Difficulty: Introductory to Intermediate | Prerequisites: Part 1 of these notes (alkane basics, unbranched naming)
Once you move beyond straight-chain alkanes, molecules can branch, and you need a systematic way to name them so that every chemist in the world arrives at the same name for the same structure. That system is IUPAC nomenclature. The naming rules here are not just an academic exercise: they form the backbone of how you will name every organic molecule for the rest of the course. Mastering the procedure now saves enormous time later.
Branched alkanes are named by identifying the longest continuous carbon chain (the parent), numbering it to give substituents the lowest possible locants, and listing substituents alphabetically before the parent name. Carbon atoms are classified as primary, secondary, tertiary or quaternary based on how many other carbons they are bonded to.
Parent chain (longest continuous chain)
The longest unbroken sequence of carbon atoms in the molecule. It determines the base name of the alkane (pentane for 5 carbons, hexane for 6, and so on). Think of it as: the spine of the molecule.
Substituent
Any group attached to the parent chain. For alkanes, these are alkyl groups. The suffix "-yl" replaces "-ane" (methane becomes methyl, ethane becomes ethyl). Think of it as: the branches hanging off the spine.
Locant
The number assigned to the carbon on the parent chain where a substituent is attached. You always choose the numbering direction that gives the lowest set of locants.
Isopropyl (1-methylethyl)
A three-carbon substituent where the point of attachment is the middle carbon. Common name for the branched C₃ group. In simple terms, a Y-shaped three-carbon branch.
Tert-butyl (1,1-dimethylethyl)
A four-carbon substituent where the point of attachment is a carbon bonded to three methyl groups. The most highly branched C₄ substituent.
Primary carbon (1°)
A carbon bonded to exactly one other carbon atom. Think of it as: the end of a chain.
Secondary carbon (2°)
A carbon bonded to exactly two other carbon atoms. Think of it as: a carbon in the middle of a chain.
Tertiary carbon (3°)
A carbon bonded to exactly three other carbon atoms. Think of it as: a branching point.
Quaternary carbon (4°)
A carbon bonded to four other carbon atoms (no hydrogens). Think of it as: a fully surrounded junction.
Find the longest continuous chain of carbon atoms. This is the parent chain. Its length gives the base name (5 carbons = pentane, 7 carbons = heptane). Be careful: the longest chain is not always drawn horizontally. Trace every possible path through the structure.
Number the parent chain from 1 to n. Choose the direction that gives the substituents the lowest possible set of locants.
Identify and name each substituent. An alkyl substituent is named by its carbon count plus the suffix "-yl."
Combine: write the locant, a hyphen, the substituent name, then the parent name. No spaces between the substituent and the parent. Example: 3-methylpentane.
Same substituent appears more than once: use a multiplying prefix (di-, tri-, tetra-, penta-, etc.) and list each locant separated by commas.
Example: 2,4-dimethylhexane (two methyl groups at positions 2 and 4 on a hexane chain)
Different substituents: list them in alphabetical order in the name.
Example: 4-ethyl-2-methylhexane (ethyl comes before methyl alphabetically)
Multiplying prefixes (di-, tri-, etc.) do not count when alphabetising. "Dimethyl" is alphabetised under "m" for methyl, not "d" for di.
If there is a tie in numbering: the substituent that comes first alphabetically gets the lower locant.
Example: 3-ethyl-5-methylheptane, not 5-ethyl-3-methylheptane
Name | Abbreviation | Condensed Formula |
|---|---|---|
Methyl | Me | —CH₃ |
Ethyl | Et | —CH₂CH₃ |
Propyl | Pr | —CH₂CH₂CH₃ |
Isopropyl | iPr | —CH(CH₃)₂ |
Butyl | Bu | —CH₂CH₂CH₂CH₃ |
Isobutyl | iBu | —CH₂CH(CH₃)₂ |
sec-Butyl | s-Bu | —CH(CH₃)CH₂CH₃ |
tert-Butyl | t-Bu | —C(CH₃)₃ |
Substituents themselves can be branched. When that happens, the substituent is named using the same rules (find its longest chain, number, identify sub-substituents) and enclosed in parentheses.
