Organic Chemistry · Organic Compounds: Alkanes and Their Stereochemistry
Naming Alkanes
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In 30 seconds
IUPAC (International Union of Pure and Applied Chemistry) nomenclature gives every organic molecule a unique, systematic name that any chemist in the world can convert back into a structure. For alkanes the system has four steps: (1) find the longest continuous carbon chain — the parent; (2) number the chain from the end that gives the first Substituent A group (e.g., alkyl) replacing an H on the parent chain Full entry → the lowest possible Locant Number showing where a substituent attaches to the parent Full entry →; (3) name each substituent (alkyl groups from the previous topic) and attach its position number; (4) list substituents alphabetically, ignoring di-, tri-, and tetra- multipliers, and assemble the name with hyphens between numbers and words and commas between numbers. For example, CH3CH(CH3)CH2CH3 is 2-methylbutane: a four-carbon parent (butane) with a methyl group on carbon 2. The rules sound simple but hide several traps — choosing the wrong "longest chain," numbering from the wrong end, and alphabetizing incorrectly — so this topic works through each rule and the classic errors that appear on exams.
Why this matters
- Universal communication: A systematic name uniquely identifies a molecule; "isooctane" or "petroleum ether" are ambiguous, but 2,2,4-trimethylpentane is not.
- Safety and regulation: Chemical inventories, SDS (safety data sheets), and pharmaceutical labels rely on systematic names — misreading a name can mean handling the wrong substance.
- Foundation for all later naming: Alkenes, alcohols, aldehydes, and every functional group build on alkane parent names and the same numbering/alphabetizing logic.
- Structure–property prediction: Once a name tells you the connectivity, you can predict boiling points, polarity, and reactivity.
- Exams: Name-to-structure and structure-to-name conversions are guaranteed questions in every organic course; the traps below are the ones exam writers reuse.
The college version
Core Concepts
Step 1: Find the longest continuous chain
The parent name comes from the longest continuous chain of carbons, regardless of how the molecule is drawn. With CH3CH2CH(CH2CH3)CH2CH3, students often pick the horizontal row of four carbons, but the longest path is five carbons (through the ethyl branch), so the parent is pentane, not butane. When two chains tie for length, choose the one with more substituents.
Step 2: Number to give the first substituent the lowest locant
Number the Parent chain Longest continuous carbon chain; gives the root name Full entry → from the end that gives the first substituent encountered the smallest number. If the first substituents tie (e.g., methyls at C2 from both ends), compare the next substituent, and so on — the First point of difference Compare locants in order; the first lower number wins Full entry → rule. A common error is choosing the direction that gives the sum of locants the smallest; that is wrong — it is the first difference that decides.
Step 3: Name the substituents and assign locants
Each branch off the parent is an alkyl group (methyl, ethyl, propyl…). Write its position number (locant) immediately before its name, separated by a hyphen: 2-methyl, 3-ethyl. If the same group appears more than once, use multipliers di-, tri-, tetra- with a comma-separated list of locants: 2,2-dimethyl, 3,4,5-trimethyl.
Step 4: Alphabetize and assemble
List substituents alphabetically by name — ethyl before methyl — ignoring the di-/tri-/tetra- multipliers for alphabetizing (dimethyl counts as "methyl"). Assemble as: locant(s)–substituent, locant(s)–substituent parent. Commas separate numbers; hyphens separate numbers from words. Example: 3-ethyl-2-methylpentane (ethyl listed first alphabetically, even though the methyl has the lower locant).
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| Longest chain | The horizontal row you see first | Trace every continuous path; the ethyl group may extend the chain |
| First-point-of-difference rule | Smallest sum of locants | 2,3-dimethylpentane is correct; 3,4-dimethylpentane (sum 7 vs 5) is not — but compare first locants, not sums |
| Alphabetical order of substituents | Numerical order of locants | 3-ethyl-2-methylpentane (E before M), even though methyl has locant 2 |
| Alphabetizing "dimethyl" | Alphabetizing by "d" | Multipliers are ignored: 2,2-dimethyl comes after 3-ethyl |
| 2-ethyl-butane-style names | Valid names | A "2-ethyl" on a short chain means you missed the longer parent; ethyl must appear at C3 or beyond on a valid chain |
| Locant placement | Anywhere in the name | Locants go immediately before their substituent name: 2-methyl, not methyl-2 |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Naming an alkane is like giving an address to a train. First, count the longest line of train cars — that's the family name (pentane = five cars). Then number the cars from whichever end reaches the first branch sooner. Each side-branch gets a label — a methyl car attached to car 2 — and you write it as "2-methyl." If branches are different kinds, list them in ABC order. The full name is the train's exact address: 3-ethyl-2-methylpentane tells you exactly how to rebuild the train.
