A molecule with one functional group is easy to name. A molecule with three has to decide which of them the name is about, and that decision is the single most important rule in nomenclature, because it sets the suffix, it can change which chain is the parent, and it overrides the numbering rule you just learned.
One group gets the suffix. Everything else becomes a prefix.
Exactly one functional group in a name is the principal characteristic group. It is named as the suffix at the end of the name, and it is chosen by a fixed priority order — not by which one you think is more interesting, and not by which one is drawn first. Every other functional group in the molecule is demoted and cited as a prefix, alongside the alkyls and halogens from the previous section.
This is why the same group has two names depending on company it keeps. An –OH is the suffix -ol when it is the highest-priority group present, and the prefix hydroxy- when something outranks it. Nothing about the oxygen changed; its rank did.
The priority order
Highest first. The top four are the carboxylic acid and the three things you can make directly from it.
Below the line, and never a suffix under any circumstances: ethers, halides, nitro groups.
Learning it as a list of ten is harder than learning it in three blocks. The acid derivatives come first, in the order acid, ester, amide, nitrile. Then the carbonyls that are not derivatives, aldehyde before ketone. Then the ones that are just a heteroatom carrying a hydrogen: alcohol, then amine. Multiple bonds rank below all of them, and the three groups on the second line cannot be suffixes at all.
It is worth being clear about what this order is and is not. Those top four carbons are all at the same oxidation level — the carbon of a carboxylic acid, an ester, an amide and a nitrile each sits at +3 — so the sequence within that block is not an oxidation ranking. It is a convention, and the useful thing about it is that it happens to run in the same direction as their reactivity: an acyl chloride or an acid converts to an ester, an ester to an amide, and never back up. The aldehyde-before-ketone pair is an oxidation difference, though, since an aldehyde carbon is at +1 and a ketone carbon at 0.
The suffix each one takes, and the prefix it is demoted to
| Group | As suffix | As prefix |
|---|---|---|
| Carboxylic acid | -oic acid | carboxy- |
| Ester | alkyl …-oate | alkoxycarbonyl- |
| Amide | -amide | carbamoyl- |
| Nitrile | -nitrile | cyano- |
| Aldehyde | -al | oxo- (or formyl-) |
| Ketone | -one | oxo- |
| Alcohol | -ol | hydroxy- |
| Amine | -amine | amino- |
| Ether | — | alkoxy- |
| Halide | — | fluoro-, chloro-, bromo-, iodo- |
When the suffix begins with a consonant the parent's final e is kept, and when it begins with a vowel the e is dropped: propane becomes propanol, not propaneol, but it becomes propanenitrile with the e intact. This is spelling rather than chemistry, and it is worth getting right because it is visible.
The principal group takes the lowest locant, and it outranks everything
In the previous section the numbering was chosen to give the substituents the lowest locants, because an alkane has nothing else to number. Once a principal characteristic group is present, that rule is demoted. The order is now:
- First, the principal characteristic group gets the lowest possible number. This decides the direction on its own, whenever it can.
- Then double and triple bonds.
- Then the substituents cited as prefixes, by first point of difference.
- Then, as a last tie-break, alphabetical order.
So a six-carbon chain with an OH on C2 counting from the right and a methyl on C2 counting from the left is 5-methylhexan-2-ol, not 2-methylhexan-5-ol. The alcohol wins the low number and the methyl takes whatever is left. Giving the lowest locant to the substituent out of habit is one of the most common errors on this material.
There is a second consequence that is easy to miss: the parent chain must contain the principal characteristic group. If the longest chain in the molecule does not pass through the carbon bearing the –OH, then the longest chain is not the parent — you take the longest chain that does contain it, even if that chain is shorter. Priority beats length.
A five-carbon chain carries a carboxylic acid at one end, a ketone at C3 and a bromine at C4.
Rank the groups. Carboxylic acid outranks ketone, which outranks nothing here; bromine is a prefix always. So the acid is the principal characteristic group and takes the suffix -oic acid. The ketone is demoted to oxo-.
Number. The acid carbon is C1 by necessity, which fixes the direction with nothing left to decide.
Assemble, alphabetically. bromo before oxo: 4-bromo-3-oxopentanoic acid.
Notice that a molecule most chemists would describe out loud as "a keto acid" contains no mention of a ketone in its systematic name. The word oxo is where it went.
The common names that never went away
IUPAC retains a set of older names because they are too entrenched to dislodge, and you will meet these far more often than their systematic equivalents — including in the rest of this course, which uses them the way chemists do.
| Common name | Systematic name |
|---|---|
| Formic acid | Methanoic acid |
| Acetic acid | Ethanoic acid |
| Formaldehyde | Methanal |
| Acetaldehyde | Ethanal |
| Acetone | Propan-2-one |
| Benzoic acid | (retained — benzenecarboxylic acid is not used) |
| Phenol | (retained — hydroxybenzene is not used) |
| Aniline | (retained — benzenamine is rare) |
| Toluene | Methylbenzene |
| Styrene | Ethenylbenzene (vinylbenzene) |
Acetone is the one worth pausing on. It is a ketone and it is the principal characteristic group of its own molecule, yet its name contains no suffix at all — a reminder that a retained name is a word, not a derivation, and cannot be taken apart the way a systematic name can.
What carries forward
This priority order is not only a naming rule. It is very close to a reactivity order for the acid derivatives at the top of it, and the same ranking reappears in the Carboxylic Acids chapter as the ladder that says an acyl chloride converts to an ester and never the reverse. When you meet it there, it will already be familiar — you will have been using it to decide suffixes for eight chapters.