cyclopentane, cyclohexane).
Cis / trans
Prefixes that describe the relative positions of two substituents on a ring. Cis means both substituents are on the same face of the ring; trans means they are on opposite faces.
tert-butyl (t-butyl)
A branched four-carbon substituent, (CH₃)₃C-. The "tert-" is not alphabetised when ordering substituents (you alphabetise by "butyl").
Find the longest continuous carbon chain. When two chains tie in length, pick the one with more substituents.
Number the chain from the end that gives the lowest set of locants to the substituents.
Name each substituent and assign it the number of the carbon it is attached to.
List substituents in alphabetical order in the final name. Use prefixes di-, tri-, tetra- for repeated groups, but do not alphabetise the prefix ("dimethyl" is alphabetised under "m").
Combine: locants-substituent names + parent chain name + "-ane".
Example from the exam: a branched hexane with a methyl group at C2 and an ethyl group at C3 is named 3-ethyl-2-methylhexane. The longest chain has six carbons (hexane). Numbering from the end nearest the first branch gives the lowest locants.
Name the ring by its size: cyclopropane (3C), cyclobutane (4C), cyclopentane (5C), cyclohexane (6C).
If the ring has substituents, number the ring carbons to give the lowest locants.
If there are two substituents, assign C1 to one and number around the ring so the other gets the lowest number.
Example: 2,3-dicyclopropylbutane means a four-carbon chain (butane) with cyclopropyl groups on C2 and C3.
Example: trans-1-ethyl-3-tert-butylcyclopentane means a cyclopentane ring with an ethyl group on C1 and a tert-butyl group on C3, with those two substituents on opposite faces of the ring (trans).
Cis: both substituents point to the same face (both "up" or both "down" relative to the ring plane).
Trans: substituents point to opposite faces (one "up", one "down").
This only applies to ring compounds. Open-chain alkanes rotate freely, so cis/trans does not apply to them.
Students often pick the wrong parent chain by choosing the straightest chain instead of the longest. The parent chain can bend and twist through the structure; it just has to be the longest continuous sequence of carbons.
A common error is alphabetising prefixes like "di-" and "tert-". Multiplying prefixes (di-, tri-) are ignored in alphabetical ordering. Hyphenated prefixes like tert- and sec- are also ignored (alphabetise by the root: "butyl", not "tert-butyl"). However, "iso-" and "cyclo-" are part of the name and are included in alphabetical ordering.
Students sometimes number from the wrong end, giving higher locants to substituents when lower ones are available.
⚠️ "Name this structure" and "draw this name" are standard exam questions. Practise both directions.
⚠️ The longest chain with the most substituents is the parent chain, not the longest straight line.
⚠️ Cis/trans designation on rings is commonly tested alongside chair conformations.
⚠️ Know common substituent names cold: methyl, ethyl, propyl, isopropyl, butyl, tert-butyl, cyclopropyl.
True or false: "dimethyl" is alphabetised under "d" in an IUPAC name. (False. Ignore "di-" and alphabetise under "m" for methyl.)
Fill in the blank: the parent chain is the ____ continuous chain of carbons. (longest)
True or false: cis means two substituents are on opposite faces of a ring. (False. Cis means same face; trans means opposite.)
Q: What is the IUPAC name for a six-carbon chain with a methyl group at C2 and an ethyl group at C3?
A: 3-ethyl-2-methylhexane. Ethyl comes before methyl alphabetically.
Q: Draw 2,3-dicyclopropylbutane.
A: Draw a four-carbon chain. Attach a cyclopropyl ring (a triangle) to C2 and another to C3.
Q: Draw trans-1-ethyl-3-tert-butylcyclopentane.
A: Draw a five-membered ring. Place an ethyl group on C1 and a tert-butyl group on C3. Because it is trans, one substituent points up and the other points down relative to the ring plane.
Nomenclature is the vocabulary you use for the entire course. When you study stereochemistry, you will add R/S and E/Z designations to these same IUPAC names. Chair conformations (coming up next) require you to know cis/trans relationships before you can place substituents axially and equatorially.
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