Example: 4-(1-methylethyl)heptane is the systematic name for what is commonly called 4-isopropylheptane.
Carbon atoms are classified by the number of other carbon atoms directly bonded to them. This classification becomes important when you study reaction mechanisms later.
Primary (1°): bonded to 1 other carbon (end of a chain or branch)
Secondary (2°): bonded to 2 other carbons (mid-chain)
Tertiary (3°): bonded to 3 other carbons (a branch point)
Quaternary (4°): bonded to 4 other carbons (no hydrogens on that carbon)
The same classification applies to hydrogens: a hydrogen on a primary carbon is a primary hydrogen, and so on. This matters for reactivity in later chapters.
Students often pick the horizontal chain as the parent, even when a longer chain runs through branches drawn at angles. Always trace every possible continuous path.
A common mistake is alphabetising using the multiplying prefix: "dimethyl" is filed under M, not D.
Students sometimes number the chain from left to right out of habit, when numbering from right to left would give lower locants. Always check both directions.
Confusing carbon classification with substituent naming is common. A tertiary carbon is defined by how many other carbons it touches, not by how large the groups are.
⚠️ Naming branched alkanes from a drawn structure (and drawing a structure from a name) is one of the most predictable exam question types in this chapter.
⚠️ Watch for tricky drawings where the longest chain is not the obvious horizontal line. Exam structures are often drawn to catch this mistake.
⚠️ Know the common names for isopropyl, isobutyl, sec-butyl and tert-butyl, along with their systematic equivalents.
⚠️ Carbon classification (1°, 2°, 3°, 4°) appears again in substitution and elimination reactions. Get comfortable identifying them now.
True or false: When alphabetising substituents, "diethyl" comes before "methyl." (False, the prefix "di-" is ignored; ethyl comes before methyl.)
Fill in the blank: A carbon bonded to three other carbon atoms is a ______ carbon. (Tertiary / 3°)
True or false: The parent chain must always be the one drawn horizontally. (False, it is the longest continuous chain regardless of how it is drawn.)
Fill in the blank: Use commas to separate ______ and hyphens to separate ______. (Numbers from numbers; numbers from letters)
True or false: tert-Butyl is a four-carbon substituent with three methyl groups on the attachment carbon. (True)
Q: Name the following compound: a seven-carbon parent chain with a methyl group at C-3 and an ethyl group at C-5.
A: 5-Ethyl-3-methylheptane. Substituents are listed alphabetically (ethyl before methyl).
Q: A molecule is drawn as CH₃CH₂CH(CH₃)CH₂CH₃. What is its IUPAC name?
A: 3-Methylpentane. The longest chain is five carbons, with a methyl substituent on carbon 3.
Q: How would you name an alkane with a six-carbon parent chain and methyl groups on C-2 and C-4?
A: 2,4-Dimethylhexane.
Q: Identify the primary, secondary, tertiary and quaternary carbons in 2,2-dimethylpropane (neopentane).
A: The central carbon is quaternary (bonded to four other carbons). The four surrounding methyl carbons are all primary (each bonded to only one other carbon).
Q: Why is 4-isopropylheptane sometimes written as 4-(1-methylethyl)heptane?
A: Both names describe the same compound. The second version uses the systematic IUPAC name for the substituent rather than the common name. The substituent has a two-carbon chain (ethyl) with a methyl branch at position 1, hence 1-methylethyl, enclosed in parentheses.
The IUPAC naming procedure you learn here extends directly to naming alkenes, alkynes, alcohols and every other functional group, with only minor modifications. Carbon classification (1°, 2°, 3°, 4°) determines reactivity in free-radical halogenation (Chapter 4) and in substitution and elimination reactions (Chapters 6 and 7). Common substituent names like isopropyl and tert-butyl appear constantly in later chapters.
IUPAC nomenclature, branched alkane naming, parent chain, substituent, locant, alkyl group, methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, tert-butyl, primary carbon, secondary carbon, tertiary carbon, quaternary carbon, 1° 2° 3° 4°, naming procedure, alphabetical order, multiplying prefix, di- tri- tetra-, CHEM 25500, organic chemistry chapter 2, Purdue