Functional group priority

Butan-2-ol and 4-hydroxybutan-2-one both carry an –OH, yet the OH is named differently in each. This section explains why.

One group gets the suffix. Everything else becomes a prefix.

4-Hydroxybutan-2-one has two functional groups, a ketone and an alcohol. Only one of them can be the suffix, the ending of the name. The ketone wins, so the name ends in -one. The OH is the same group as in butan-2-ol; only its rank changed. The alcohol is demoted to a prefix, a piece written in front of the parent's name.

The group that wins the suffix is the principal characteristic group. A molecule has at most one kind of it. A fixed priority order picks it. It is not the group you find more interesting, and not the one drawn first.

Every other functional group is cited as a prefix. It sits alongside the alkyl groups and halogens you already name that way.

This choice matters more than any other rule in naming. It sets the suffix. It can change which chain is the parent. It also overrides the lowest-locant rule for substituents. The sections below take these one at a time.

The priority order

Every group on the list below can end the name. Find the highest one present in your molecule, and it takes the suffix.

Learn the list in three blocks, not as ten separate items. First come the acid and its derivatives, the groups made by swapping the acid's –OH for something else. Their order is acid, anhydride, ester, acyl chloride, amide, nitrile. Next come the two plain carbonyl groups, aldehyde before ketone. Last come the groups joined to the chain by one single bond to O or N: the alcohol (–OH), then the amine (–NH₂, –NHR or –NR₂).

Two of the derivatives are easy to forget. An anhydride is two acyl groups (R–C=O units) sharing one oxygen. Its suffix is -oic anhydride, as in ethanoic anhydride, better known as acetic anhydride. An acyl chloride is the acid with its –OH swapped for –Cl; Foundations called it the acid chloride. Its suffix is -oyl chloride, as in ethanoyl chloride, better known as acetyl chloride.

An anhydride has no simple prefix of its own, and an acyl chloride is rarely a prefix, so many textbook lists leave both out. Know where they go anyway.

Carbon–carbon double and triple bonds are not on the list, so they never compete for the suffix. They change the parent's name instead, to -ene or -yne, which the next section covers. Many textbooks print alkene and alkyne at the bottom of this list. That gives the same answer, because any group on the list beats them.

What the order follows, and what it does not

Count the bonds from the group's own carbon to O, N or Cl. In an acid, an anhydride, an ester, an acyl chloride or an amide the count is three: two to the C=O oxygen and one to the other atom (O, Cl or N). A nitrile carbon also has three, all to nitrogen. An aldehyde or ketone carbon has two. An alcohol or amine carbon has one. An ether, an alkyl chloride and a nitro compound also have one such bond at their carbon, but none of them is ever a suffix, so they sit outside the list. So the three blocks run three, two, one. The row labels in the figure above show the count.

That count is the carbon's oxidation level. Oxidation levels are made precise in Oxidation & Reduction. Here you only need the count.

Inside a block the count is the same, so the order there is a convention. Take the aldehyde and the ketone. Each carbon has two bonds to oxygen. The aldehyde carbon also carries a hydrogen where the ketone carbon carries a carbon. That hydrogen is why only the aldehyde can be oxidized further, but the count stays at two.

The order is not a reactivity order either. The acyl chloride is the most reactive acid derivative, yet it ranks below the ester. For reactivity the derivatives run acyl chloride > anhydride > ester > amide. For naming they run anhydride > ester > acyl chloride > amide.

The suffix each one takes, and the prefix it is demoted to

A demoted aldehyde has two prefix names. If its carbon is the last carbon of the parent chain, the =O on that end carbon is named oxo-, as in 4-oxobutanoic acid, OHC–CH₂–CH₂–COOH. If the –CHO hangs off the chain or a ring, so its carbon is not in the parent, the whole group is named formyl-. Formyl means a –CHO group attached through its carbon. In 2-formylbutanoic acid, CH₃CH₂CH(CHO)COOH, the four-carbon chain runs through the ethyl end, so the CHO carbon is left off the chain and becomes formyl- on C2.

An ester's name is two words, and students often swap them. Look at ethyl propanoate in the next figure. It has a chain on each side of the single-bonded oxygen.

The chain that holds the C=O is the acid part. Count it from the C=O carbon. Here it has three carbons, so it is propanoate, the second word. The chain on the other oxygen is the alkyl part. It comes from the alcohol the ester was made from. Here it has two carbons, so it is ethyl, the first word.

