Difficulty: Introductory | Prerequisites: Basic understanding of covalent bonding and molecular structure
Naming organic molecules is a language skill you will use in every chapter for the rest of the course. IUPAC nomenclature gives you a systematic way to go from a structure to an unambiguous name (and back again). This section also introduces alkane properties and carbon classification, which become essential when you start thinking about reactivity at different positions on a carbon chain. If you cannot name a molecule or identify a tertiary carbon, you will struggle with substitution and elimination mechanisms later.
Alkanes are saturated hydrocarbons with only C–C and C–H single bonds. IUPAC nomenclature follows a set of rules: find the longest chain (parent), number it to give substituents the lowest locants, and name substituents as prefixes. Carbons are classified as primary, secondary, tertiary, or quaternary based on how many other carbons they are bonded to.
Alkane
A hydrocarbon containing only single bonds (C–C and C–H). Described as "saturated" because every carbon has the maximum number of hydrogen atoms possible. Think of it as the simplest, most "filled-up" type of hydrocarbon.
Straight-chain (linear) alkane
An alkane with no branches. General formula: CₙH₂ₙ₊₂. In simple terms, the carbons form a single unbroken line.
Branched alkane
An alkane where one or more carbon chains branch off the main (longest) chain. Same molecular formula as a straight-chain isomer, different connectivity.
Cycloalkane
An alkane whose carbon chain forms a ring. General formula: CₙH₂ₙ. The ring accounts for two fewer hydrogens (one degree of unsaturation, also called one degree of hydrogen deficiency, or "DoU").
Constitutional isomers
Molecules that share the same molecular formula but differ in the connectivity of their atoms. For example, butane and isobutane are both C₄H₁₀ but have different structures.
Degree of unsaturation (DoU)
A count of how many pairs of hydrogen atoms are "missing" compared to the fully saturated formula. Each ring or double bond adds one DoU; each triple bond adds two.
Primary carbon (1°)
A carbon bonded to exactly one other carbon. It carries three hydrogens (in a simple alkane context). Called "monosubstituted" in substitution terminology.
Secondary carbon (2°)
A carbon bonded to exactly two other carbons. It carries two hydrogens.
Tertiary carbon (3°)
A carbon bonded to three other carbons. It carries one hydrogen.
Quaternary carbon (4°)
A carbon bonded to four other carbons. It carries zero hydrogens.
IUPAC nomenclature
The systematic naming system maintained by the International Union of Pure and Applied Chemistry. It removes ambiguity: one structure, one name.
Straight-chain: CₙH₂ₙ₊₂, saturated, no branches
Branched: same molecular formula as straight-chain isomers, different connectivity (constitutional isomers)
Cycloalkane: CₙH₂ₙ, one ring means two fewer hydrogens (1 DoU)
1° (primary): bonded to 1 other C, 3 H (monosubstituted)
2° (secondary): bonded to 2 other C, 2 H (disubstituted)
3° (tertiary): bonded to 3 other C, 1 H (trisubstituted)
4° (quaternary): bonded to 4 other C, 0 H (tetrasubstituted)
A quick shortcut: count the hydrogens on a given carbon. 3 H = primary, 2 H = secondary, 1 H = tertiary, 0 H = quaternary.
