Difficulty: Intermediate | Prerequisites: General chemistry, Lewis structures, basic understanding of carbon bonding.
This covers the systematic naming of alkanes and cycloalkanes using IUPAC rules, including how to handle branched substituents, multiplicative prefixes, and alphabetising. It sits at the foundation of organic chemistry: every reaction, mechanism, and synthesis problem you will meet later assumes you can name and draw structures fluently. If you are coming in cold, you should already be comfortable with how carbon forms four bonds and what a skeletal (line-angle) structure represents.
IUPAC nomenclature is a universal system for naming organic molecules so that every name maps to exactly one structure. For alkanes, you find the longest continuous carbon chain (the parent), number it to give substituents the lowest possible locants, then list substituents alphabetically with the right prefixes. Master the rules here and you will be able to name or draw any saturated hydrocarbon on sight.
IUPAC name
The standardised name assigned to a compound following International Union of Pure and Applied Chemistry rules. Think of it as the molecule's official, unambiguous address.
Parent chain
The longest continuous chain of carbon atoms in the molecule, which determines the base name (methane, ethane, propane, butane, pentane, hexane, heptane, etc.). In simple terms, this is the backbone you build the rest of the name around.
Locant
The number assigned to a carbon on the parent chain to specify where a substituent is attached. Think of it as a street number on the parent chain.
Substituent
Any group branching off the parent chain. An alkyl substituent is named by swapping the parent alkane's "-ane" ending for "-yl" (e.g. methane becomes methyl). In simple terms, it is anything hanging off the main chain.
Multiplicative prefix
A prefix indicating how many of the same substituent are present: di- (2), tri- (3), tetra- (4). These prefixes are ignored when alphabetising substituents.
Alphabetising rule
Substituents are listed in alphabetical order in the final name. Multiplicative prefixes (di-, tri-, tetra-) and the hyphenated prefixes sec- and tert- are ignored for alphabetising. However, iso- and neo- are part of the substituent name and do count.
Isopropyl (common) / 1-methylethyl (systematic)
A three-carbon branched substituent: a CH group bonded to two methyl groups. Think of it as a "Y-shaped" three-carbon fragment.
Isobutyl (common) / 2-methylpropyl (systematic)
A four-carbon substituent branching at the second carbon from the point of attachment.
sec-Butyl (common) / butan-2-yl (systematic)
A four-carbon substituent attached through its second carbon. The prefix "sec-" means the point of attachment is a secondary carbon.
tert-Butyl (common) / 1,1-dimethylethyl (systematic)
A four-carbon substituent attached through a quaternary-like carbon bonded to three methyl groups. The prefix "tert-" means the attachment point is a tertiary carbon.
Neopentyl (common) / 2,2-dimethylpropyl (systematic)
A five-carbon substituent with a central carbon bearing three methyl groups, attached through the remaining CH2.
Cycloalkane
An alkane whose carbon chain forms a ring. Named with the prefix "cyclo-" before the corresponding alkane name. When the ring has fewer carbons than an attached chain, the ring becomes a substituent (e.g. cyclopentyl).
Find the longest continuous carbon chain. This is the parent chain. Count carefully, because the longest chain may not be drawn in a straight line across the page. The parent name comes from the number of carbons: meth- (1), eth- (2), prop- (3), but- (4), pent- (5), hex- (6), hept- (7), oct- (8), non- (9), dec- (10).
Number the parent chain from the end that gives the lowest set of locants to the substituents. Compare locant sets as a whole: (2,3,5) beats (2,4,5) because the first point of difference is lower. If there is still a tie, give the lower locant to the substituent that comes first alphabetically.
Identify and name each substituent. Remove the substituent from the parent chain, count its carbons, and name it as an alkyl group (-yl ending). Branched substituents get their own internal numbering starting from the point of attachment.
Use multiplicative prefixes (di-, tri-, tetra-) when the same substituent appears more than once. Each occurrence still gets its own locant.
Alphabetise substituents in the final name. Ignore di-, tri-, tetra- and the hyphenated prefixes sec- and tert- when determining alphabetical order. Count iso- and neo- because they are inseparable parts of the substituent name.
Assemble the name: locants-substituent names (alphabetical) + parent chain name + "-ane" suffix. Separate numbers from letters with hyphens, and numbers from numbers with commas.
Ignored for alphabetising: di-, tri-, tetra- (multiplicative prefixes); sec- and tert- (hyphenated, separable prefixes).
Counted for alphabetising: iso- and neo- (fused into the substituent name, not separated by a hyphen in practice).
Example: in a molecule with both an isopropyl and a sec-butyl group at equivalent positions, isopropyl (alphabetised under "i") comes before sec-butyl (alphabetised under "b" for butyl). So isopropyl is listed first and gets the lower locant when there is a tie.
Common name | Systematic IUPAC name |
|---|---|
Isopropyl | 1-methylethyl |
Isobutyl | 2-methylpropyl |
sec-Butyl | butan-2-yl (or 1-methylpropyl) |
tert-Butyl | 1,1-dimethylethyl |
Neopentyl | 2,2-dimethylpropyl |
Systematic names use parentheses in the full compound name, e.g. 3-(1-methylethyl)-2,2-dimethylpentane.
If the ring has more carbons than any attached chain, the ring is the parent. Name it as cyclopropane, cyclobutane, cyclopentane, cyclohexane, etc.
