THE CONCEPT
Naming as a Repeatable Procedure, Not Guesswork
By this point you know the individual pieces — chain-length prefixes, functional-group suffixes, substituent naming — but assembling them into one single correct name requires following a fixed order of operations. Skip a step, or do them out of order, and you'll often still produce something that looks like a name but isn't actually correct or unambiguous.
The six-step procedure below is exactly that fixed order, and it works identically whether you're naming a two-carbon molecule or a heavily branched twelve-carbon one — the steps don't change, only how much work each step takes.
💡 Memory Trick
The hub's trick condenses the whole procedure into one memorable string of letters: LNNFAS — Longest chain, Number from closest substituent, Name substituents, Find parent, Alphabetize, State name. Run through those six letters in order every single time you name a compound, and you'll never accidentally skip a step or do them out of sequence.
WALKING THROUGH ALL SIX STEPS
The Full LNNFAS Procedure
1
Find the longest carbon chain
Identify the longest continuous chain of carbons, making sure to check for chains that bend through branch points rather than assuming the longest chain is drawn in a straight line.
2
Number from the end closest to the first substituent
Number the parent chain from whichever end gives the lowest possible locant to the first point of difference — usually the functional group if one is present, otherwise the first substituent encountered.
3
Name and number all substituents
Identify every substituent branching off the parent chain, give each its correct substituent name, and record which numbered carbon it's attached to.
4
Find (name) the parent chain
Combine the chain-length prefix with the correct functional-group suffix to generate the parent name (e.g., hexan-2-ol, pentanal).
5
Alphabetize the substituents
List every substituent in alphabetical order within the name, ignoring multiplying prefixes like di-, tri-, and tetra- when alphabetizing (as covered in the Multiple Substituents lesson).
6
State the full name
Assemble everything into the single final name: locants and substituent names first (alphabetized), then the parent chain name with its suffix, all correctly punctuated with commas and hyphens.
🧪 Lab Application
You're given a branched structure with a chlorine substituent and a methyl substituent on a six-carbon chain and need to produce its full, correct IUPAC name.
1
Apply step 1 — find the longest chain. Confirm the longest continuous chain is six carbons, giving a hexane parent.
2
Apply step 2 — number from the correct end. Number from whichever end gives the lowest locants overall to the two substituents combined.
3
Apply steps 3 and 4 — name substituents and the parent. Identify chloro- and methyl- as the two substituents with their correct carbon numbers, and confirm hexane as the parent name (no functional group suffix needed here since there's no O, N, or unsaturation to name).
4
Apply steps 5 and 6 — alphabetize and state the full name. Since 'chloro' comes before 'methyl' alphabetically, list chloro first: the final name reads, for example, 3-chloro-2-methylhexane.
📌 Exam Application
Full-naming questions are graded step by step in most courses, meaning partial credit is often available even if the final name has an error — always show your work for each of the six steps explicitly, rather than jumping straight to a final answer, so a small numbering mistake doesn't cost you credit for correctly identifying the parent chain and substituents.
⚠️ Most Common IUPAC Naming Steps Mistakes
The most common mistake is numbering the chain to give the lowest locant to whichever substituent is drawn first or seems most prominent, rather than checking both directions and choosing whichever gives the lowest locant set overall. The other frequent trap is alphabetizing using the multiplying prefix (like counting 'dimethyl' under 'd') instead of the substituent's own name (correctly alphabetized under 'm' for methyl).
✓ Quick Self-Test
1) What does each letter in LNNFAS stand for? 2) Why must you check both directions before numbering a chain? 3) Are multiplying prefixes like di- and tri- counted when alphabetizing substituents? 4) At what step in the procedure do you combine the chain-length prefix with the functional-group suffix? 5) Why is following a fixed step order important, rather than naming a compound however feels intuitive?
Next Lesson
Multiple Substituents
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