Every compound in this course has a name that was not chosen — it was derived. That is the whole point of IUPAC nomenclature: a name is a set of instructions for drawing the molecule, so two chemists who have never met describe the same structure identically, and a name you have never seen before still tells you what to draw. Learning it is not memorization. It is learning to run a procedure in a fixed order.
The procedure builds a name out of three pieces, always in this order:
prefix (what is hanging off it) + root (how long the parent chain is) + suffix (what the main functional group is)
In 2-methylbutan-1-ol, the prefix is 2-methyl, the root is but (four carbons), and the suffix is -1-ol (an alcohol on carbon 1). This section is about finding the root. The two after it cover the prefix and the suffix.
The roots: ten words that do most of the work
The root says how many carbons are in the parent chain. The first four are historical and have to be learned; from five on they are just Greek and Latin numbers, which you already half know from geometry.
Add -ane and you have named the alkanes: methane, ethane, propane, butane, pentane, and on up. The suffix -ane means "no double bonds, no triple bonds, no functional group worth naming" — it is the default, and everything in the next two sections is about replacing it.
Rule 1: the parent chain is the longest continuous chain of carbons
Find the longest unbroken path of carbon atoms through the molecule. That path is the parent; everything not on it is a substituent, named in the prefix. The word doing the work is continuous: you may turn as many corners as you like, and you may not jump a gap or pass through the same carbon twice.
The trap is that molecules are drawn for the convenience of whoever drew them, and the longest chain is very often not the one lying horizontally across the page. A structure drawn as a five-carbon row with a two-carbon branch may well have a six- or seven-carbon path through it that turns a corner. Tracing only the horizontal row is the single commonest way a first attempt goes wrong, and it goes wrong early: pick the wrong parent and every number after it is wrong too.
When two chains tie
Sometimes two different paths through the molecule are the same length. The tie-break is: choose the chain with more substituents attached. It reads backwards — you are deliberately choosing the messier-looking parent — but it is the rule, and the reason is that a name should describe as much of the molecule as possible with locants rather than burying it inside a complicated branch name.
So a seven-carbon molecule with two chains of length seven, one carrying a single ethyl group and the other carrying two methyls, is named as the heptane with two substituents, not the one with one.
Rule 2: number the chain to give the lowest locants
Once the parent is chosen, number its carbons from one end to the other. You have two choices — start at either end — and you pick the one that gives the lowest set of locants to the things that need numbering.
"Lowest set" is decided at the first point of difference, comparing the two numbering schemes term by term. A molecule whose substituents would be at {2, 3, 6} counting from the left and {2, 5, 6} counting from the right is numbered from the left, because the sets agree at 2 and the left wins at the second term, 3 against 5. You do not add the sets up and compare totals — a common invention, and wrong: {2, 5, 6} sums to 13 and {2, 3, 6} sums to 11, which happens to give the right answer here and does not in general.
Take a chain drawn as six carbons in a row with a methyl group on the third carbon from the left.
Find the parent. The longest continuous path is the six-carbon row itself; the methyl is a one-carbon branch and adding it to the path would mean turning off the main chain and back, which is not a path. Six carbons: hex. No functional group, no multiple bonds: hexane.
Number it. From the left the methyl is on C3; from the right it is on C4. Three beats four, so number from the left.
Assemble. 3-methylhexane. Hyphen between the number and the word, and the substituent name runs straight into the parent with no space.
Punctuation is part of the name
Names are written to be machine-readable, and the punctuation carries meaning rather than decorating it. Numbers are separated from letters by hyphens and from each other by commas: 2,3-dimethylpentane, never 2 3 dimethylpentane or 2-3-dimethyl pentane. There is no space anywhere inside the name of a single compound, with the exception of names like "acetic acid" where the second word names a class.
Modern IUPAC also puts a locant immediately before the part of the name it refers to, which is why this book writes but-2-ene and butan-1-ol rather than 2-butene and 1-butanol. Both styles are in wide use and older textbooks use the second, so you should read both; write the first.
What carries forward
Finding the parent chain and numbering it is the first half of every name in the rest of the course, and the two rules above never change — what changes is what you are giving the lowest number to. In this section it was the substituents, because an alkane has nothing else. Once a functional group is present it outranks them and takes the low number first, which is the subject of the functional group priority section. Get the parent and the numbering into your hands now, on molecules where nothing competes, and that later rule is a small addition rather than a new skill.