Difficulty: Introductory to Intermediate | Prerequisites: Basic understanding of carbon bonding (four bonds per carbon), structural formulas, and the concept of constitutional isomers.
Naming organic molecules is the backbone of organic chemistry communication. If you cannot name a compound from its structure (or draw it from its name), every subsequent topic, from reactions to spectroscopy, becomes harder to follow. This set of notes covers how substituent groups (branches off the main chain) are named and how those names slot into the full IUPAC name of a branched alkane. You should already be comfortable drawing Lewis structures for simple hydrocarbons and identifying primary, secondary, and tertiary carbons before working through this material.
To name a branched alkane, find the longest continuous carbon chain (the parent), number it to give substituents the lowest possible locants, then list the substituents alphabetically before the parent name. Substituents are named by taking the corresponding alkane and swapping "-ane" for "-yl." Identical substituents get a multiplying prefix (di-, tri-) that is ignored when alphabetising.
Parent chain
The longest continuous chain of carbon atoms in the molecule. It determines the base name of the compound (e.g. octane for eight carbons). In simple terms, it is the "spine" of the molecule that everything else hangs off.
Substituent (alkyl group)
Any branch attached to the parent chain. Named by replacing the "-ane" ending of the corresponding alkane with "-yl." Think of it as a fragment left over when you conceptually remove one hydrogen from an alkane.
Primary carbon (1° C)
A carbon bonded to only one other carbon. Hydrogens on a primary carbon are called primary hydrogens.
Secondary carbon (2° C)
A carbon bonded to exactly two other carbons.
Tertiary carbon (3° C)
A carbon bonded to three other carbons. Removing the lone hydrogen from a tertiary carbon gives a tertiary alkyl group (e.g. tert-butyl).
Constitutional isomers
Compounds with the same molecular formula but different connectivity of atoms. n-Butane and isobutane are constitutional isomers of C₄H₁₀.
Locant
The number assigned to a carbon on the parent chain to specify where a substituent is attached. In simple terms, it is the address of a branch.
Multiplying prefix (di-, tri-, tetra-)
Indicates that the same substituent appears more than once. These prefixes are ignored when placing substituents in alphabetical order.
The general rule: parent alkane name + "-yl" suffix.
R = the rest of the molecule (the parent chain). Whenever a hydrogen is removed from an alkane, the fragment left behind is the alkyl group.
The point of attachment to the parent chain is always shown as R-.
Methyl (CH₃-): derived from methane. One carbon, three hydrogens. The simplest and most common substituent.
Ethyl (CH₃CH₂-): derived from ethane. Two carbons in a row.
Propane has two types of hydrogen: primary (on the end carbons) and secondary (on the middle carbon). The substituent you get depends on which hydrogen you remove.
Propyl (n-propyl, CH₃CH₂CH₂-): remove a primary hydrogen from an end carbon. The chain stays straight.
Isopropyl ((CH₃)₂CH-): remove the secondary hydrogen from the central carbon. The attachment point is the middle carbon, giving a forked shape.
Two constitutional isomers of C₄H₁₀ exist, n-butane and isobutane, and each can lose different types of hydrogen, giving four distinct substituent names.
From n-butane (CH₃CH₂CH₂CH₃):
Butyl (n-butyl): remove a 1° H from an end carbon. Straight four-carbon chain attached at the terminal carbon.
sec-Butyl (secondary butyl): remove a 2° H from carbon 2. Attachment at an internal carbon; the chain branches at the point of attachment.
From isobutane ((CH₃)₃CH):
Isobutyl: remove a 1° H from one of the terminal methyl groups. The attachment is at a primary carbon, and the chain still forks one carbon in.
tert-Butyl (tertiary butyl): remove the single 3° H from the central carbon. Three methyl groups radiate from the attachment point. This is the bulkiest four-carbon substituent.
Find the longest continuous carbon chain. Count carefully; the longest chain may not be drawn horizontally. The length determines the parent name (e.g. 7 C = heptane, 8 C = octane, 9 C = nonane).
Number the chain from the end that gives the substituents the lowest set of locants.
Identify and name each substituent using the rules above.
Combine identical substituents with multiplying prefixes: di- (two), tri- (three), tetra- (four). The locant for each instance is listed, separated by commas.
Alphabetise the substituents. Multiplying prefixes (di-, tri-) are ignored when alphabetising. The prefixes "iso" and "tert-" (and sec-) are also typically ignored, except when comparing groups like tert-butyl and sec-butyl directly.
Write the name as one word. Locants are separated from words by hyphens, and from each other by commas.