If the ring has fewer carbons than the longest attached chain, the ring becomes a substituent: cyclopropyl, cyclobutyl, cyclopentyl, etc.
Number the ring to give substituents the lowest locants. For two substituents, give the lower locant to the one that comes first alphabetically.
Example: a cyclohexane with an ethyl group at C-1 and a methyl group at C-3 is 1-ethyl-3-methylcyclohexane (ethyl gets C-1 because "e" comes before "m").
When given an IUPAC name, work backwards:
Draw the parent chain with the correct number of carbons.
Number the chain.
Attach each substituent at its specified locant.
Fill in hydrogens to satisfy carbon's four bonds (usually left implicit in skeletal drawings).
For systematic substituent names in parentheses, decode first: 1-methylethyl = isopropyl, 1,1-dimethylethyl = tert-butyl, butan-2-yl = sec-butyl.
Students often alphabetise sec-butyl under "s" and tert-butyl under "t." They should not. The prefixes sec- and tert- are ignored for alphabetising; the relevant letter is "b" for butyl.
Students often ignore iso- and neo- for alphabetising, treating them the same way as sec- and tert-. Iso- and neo- are part of the name and do count. Isopropyl is alphabetised under "i," not "p."
Students sometimes pick the parent chain as the horizontal line drawn on the page, rather than the longest continuous chain. The longest chain can zigzag through the structure. Always count carbons carefully.
Students sometimes forget that each identical substituent still needs its own locant, even when a multiplicative prefix is used. For example, 2,3-dimethyl requires both "2" and "3," not just "dimethyl" with no locants.
Nomenclature questions appear on virtually every organic chemistry exam and are considered free marks if you know the rules. They also underpin every later topic: you cannot discuss a reaction mechanism if you cannot name the starting material and the product.
Expect questions that test whether you can find a non-obvious longest chain in a heavily branched structure.
Expect tie-breaking scenarios: two numbering directions give the same locant set, so you must use alphabetical order to break the tie.
The distinction between which prefixes count for alphabetising (iso-, neo-) and which do not (di-, tri-, sec-, tert-) is a perennial exam favourite.
Systematic substituent names in parentheses (e.g. 1,1-dimethylethyl) will appear. You need to decode them fluently.
Cycloalkane questions often test whether the ring or the chain is the parent. The rule: whichever has more carbons is the parent.
True or false: the prefix "di-" is considered when alphabetising substituents in an IUPAC name. (False.)
True or false: "iso-" is ignored for alphabetising, just like "sec-" and "tert-." (False. Iso- counts.)
Fill in the blank: the systematic IUPAC name for a tert-butyl substituent is ______. (1,1-dimethylethyl.)
True or false: if a cyclopentane ring is attached to an octane chain, the ring is the parent. (False. The chain with more carbons is the parent; cyclopentane has 5 carbons, octane has 8.)
Fill in the blank: when two numbering directions give the same locant set, the tie is broken by giving the lower number to the substituent that comes first ______. (Alphabetically.)
Q: Name a disubstituted cyclohexane bearing one ethyl and one methyl group in a 1,3-relationship. Apply lowest-locant and tie-breaking rules.
A: 1-ethyl-3-methylcyclohexane. Ethyl receives C-1 because "e" precedes "m" alphabetically, and 1,3 is lower than any alternative numbering.
Q: A parent chain carries both an isopropyl and a sec-butyl group at equivalent positions. Which substituent gets the lower locant, and why?
A: Isopropyl gets the lower locant. When locant sets are tied, the substituent that comes first alphabetically receives the lower number. For alphabetising, sec- is ignored (so sec-butyl is alphabetised under "b"), and iso- counts (so isopropyl is alphabetised under "i"). "B" comes before "i," so sec-butyl (as "butyl") comes first alphabetically and gets the lower locant. (Correction to the instinct: butyl under "b" beats isopropyl under "i.")
Q: Give the systematic IUPAC names for: isopropyl, isobutyl, sec-butyl, tert-butyl, neopentyl.
A: Isopropyl = 1-methylethyl. Isobutyl = 2-methylpropyl. sec-Butyl = butan-2-yl (or 1-methylpropyl). tert-Butyl = 1,1-dimethylethyl. Neopentyl = 2,2-dimethylpropyl.
Q: Name a cyclopentane bearing both an isopropyl and a tert-butyl group trans to each other (stereochemistry descriptors not required).
A: 1-(1,1-dimethylethyl)-2-(1-methylethyl)cyclopentane (using systematic names). The tert-butyl group, alphabetised under "d" for dimethylethyl, gets the lower locant. Or in common-name form: 1-tert-butyl-2-isopropylcyclopentane.
Q: You are given the name 3-cyclopentyl-2-methylheptane. How do you confirm the parent chain is correct?
A: Count the longest continuous chain: heptane = 7 carbons. The cyclopentane ring has only 5 carbons, so it is the substituent, not the parent. The name is correct because the parent is the longest chain.
Nomenclature rules for alkanes extend directly to naming alkenes, alkynes, alcohols, and other functional groups. The same parent-chain and substituent logic applies; only the suffix changes. Understanding carbon classification (primary, secondary, tertiary, quaternary) from this chapter feeds directly into reaction selectivity in later chapters on substitution and elimination.
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