The same reading works for ethyl ethanoate, CH₃COOCH₂CH₃, where both parts happen to have two carbons. Find the C=O, count its chain for the second word, and name whatever hangs off the other oxygen as the first word.

An amine's substituents on nitrogen take the locant N, as in N-methylpropan-1-amine. That convention is developed in Amines.

Propane plus -ol gives propanol, not propaneol. Propane plus -nitrile gives propanenitrile, with the e kept. The rule: drop the parent's final e before a suffix that begins with a vowel, and keep it before a consonant. This is spelling rather than chemistry, but it is visible, so get it right.

Two of the same principal group take a multiplying prefix on the suffix: -diol, -dione, -dioic acid. Di begins with a consonant, so the parent keeps its e: butane-1,4-diol, pentane-2,4-dione, hexanedioic acid. Butan-1,4-diol is the misspelling this rule exists to catch. It is common because butan-1-ol is right.

The principal group takes the lowest locant, and it outranks everything

Take a six-carbon chain with an OH near one end and a methyl near the other. Counted from one end, the OH is on C2. Counted from the other end, the methyl is on C2. Only one of them can have the 2.

The OH wins, so the name is 5-methylhexan-2-ol, not 2-methylhexan-5-ol. The alcohol is the principal characteristic group, so it claims the lowest number first. The methyl takes whatever is left. Giving the low number to the substituent out of habit is one of the commonest errors on this material.

In an alkane the substituents got the lowest locants, because nothing else needed a number. With a principal group present, the claims on a low number come in this order:

• First, the principal characteristic group gets the lowest possible number. Whenever it can, this decides the direction on its own.
• Then double and triple bonds (the next section names them).
• Then the substituents cited as prefixes, by first point of difference.
• Then, as a last tie-break, alphabetical order.

The parent chain must contain the principal group

Now take the alcohol in the next figure. Its longest chain has six carbons. The OH, though, sits on a CH₂ branch hanging off that chain.

The six-carbon chain is not the parent. The parent chain must contain the principal characteristic group, which here means the carbon that carries the OH. Take the longest chain that does contain it, even if that chain is shorter. The five-carbon chain through the CH₂OH carbon gives 2-ethylpentan-1-ol. Priority beats length.

Where C1 is fixed: chain ends and rings

Rings work differently. Cyclohexane is a ring of six carbons; the next section names rings in full. A COOH or a CHO on a ring has its own carbon, bonded to a ring carbon. That carbon cannot be part of the ring. So the ring is the parent, and the group's carbon is not counted in it.

The suffix then becomes -carboxylic acid or -carbaldehyde. These endings include the group's own carbon. The names are cyclohexanecarboxylic acid and cyclohexanecarbaldehyde, and the ring carbon bearing the group is C1.

A ketone or an alcohol on a ring needs no such device, because its carbon is a ring carbon. The names are cyclohexanone and cyclohexanol. C1 is the carbon at the group, and no locant is written when the group is the only thing on the ring.

Reading a name back into a structure

Exams often run the procedure backwards: "draw 3-hydroxybutanal". The trap is reading the name left to right. You draw the prefix first and then have to guess where the suffix goes.

Read the name from the end instead. The suffix says which group sits at C1, or at the locant it carries. The root says how long the chain is. Each prefix then lands on its number.

The common names that never went away

Acetic acid is CH₃COOH, the acid in vinegar. Its systematic name is ethanoic acid, but few chemists say it. IUPAC keeps a set of older names like this because they are too entrenched to replace. You will meet them far more often than their systematic equivalents, and the rest of this course uses them the way chemists do.

The last column is the one name that the 2013 IUPAC rules pick as preferred. For seven of these compounds the old name is itself the preferred one. The systematic names in the middle column are accepted in general use.

Acetone is the one worth pausing on. It is a ketone, and the ketone is its principal characteristic group. Yet nothing in the word acetone gives the chain length or a locant.

So a retained name is a word, not a derivation. You cannot take it apart the way you take apart a systematic name.

What carries forward

You will use this priority order every time you name a molecule with more than one group. The reactivity ladder of the acid derivatives in Carboxylic Acids & Derivatives is a different list, as shown above. That ladder says an acyl chloride converts to an ester (so does the acid itself), and an ester to an amide. An ester does not go back to an acyl chloride, nor an amide to an ester, directly. To go back, an ester or an amide is first hydrolyzed to the acid.