5-Ethyl-2,3-dimethyloctane: longest chain = 8 C (octane). Methyl groups at positions 2 and 3 (prefix: di-). Ethyl at position 5. Ethyl comes before methyl alphabetically.
2,3,5-Trimethylheptane: longest chain = 7 C (heptane). Three methyl groups at positions 2, 3, and 5.
3-Ethyl-4,5-dimethyloctane: longest chain = 8 C. Ethyl at 3, methyls at 4 and 5.
6-Isopropyl-2,3-dimethylnonane: longest chain = 9 C (nonane). Isopropyl at 6, methyls at 2 and 3. "Isopropyl" is alphabetised under "i."
Students often assume the parent chain is the one drawn left to right. It is not always. The longest chain can bend around corners in a zig-zag drawing. Always count every possible path.
"Di-" and "tri-" are frequently treated as part of the substituent name when alphabetising. They are not. Dimethyl is still alphabetised under "m" for methyl.
Confusing sec-butyl with isobutyl is common because both are four-carbon groups. The difference is where the attachment point sits: sec-butyl attaches at a secondary carbon of n-butane; isobutyl attaches at a primary carbon of isobutane.
Students sometimes think "iso" is always ignored when alphabetising. In IUPAC practice, "iso" is part of the name when it is fused into a single word (isopropyl, isobutyl) and is considered in alphabetical ordering. Check your course's convention, as some instructors treat it differently.
⚠️ Expect to be given a structural drawing and asked to produce the full IUPAC name, or vice versa. Both directions are fair game.
⚠️ Questions that test whether you can distinguish propyl from isopropyl (and the four butyl isomers) are a staple of early exams. Know the attachment point for each.
⚠️ Alphabetisation errors are easy marks lost. Practise ordering substituents until it is automatic.
⚠️ Lowest-locant-set rule: if numbering from either end gives the same first locant, compare the second, then the third, and so on.
True or False: The parent chain is always the horizontal chain in a line drawing. (False: it is the longest continuous chain regardless of how it is drawn.)
Fill in the blank: An alkyl substituent is named by replacing "-ane" with "______". (-yl)
True or False: "tri-" counts when placing substituents in alphabetical order. (False: multiplying prefixes are ignored.)
Fill in the blank: Removing the sole tertiary hydrogen from isobutane gives the ______ group. (tert-butyl)
True or False: sec-Butyl and isobutyl are the same group. (False: they differ in the point of attachment and the parent isomer.)
Q: A molecule has a nine-carbon parent chain, methyl groups at carbons 2 and 3, and an isopropyl group at carbon 6. What is its IUPAC name?
A: 6-Isopropyl-2,3-dimethylnonane.
Q: You are given the structure of 2,3,5-trimethylheptane. How many carbons are in the parent chain, and how many methyl substituents does it have?
A: Seven carbons in the parent chain (heptane) and three methyl substituents (tri-).
Q: Explain the difference between n-butyl and tert-butyl.
A: n-Butyl is a straight four-carbon chain attached at a terminal (primary) carbon. tert-Butyl has the attachment at a tertiary carbon, with three methyl groups branching from it.
Q: When writing the name 5-ethyl-2,3-dimethyloctane, why does "ethyl" appear before "dimethyl"?
A: Substituents are listed in alphabetical order. "Ethyl" (e) comes before "methyl" (m). The prefix "di-" is ignored for alphabetisation purposes.
Q: Draw the structural formula for 3-ethyl-4,5-dimethyloctane.
A: Start with an eight-carbon chain. Attach an ethyl group at carbon 3 and a methyl group at each of carbons 4 and 5.
This naming system is the foundation for every subsequent naming topic in organic chemistry. Once you add functional groups (alcohols, ketones, carboxylic acids), the same parent-chain-plus-substituent logic applies, with the functional group taking priority in numbering. Cycloalkane nomenclature follows a parallel pattern: find the ring, name branches the same way. Understanding primary, secondary, and tertiary carbons here also feeds directly into reaction topics such as radical halogenation and SN1/SN2 substitution, where reactivity depends on carbon classification.
IUPAC nomenclature, alkyl groups, naming alkanes, substituent naming, methyl ethyl propyl butyl, isopropyl, sec-butyl, tert-butyl, isobutyl, n-butyl, parent chain, longest carbon chain, branched alkanes, constitutional isomers, organic chemistry naming rules, locant, lowest locant rule, multiplying prefixes, di- tri- tetra-, alphabetical order substituents, Organic Chemistry I, OChem nomenclature