Start with one name: 2-methylbutan-1-ol. Every piece of it points at part of a drawing. The 2 and the 1 are locants: numbers that say which carbon something sits on. Methyl is a one-carbon branch, but says the main chain has four carbons, and ol says an OH group is attached.
Nobody chose that name. It was derived. A name is a set of instructions for drawing the molecule. Two chemists who have never met write the same name for the same structure, and a name you have never seen still tells you what to draw.
So learning names is not memorization. It is learning to run one procedure in a fixed order.
The procedure builds every name from three pieces, always in this order:
prefix (what hangs off the chain) + root (how long the parent chain is) + suffix (the main functional group)
The parent chain is the main chain of carbons the name is built on, and the root names its length. The figure shows one more small piece: the an between root and suffix. It is left over from butane, and it says the chain has only single bonds.
This section is about finding the root. The next two sections cover the prefix and the suffix.
The roots: ten words that do most of the work
Hexane has six carbons, and hex is the root for six. The root always says how many carbons are in the parent chain.
The first four roots are historical, so learn them by heart. From five on they are Greek and Latin numbers, which you already half know from geometry: pentagon, hexagon, octagon.
Add -ane to a root and you have named an alkane, a compound of only carbon and hydrogen joined by single bonds: methane, ethane, propane, butane, pentane, and on up.
A double or triple bond changes -ane to -ene or -yne. A functional group adds its own suffix after the an, as in butan-1-ol. Later sections of this chapter cover both.
Rule 1: the parent chain is the longest continuous chain of carbons
Look at the skeleton below. Its row across the page has five carbons. A different path climbs the branch and then runs along the row, and that path has seven. The seven-carbon path is the parent. The molecule is a heptane with one methyl on it: 4-methylheptane.
That is Rule 1. Find the longest unbroken path of carbon atoms through the molecule. That path is the parent chain.
The key word is continuous. You may turn as many corners as you like. You may not jump a gap, and you may not pass through the same carbon twice.
Everything not on the parent chain is a substituent: a group attached to the chain in place of a hydrogen. Substituents are named in the prefix.
A carbon branch takes its name from its length: the root plus -yl. A one-carbon branch is methyl, a two-carbon branch is ethyl, and a three-carbon branch is propyl. The next section names branches in full. For now, a branch is simply what is left over.
The longest chain is often not the row across the page, as in the figure. Tracing only the row is the commonest first mistake. Pick the wrong parent and every number after it is wrong.
Check every end, not just the one you started from
The three drawings below look alike. Each is a row of carbons with a short branch. Each parent is found a different way.
List the ends. An end carbon is a carbon bonded to only one other carbon. Each drawing has three: the two ends of the row and the far end of the branch.
Left drawing. Row end to row end: six. Methyl to the left end of the row: four. Methyl to the right end: five. The row wins, and the name is 3-methylhexane.
Middle drawing. Row end to row end: four. Ethyl end to the near row end: four. Ethyl end to the far row end: two ethyl carbons, then the carbon they hang from, then two more, which makes five. The longest route turns the corner, so the parent is pentane. The carbon left over, the near end of the row, is a methyl on C3: 3-methylpentane. "2-ethylbutane" describes the drawing, not the molecule.
Right drawing. Row end to row end: five. Ethyl end to either row end: two, then the middle carbon, then two, which is also five. Every route ties. Each of these chains carries one ethyl, so the tie changes nothing. The row is a fair parent and the ethyl is a real ethyl: 3-ethylpentane.
The middle drawing shows a general fact. An ethyl on the second carbon of a row can never stay an ethyl, because the route through it is always one carbon longer than the row. An ethyl on the middle carbon of a five-carbon row exactly ties. Counting from every end tells you which case you have; looking at the drawing does not.
So the test is mechanical. A chain has to start and finish at end carbons. List the ends, and for each pair count the carbons on the route between them. The longest of those routes is the parent.
A drawing with three ends has three routes to check. That takes ten seconds, and it is the ten seconds most wrong names would have needed.
When two chains tie
Sometimes two paths through a molecule have the same length. The skeleton below has two seven-carbon paths. The row across the top leaves one branch behind: a three-carbon group attached by its middle carbon, called isopropyl. The other path starts at one end of that branch. It leaves two branches behind: a methyl on C2 and an ethyl on C3.
