Once the parent chain is found and numbered, everything hanging off it needs a name and a number. The names form the prefix, the part of the name in front of the parent. The number is its locant, as in The parent chain.
The rules on this page are conventions rather than chemistry. None of them is hard on its own. They go wrong when they are applied in the wrong order, so a later section sets out that order.
Alkyl groups: an alkane minus one hydrogen
The parent chain introduced the simple branches: methyl, ethyl, propyl, each the root for its length plus -yl. This section adds the branches that can attach in more than one way.
From three carbons up, a branch can attach through different carbons. Propane shows it. Remove a hydrogen from an end carbon, and the branch attaches through that end: propyl. Remove one from the middle carbon, and the branch attaches through the middle: isopropyl. These are two different groups with two different names.
The carbon of the branch that bonds to the parent chain is its point of attachment. Four carbons can be arranged into four different groups, and five into more. The table lists the ones you will meet, and the figure under it draws each one.
| Common name | Systematic name (IUPAC 2013 style) | Carbons | Point of attachment |
|---|---|---|---|
| propyl | propyl (propan-1-yl) | 3 | an end carbon |
| isopropyl | propan-2-yl | 3 | the middle carbon |
| butyl | butyl (butan-1-yl) | 4 | an end carbon |
| sec-butyl | butan-2-yl | 4 | the second carbon of a four-carbon chain |
| isobutyl | 2-methylpropyl | 4 | the end of a three-carbon chain whose middle carbon carries a methyl |
| tert-butyl | 2-methylpropan-2-yl | 4 | a carbon that carries three methyls |
| neopentyl | 2,2-dimethylpropyl | 5 | a CH₂ bonded to a carbon that carries three methyls |
Read a systematic name as "which chain, and where it attaches". Propan-2-yl is a three-carbon chain attached through its C2. 2-Methylpropyl is a three-carbon chain attached through its C1, with a methyl on its C2.
The italic prefixes are abbreviations. sec stands for secondary: in butane, the carbon that becomes the attachment point is bonded to two other carbons. tert stands for tertiary: in 2-methylpropane, that carbon is bonded to three.
The common names do not all have the same standing. Under IUPAC's current (2013) recommendations, tert-butyl is still the preferred name when it carries no further substituents. Isopropyl is acceptable in general use, as long as it carries no substituent of its own. sec-Butyl, isobutyl and neopentyl are common names in wide use that current IUPAC no longer recommends. Most lecturers, papers and exams still use all of them. Learn both directions, and answer in whichever the question uses.
Halogens and other simple prefixes
Halogens are always named as prefixes: fluoro, chloro, bromo, iodo. A halogen never becomes the suffix of a name.
Take a hexane that carries a methyl and a chlorine. Numbered from the left, the two groups sit on C2 and C4. Numbered from the right, they sit on C3 and C5. Compare the two sets term by term; the first place they differ decides (the first point of difference rule from The parent chain). The first terms are 2 and 3, and 2 is lower, so C1 is the left end. The name is 4-chloro-2-methylhexane. The prefixes are written in alphabetical order, which a later section explains.
So in an alkane a halogen has no special claim on the low numbers. It competes with the alkyl groups as an equal. When both numberings give exactly the same set, the alphabet breaks the tie; the numbering section below shows how.
Simple halides (here, a halide is a carbon group bonded to a halogen) also have common names built from the alkyl group, and the substitution and elimination chapters use them constantly. Read them as "alkyl group, then halide": isopropyl bromide is 2-bromopropane, tert-butyl chloride is 2-chloro-2-methylpropane, and ethyl iodide is iodoethane.
Two more groups behave the same way and are never suffixes. Nitro is –NO₂. Alkoxy is –OR, an oxygen bonded to a carbon group: –OCH₃ is methoxy and –OCH₂CH₃ is ethoxy.
An ether is two carbon groups joined through one oxygen. In substitutive names, the kind this chapter builds, an ether has no suffix of its own. It is always named as an alkoxy substituent on something else.
Multiplying prefixes, and the locant that must be repeated
Take a pentane with two methyls, both on C2. Its name is 2,2-dimethylpentane. The di- says there are two methyls, and each methyl gets its own locant, even though both locants are 2.
Writing 2-dimethylpentane is not a shorthand. It is missing a locant, and the reader cannot tell where the second methyl went.
The general rule: identical substituents are collected with di- (two), tri- (three), tetra- (four) or penta- (five), and every one of them keeps its own locant.
One variant to recognize rather than produce. When the repeated substituent is itself substituted (a complex substituent, named in parentheses; see Complex substituents below), the multipliers become bis-, tris- and tetrakis-: bis(2-methylpropyl), not di(2-methylpropyl). A plain "di" in front of a name that already holds a number could be misread as part of that name.
Alphabetical order, and what does not count
Look at 4-ethyl-2,2-dimethylhexane. The ethyl is on C4 and the two methyls are on C2, yet ethyl is written first.