Carbons | Base name | Substituent name |
|---|---|---|
1 | Methane | Methyl |
2 | Ethane | Ethyl |
3 | Propane | Propyl |
4 | Butane | Butyl |
5 | Pentane | Pentyl |
6 | Hexane | Hexyl |
7 | Heptane | Heptyl |
8 | Octane | Octyl |
9 | Nonane | Nonyl |
10 | Decane | Decyl |
Isopropyl: a three-carbon group branching from the middle carbon
Isobutyl: a four-carbon group with a branch at the second carbon from the point of attachment
Sec-butyl: a four-carbon group attached at a secondary carbon
Tert-butyl (t-butyl): a four-carbon group attached at a tertiary carbon (three methyl groups on one carbon)
Neopentyl: a five-carbon group with a quaternary carbon at the point of attachment
Find the longest continuous carbon chain (this is your parent chain)
Number the chain so that substituents receive the lowest possible locants
Name each substituent and place it alphabetically before the parent name
Use prefixes (di-, tri-, tetra-) for identical substituents; these prefixes do not affect alphabetical order
For now, the functional groups to know are alkanes and haloalkanes
Weak intermolecular forces (London dispersion only)
Generally unreactive due to strong, non-polar C–H and C–C bonds
Non-polar covalent bonds are electron-rich (important later for electrophilic reactions)
Larger molecule → increased surface area → increased boiling point (BP) and melting point (MP)
Branched alkanes have lower BP and MP compared to their straight-chain isomers, because branching decreases the surface area available for intermolecular stacking
IUPAC nomenclature is the universal language chemists use to communicate structures unambiguously, whether in a research paper, a pharmaceutical patent, or a safety data sheet. The trend of boiling points with molecular size and branching explains why shorter-chain hydrocarbons (like propane and butane) are gases at room temperature while longer ones (like octane) are liquids.
Students often forget to find the longest continuous chain, not just the one drawn horizontally. Redraw the molecule if needed.
Students mix up sec-butyl and isobutyl. Sec-butyl is attached at a secondary carbon; isobutyl branches at the carbon next to the attachment point.
Students sometimes think more carbons always means more branching. Branching and chain length are independent properties.
Students overlook that alphabetical ordering of substituents ignores multiplying prefixes (di-, tri-). "Dimethyl" is alphabetised under "m," not "d."
⚠️ You are expected to know base names (methane through decane) and their substituent equivalents cold.
⚠️ Naming and drawing structures from IUPAC names is tested frequently, often as the opening question on an exam.
⚠️ Carbon classification (1°, 2°, 3°, 4°) comes back repeatedly when discussing carbocation stability, substitution, and elimination.
⚠️ Know how branching affects boiling point and melting point, and be able to explain why (surface area for London dispersion forces).
Fill in the blank: The general formula for an acyclic alkane is CₙH______.
True or false: A quaternary carbon carries one hydrogen.
Fill in the blank: The IUPAC base name for a seven-carbon chain is ________.
True or false: Branched alkanes have higher boiling points than their straight-chain isomers.
Fill in the blank: When two methyl groups are present on a parent chain, the prefix used is ________.
Answers: 1. 2n+2. 2. False (it carries zero hydrogens). 3. Heptane. 4. False (branching lowers boiling point due to decreased surface area). 5. Dimethyl.
Q: What is the IUPAC base name and substituent name for a four-carbon unit?
A: Base name is butane; substituent name is butyl.
Q: A carbon is bonded to three other carbons and one hydrogen. What is its classification?
A: Tertiary (3°).
Q: Explain why pentane has a higher boiling point than neopentane, even though they share the molecular formula C₅H₁₂.
A: Pentane is a straight chain with more surface area available for London dispersion interactions. Neopentane is compact and roughly spherical, reducing contact area between molecules, which weakens intermolecular forces and lowers the boiling point.
Q: In IUPAC nomenclature, if a compound has both an ethyl and a dimethyl substituent, which comes first in the name?
A: Ethyl comes first. Alphabetical order uses "e" for ethyl and "m" for methyl; the "di-" prefix on dimethyl is ignored for alphabetical purposes.
Carbon classification ties directly into carbocation stability (Chapter 3+), where tertiary carbocations are more stable than secondary or primary. Nomenclature will expand to include alkenes, alkynes, alcohols, and other functional groups in later chapters, all building on the IUPAC rules introduced here. Intermolecular forces revisit themes from general chemistry and become important again when discussing solubility and separation techniques.
alkane, nomenclature, IUPAC naming, straight chain, branched alkane, cycloalkane, constitutional isomers, degree of unsaturation, DoU, primary carbon, secondary carbon, tertiary carbon, quaternary carbon, methyl, ethyl, propyl, butyl, isopropyl, tert-butyl, sec-butyl, isobutyl, neopentyl, boiling point, London dispersion forces, organic chemistry chapter 2