The second path is the parent, and the name is 3-ethyl-2-methylheptane. (The chain is numbered from the end nearer the branches; Rule 2 below explains why. Ethyl is written before methyl because substituents are listed in alphabetical order; the next section covers that.)
Here is the rule: when two chains tie for length, choose the chain with more substituents.
The rule reads backwards, because you pick the messier-looking parent on purpose. The reason is that a name should describe as much of the molecule as it can with locants. It should not bury part of the molecule inside one complicated branch name.
If the substituent counts tie as well, the numbering rule below breaks the tie. Take the chain whose substituents get the lower set of locants.
Rule 2: number the chain to give the lowest locants
Take the molecule in the figure below: six carbons in a row, with a methyl on the third carbon from the left.
Find the parent. The row has six carbons. The methyl has two routes of its own: four carbons to the left end and five to the right end. Both are shorter than six, so the row is the parent. Six carbons gives hex. There is no functional group and no multiple bond, so the parent is hexane.
Number it. From the left the methyl is on C3. From the right it is on C4. Three is lower than four, so number from the left.
Assemble. 3-methylhexane. A hyphen joins the number to the word, and the substituent name runs straight into the parent with no space.
That is Rule 2. Once the parent is chosen, number its carbons from one end to the other. You can start at either end. Pick the end that gives the lowest set of locants to the things that need numbers.
With more than one substituent, you compare two sets. The heptane below carries three methyls, so its name will say trimethyl. (The prefixes di- and tri- count repeated groups; the next section covers them.) Its methyls sit at {2, 3, 6} counting from the left, and at {2, 5, 6} counting from the right.
The sets agree at the first term, 2. They differ at the second term, 3 against 5, and 3 is lower. So the chain is numbered from the left, and the name is 2,3,6-trimethylheptane. This is the first point of difference rule: compare the sets term by term, and the first term where they differ decides.
Do not add the sets up and compare totals. That shortcut is common, and it is wrong. For the heptane it happens to agree: {2, 3, 6} adds to 11 and {2, 5, 6} adds to 13. The decane below shows it failing.
Counted from the left, the decane's methyls sit at {2, 7, 8}, which add to 17. Counted from the right, they sit at {3, 4, 9}, which add to 16. The lower sum points right. The first term, 2 against 3, points left, and the first term is the rule. The name is 2,7,8-trimethyldecane.
Punctuation is part of the name
Look at 2,3-dimethylpentane. A comma separates the two numbers. A hyphen separates the numbers from the word. There is no space anywhere.
That is the rule. Numbers are separated from each other by commas and from letters by hyphens. So "2 3 dimethylpentane" and "2-3-dimethyl pentane" are both wrong. The punctuation carries meaning; it does not decorate.
The name of a single compound contains no spaces. The exceptions are two-word names such as acetic acid and ethyl acetate, which later sections explain.
Current IUPAC places each locant right before the part of the name it refers to. So this course writes butan-1-ol, where the 1 locates the -ol, and but-2-ene, where the 2 locates the -ene. The ending -ene marks a carbon–carbon double bond, and its locant is the lower-numbered of the two carbons that the double bond joins.
Older books write the same two compounds as 1-butanol and 2-butene, with the locant at the front. Both styles are in wide use, so learn to read both. Write the first.
What carries forward
Every name in the rest of the course starts with this page's two rules: find the parent, then number it. The rules never change. What changes is what gets the lowest number.
On this page the substituents got the lowest numbers, because an alkane has nothing else to number.
A functional group changes that. Take the alcohol below. Its longest chain has six carbons, but the carbon that carries the OH is not on that chain. The parent must contain the carbon that carries the OH, so the parent is a five-carbon chain. The OH carbon takes C1, ahead of the ethyl on C2, and the name is 2-ethylpentan-1-ol.
So a functional group outranks substituents in two ways. The parent must contain it, even at the cost of length. And it takes the lowest number first. The functional-group section of this chapter makes both rules explicit.
Most textbooks and exams treat double and triple bonds the same way: the parent must contain them, and they take a low number. The 2013 IUPAC recommendations put chain length ahead of the multiple bond instead. The rings-and-unsaturation section of this chapter covers both.
Practice finding and numbering the parent now, on molecules where nothing competes. Each later rule is then a small addition, not a new skill.