With the parent chain found and numbered, everything hanging off it has to be named and placed. That is the prefix, and it is where most of the fiddly rules live — not because the chemistry is hard, but because the conventions are arbitrary and have to be applied in a fixed order.
Alkyl groups: an alkane minus one hydrogen
A carbon branch is named by taking the root for its length and adding -yl. One carbon is methyl, two is ethyl, three is propyl, four is butyl, and so on up the same list of roots. The suffix -yl means "this is attached to something else rather than standing alone", so methane is a molecule and methyl is a piece of one.
From three carbons up, a branch can attach through different carbons, and those are genuinely different groups with different names:
The systematic names in italics are what current IUPAC prefers and what you should be able to decode; the common names are what almost every lecturer, paper and exam actually says. tert-Butyl in particular is universal — nobody says 2-methylpropan-2-yl out loud. Learn both directions and use whichever the question uses.
Halogens and other simple prefixes
Halogens are named as prefixes, always: fluoro, chloro, bromo, iodo. They never become a suffix and never outrank anything, which makes them the easiest substituent in the language — a chlorine is a chloro wherever it sits, and its only job is to collect a locant.
Two others behave the same way and are worth meeting now, because they turn up constantly and are never suffixes: nitro (–NO₂) and alkoxy (–OR, so –OCH₃ is methoxy and –OCH₂CH₃ is ethoxy). An ether has no suffix of its own in substitutive nomenclature; it is always named as an alkoxy substituent on something else.
Multiplying prefixes, and the locant that must be repeated
Two identical substituents take di-, three take tri-, four take tetra-, five penta-. Each one still gets its own number, even when two of them sit on the same carbon: a pentane with two methyls both on C2 is 2,2-dimethylpentane, and writing 2-dimethylpentane is not a shorthand — it is missing a locant, and the reader cannot tell where the second methyl went.
Alphabetical order, and what does not count
Substituents are cited in the name in alphabetical order, regardless of their locants. So 4-ethyl-2-methylhexane, with ethyl first, even though its number is higher.
The question is what you alphabetize by, and the rule has a sharp edge:
- Ignore the multiplying prefixes di-, tri-, tetra-. "Dimethyl" files under m, not d. So in 4-ethyl-2,2-dimethylhexane, ethyl still comes before dimethyl because e precedes m.
- Ignore the italicized structural prefixes sec- and tert-. tert-Butyl files under b.
- Do not ignore iso, neo or cyclo. These are treated as part of the name: isopropyl files under i, and cyclohexyl under c. This is the inconsistency people trip on — iso is joined to the word, tert- is hyphenated and italic, and that typographical difference is exactly what marks which ones count.
The order in which the rules apply
These rules interact, and applying them in the wrong order produces a wrong name that looks reasonable. The order is:
- First, number for lowest locants. The numbering is decided by the structure, not by the alphabet.
- Then, if and only if both directions give the identical locant set, break the tie by giving the lower number to the substituent that comes first alphabetically.
- Last, write the substituents in alphabetical order in the name, whatever their numbers turned out to be.
The middle rule is narrow and is often misremembered as "the alphabetically first substituent always gets the lowest number", which is false: it only applies to a genuine tie.
A seven-carbon chain carries a chlorine, an ethyl group and a methyl group. Numbered from the left they sit at C2 (methyl), C4 (ethyl) and C5 (chloro); numbered from the right, at C3, C4 and C6.
Number. Left gives {2, 4, 5}, right gives {3, 4, 6}. First point of difference: 2 beats 3, so number from the left. The alphabet plays no part in this step.
Name the pieces. chloro at 5, ethyl at 4, methyl at 2.
Order them alphabetically. c, then e, then m.
Assemble. 5-chloro-4-ethyl-2-methylheptane. Note the numbers are not in order in the finished name, and that is correct — the alphabet sets the order of the words, the structure set the numbers.
Complex substituents
A branch can itself be branched. When it is, the branch gets a name in parentheses, numbered from its own point of attachment — the attaching carbon is C1 of the substituent, always. A three-carbon branch bearing a methyl on its second carbon is a (2-methylpropyl) group, and the parentheses matter: they tell the reader that the 2 counts along the branch and not along the parent chain.
For alphabetizing, a complex substituent files under the first letter of its complete name, so (2-methylpropyl) files under m.
What carries forward
Every substituent in this section is a prefix and none of them can ever be anything else — that is what makes them easy. The next section introduces groups that compete to be the suffix, and the numbering rule changes to serve whichever one wins. The mechanics you have just learned do not change: you still find the longest chain, still number for lowest locants, still alphabetize the prefixes. One more question simply gets asked first.