That is the rule: substituents are cited in the name in alphabetical order, whatever their locants. 4-Ethyl-2-methylhexane, with one methyl fewer, is written in the same order for the same reason.
The harder 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. That is why ethyl comes before dimethyl above: e precedes m.
- Ignore the italic structural prefixes sec- and tert-. tert-Butyl files under b.
- Do not ignore iso, neo or cyclo. They count as part of the name. Isopropyl files under i, neopentyl under n, and cyclohexyl (a six-carbon ring used as a branch; rings are named in Rings & unsaturation) under c.
This is the inconsistency people trip on. Multiplying prefixes are always skipped. Of the structural prefixes, the italic, hyphenated ones (sec-, tert-) are skipped, and the ones joined straight onto the word (iso, neo, cyclo) count.
Numbering: lowest locants first, the alphabet only on a tie
With several substituents on the chain, two rules could seem to decide the numbering: lowest locants, and alphabetical order. The two worked examples show which one wins, and when.
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, they sit 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. The numbers in the finished name are not in ascending order, and that is correct. The structure set the numbers; the alphabet set the order of the words.
A heptane carries an ethyl on C3 and a methyl on C5, counting from the left. Counting from the right, the methyl is on C3 and the ethyl on C5.
Number. Left gives {3, 5}; right gives {3, 5}. The sets are identical, so the structure cannot choose a direction.
Break the tie. Ethyl comes before methyl in the alphabet, so ethyl takes the lower number. Count from the left.
Assemble. 3-ethyl-5-methylheptane, not 5-ethyl-3-methylheptane. Both look finished, and only one follows the rule.
Together the two examples give the order in which the rules apply. Apply them in a different order and you get a wrong name that looks reasonable.
- First, number for lowest locants. The structure decides the numbering, not 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. It is often misremembered as "the alphabetically first substituent always gets the lowest number". That is false. In the first example chloro comes first in the alphabet, and it still gets 5.
Complex substituents
A branch can itself carry a branch, or any other substituent. A branch like that is a complex substituent, and it gets a name of its own, in parentheses.
A nine-carbon row carries, on its fifth carbon, a three-carbon branch whose middle carbon bears a methyl.
Find the parent. The row is nine carbons. The longest route through the branch is three branch carbons, then C5, then four more: eight. So the parent is nonane, and the branch is a substituent.
Name the branch. The carbon bonded to C5 is an end of the branch's three-carbon chain, so it is the branch's C1. The middle carbon is its C2, and the far carbon its C3. The methyl sits on C2, so the branch is 2-methylpropyl.
Number the parent. The branch is on C5 from either end, because nine is odd and the branch is in the middle. There is no direction to choose.
Assemble. 5-(2-methylpropyl)nonane is the IUPAC name. You will also see 5-isobutylnonane, built from the common name isobutyl, and many people still say it.
The parentheses show which chain each number belongs to. In 5-(2-methylpropyl)nonane, the 5 counts along the parent and the 2 counts along the branch. Leave the parentheses out, and a reader cannot tell which chain the 2 belongs to.
The example was easy because the attached carbon sits at an end of the branch's chain. Every convention then agrees: the attached carbon is C1. When the attached carbon sits in the middle of the branch, as in the lower two drawings, you will meet two conventions. They name the same group in different words.
Classic style (older IUPAC rules, and still what most courses and exams use). The attached carbon is always the branch's C1. Take the longest chain that starts there, and name everything else as substituents on it. The branch in the drawings becomes (1-methylpropyl). In this style isopropyl is (1-methylethyl) and tert-butyl is (1,1-dimethylethyl).
IUPAC 2013 style. Take the longest chain in the branch that contains the attached carbon, and number it so the attached carbon gets the lowest locant it can. The same branch becomes butan-2-yl, never butan-3-yl. When the attached carbon is a chain end it gets 1, and the name ends in plain -yl, so 2-methylpropyl is the same in both styles. The systematic names in the table above (propan-2-yl, butan-2-yl, 2-methylpropan-2-yl) follow this style.
Check which style your course grades by. A name in either style tells the reader exactly what to draw.
The 2013 style also explains propan-1-yl and propan-2-yl. They are not two numberings of one group. They are two different groups: propan-1-yl is propyl, attached at an end, and propan-2-yl is isopropyl, attached in the middle.
For alphabetizing, a complex substituent files under the first letter of its complete name, so (2-methylpropyl) and (1-methylpropyl) file under m. A multiplying prefix inside the parentheses counts here: (2,2-dimethylpropyl) and (1,1-dimethylethyl) file under d. The common names file differently, isobutyl under i and neopentyl under n, so the name you choose for a group can change where it sits in a longer name.
What carries forward
Every substituent on this page is a prefix, and none of them can ever be anything else. That is what makes them easy. The next section, Functional group priority, introduces groups that compete to be the suffix, and the numbering then serves whichever group wins. The mechanics from this page do not change. You still find the longest chain, still number for lowest locants, and still alphabetize the prefixes. One more question simply gets asked first.