Worked example
Example 1: Name a branched alkane from its structure
Name CH3CH(CH3)CH2CH3.
Step 1 — longest chain: four carbons → butane. Step 2 — numbering: a methyl branch is on carbon 2 either way (the molecule is symmetric), so numbering is unambiguous. Step 3 — substituent: one methyl at C2 → 2-methyl.
Answer: 2-methylbutane. (Common name: isopentane.)
Example 2: Choose the correct parent chain
Name the molecule CH3CH2CH(CH2CH3)CH2CH3.
Step 1 — longest chain: the longest continuous path is 5 carbons (through the ethyl "branch"), not 4. Step 2 — numbering: with a five-carbon parent, the ethyl substituent sits on carbon 3 from either end.
Answer: 3-ethylpentane. The most common wrong answer, 2-ethylbutane, fails the first rule: the parent must be the longest chain, and a 2-ethyl substituent on a four-carbon chain is always really part of a five-carbon chain.
Example 3: Name a multiply branched alkane
Name CH3CH(CH3)CH2CH(CH3)CH2CH3.
Step 1 — longest chain: six carbons → hexane. Step 2 — numbering: methyls appear on carbons 2 and 4 from the left, or 3 and 5 from the right — the first point of difference is C2 vs C3, so number from the left. Step 3 — substituents: methyls at 2 and 4 → 2,4-dimethyl. Alphabetical order irrelevant here (only one substituent type).
Answer: 2,4-dimethylhexane.
Example 4: Convert a name to a structure
Draw 3-ethyl-2-methylpentane.
Parse the name: parent pentane = 5 carbons; ethyl at C3; methyl at C2. Build: CH3-CH(CH3)-CH(CH2CH3)-CH2-CH3. Verify: the longest chain through this structure is 5 carbons (the ethyl branch would extend it to 6 if counted as part of the parent — but the parent must contain the most substituted path; here the 5-carbon chain with two substituents is correct), and numbering from the left gives methyl at 2, ethyl at 3 — matching the name.
Answer: CH3CH(CH3)CH(CH2CH3)CH2CH3 — always rebuild the structure from the name and re-check that the name describes it uniquely.
Key takeaways
- Parent = longest continuous carbon chain; ties broken by "more substituents."
- Number from the end giving the first substituent the lowest locant; use the first-point-of-difference rule, never the locant sum.
- Substituents: alkyl groups with locants (2-methyl, 3-ethyl); multipliers di-, tri-, tetra- with comma-separated locants (2,2-dimethyl).
- Alphabetize substituent names, ignoring di-/tri-/tetra-: 3-ethyl-2-methyl (ethyl first) is correct; 2-methyl-3-ethyl is not.
- Punctuation: hyphens between numbers and words, commas between numbers (2,2,4-trimethylpentane).
- Common names (isobutane, neopentane) exist, but IUPAC names are required for systematic work; isobutane = 2-methylpropane, neopentane = 2,2-dimethylpropane.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
Name CH3CH2CH(CH3)CH2CH2CH3.
Show answer
3-methylhexane — longest chain is 6 carbons; numbering from the left puts the methyl at C3 (from the right it would be C4).
Which is correct: 2,3-dimethylpentane or 3,4-dimethylpentane? Explain.
Show answer
2,3-dimethylpentane — the first point of difference favors locants 2,3 over 3,4; do not compare sums.
Why is "2-ethylbutane" an invalid IUPAC name?
Show answer
The longest continuous chain through that structure is 5 carbons, so the ethyl group must be part of the parent (3-ethylpentane); an ethyl can never sit at C2 of a chain that short.
Arrange the substituents in the name: methyl at C2, ethyl at C3, methyl at C5 (parent heptane). Write the full name.
Show answer
3-ethyl-2,5-dimethylheptane — ethyl alphabetically precedes methyl; dimethyl uses the multiplier with locants 2 and 5.
Draw the structure of 2,2-dimethylpropane and give its common name.
Show answer
(CH3)4C, a central carbon with four methyls — common name neopentane.
True or false: in 3-ethyl-2-methylpentane, the methyl is listed before the ethyl because methyl has a lower locant.
Show answer
False — ethyl is listed first because substituents are alphabetized (E before M); locant order does not control alphabetization.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Parent chain
- Longest continuous carbon chain; gives the root name
- Locant
- Number showing where a substituent attaches to the parent
- Substituent
- A group (e.g., alkyl) replacing an H on the parent chain
- First point of difference
- Compare locants in order; the first lower number wins
- Multiplier (di-, tri-, tetra-)
- Counts repeated identical substituents
- n-, sec-, iso-, tert-
- Common-name prefixes for alkyl groups
Sources & references
This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.
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