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IUPAC Naming
Alkanes & Cycloalkanes

Master the systematic naming rules for straight-chain, branched, and cyclic hydrocarbons using the IUPAC nomenclature system.

Organic Chemistry 1

What Are Alkanes?

Alkanes are saturated hydrocarbons: they contain only carbon and hydrogen atoms connected by single bonds. They follow the general molecular formula \( \mathrm{C}_n\mathrm{H}_{2n+2} \).

C1 C2 C3 C4 C5
Pentane (\( \mathrm{C}_5\mathrm{H}_{12} \))

The First 10 IUPAC Prefixes

Memorizing these prefixes is the foundation of organic nomenclature. Each prefix corresponds to a specific carbon count.

Prefix Carbons Formula Name
Meth-1\( \mathrm{CH}_4 \)Methane
Eth-2\( \mathrm{C}_2\mathrm{H}_6 \)Ethane
Prop-3\( \mathrm{C}_3\mathrm{H}_8 \)Propane
But-4\( \mathrm{C}_4\mathrm{H}_{10} \)Butane
Pent-5\( \mathrm{C}_5\mathrm{H}_{12} \)Pentane
Hex-6\( \mathrm{C}_6\mathrm{H}_{14} \)Hexane
Hept-7\( \mathrm{C}_7\mathrm{H}_{16} \)Heptane
Oct-8\( \mathrm{C}_8\mathrm{H}_{18} \)Octane
Non-9\( \mathrm{C}_9\mathrm{H}_{20} \)Nonane
Dec-10\( \mathrm{C}_{10}\mathrm{H}_{22} \)Decane

What Are Cycloalkanes?

Cycloalkanes are ring-shaped saturated hydrocarbons. They follow the general formula \( \mathrm{C}_n\mathrm{H}_{2n} \) because closing the ring removes two hydrogens.

Cyclopentane Cyclohexane
\( \mathrm{C}_5\mathrm{H}_{10} \)
\( \mathrm{C}_6\mathrm{H}_{12} \)

Substituents: Alkyl Groups

When a hydrogen on the parent chain is replaced by a carbon branch, that branch is called an alkyl group. Alkyl groups use the same prefixes but end in -yl instead of -ane.

Methyl
\( \text{-}\mathrm{CH}_3 \), 1 carbon branch
Ethyl
\( \text{-}\mathrm{C}_2\mathrm{H}_5 \), 2 carbon branch
Propyl
\( \text{-}\mathrm{C}_3\mathrm{H}_7 \), 3 carbon branch
Isopropyl
\( \text{-}\mathrm{CH}(\mathrm{CH}_3)_2 \), branched 3C

The 4-Step IUPAC Naming Rules

1
Find the longest continuous carbon chain. This is your parent chain. Name it using the prefix + "-ane" suffix.
2
Number the chain from the end nearest to the first substituent, giving the lowest set of locants.
3
Identify and name each substituent. Use the position number (locant) followed by the alkyl name (e.g., 2-methyl).
4
Assemble the name: list substituents in alphabetical order, then attach them to the parent name. Use di-, tri-, tetra- for multiples of the same group.

The Lowest Locant Rule

When numbering the parent chain, choose the direction that gives the lowest set of locants at the first point of difference.

1 2 3 4 5 6
\( \mathrm{CH}_3 \)
 ✓ 3-methylhexane (locant = 3) ✕ 4-methylhexane (locant = 4)

Alkane vs Cycloalkane Naming

Feature Alkane Cycloalkane
General Formula\( \mathrm{C}_n\mathrm{H}_{2n+2} \)\( \mathrm{C}_n\mathrm{H}_{2n} \)
StructureOpen chainClosed ring
Parent NamePrefix + aneCyclo + prefix + ane
Numbering StartEnd nearest substituentSubstituent carbon = C1
DirectionLowest locant setLowest locant set around ring
No SubstituentsJust parent nameJust "cyclo" + parent

Alphabetical Order & Multipliers

When a molecule has multiple different substituents, list them in alphabetical order. If the same substituent appears more than once, use a multiplying prefix.

di-
2 of the same group (e.g., 2,3-dimethyl)
tri-
3 of the same group (e.g., 2,3,4-trimethyl)
tetra-
4 of the same group (e.g., 2,2,3,3-tetramethyl)

Naming Cycloalkanes with Branches

1
Identify the ring as the parent if it has more carbons than any substituent chain (or equal carbons).
2
If only one substituent: no locant needed. Just write substituent name + cycloparent (e.g., methylcyclohexane).
3
If two or more substituents: assign C1 to give the lowest set of locants. Number in the direction that minimizes subsequent locants.
4
If a chain has more carbons than the ring: the chain becomes the parent, and the ring is a "cyclo-" substituent (e.g., cyclopropylhexane).

Example 1: Simple Branched Alkane

1 2 3 4 5
\( \mathrm{CH}_3 \)
 2-methylpentane

Example 2: Multiple Substituents

1 2 3 4 5 6 7 ethyl methyl 3-ethyl-5-methylheptane

Example 3: Substituted Cycloalkane

1 2 3 4 5 6
\( \mathrm{CH}_3 \)
\( \mathrm{C}_2\mathrm{H}_5 \)
 1-ethyl-3-methylcyclohexane

Your Naming Checklist

1
Draw or identify the structure. Look for the longest continuous chain (or largest ring).
2
Count carbons in the parent chain/ring. Apply the correct prefix (meth-, eth-, prop-, etc.).
3
Number the chain/ring to give the lowest locants to substituents.
4
Name each substituent with its locant. Use di-, tri-, tetra- for repeated groups.
5
Alphabetize substituents (ignore di-, tri-, etc. in alphabetizing). Combine with hyphens and commas.
6
Write the final name as one word: locants-substituent + parent name (e.g., 3-ethyl-5-methylheptane).

Common Mistakes to Avoid

✕ Wrong Longest Chain
Choosing a straight-looking chain that is not actually the longest. Always trace every possible path through the structure.
✕ High Locant Numbers
Numbering from the wrong end, giving substituents higher locants when lower ones are possible.
✕ Alphabetizing Multipliers
Treating "di-" or "tri-" as part of the alphabetical name. Only the root alkyl name (methyl, ethyl) counts for ordering.
✕ Missing Cyclo- Prefix
Forgetting to add "cyclo-" before the parent name for ring compounds: cyclohexane, not hexane.

Question 1 of 5

What is the IUPAC name for a straight-chain alkane with 8 carbon atoms?

Question 2 of 5

A molecule has a 6-carbon main chain with a methyl group at carbon 3. What is its IUPAC name?

Question 3 of 5

Which compound has the molecular formula \( \mathrm{C}_5\mathrm{H}_{10} \)?

Question 4 of 5

For the compound 4-ethyl-2,2-dimethylhexane, how many substituents are attached to the parent chain?

Question 5 of 5

A cyclohexane ring has an ethyl group and a methyl group. Using IUPAC rules, which substituent gets carbon 1?