Basic IUPAC Organic Nomenclature Rules

IUPAC nomenclature is the systematic way of naming organic compounds, in which the name is built from a prefix for substituents, a root for the longest carbon chain and a suffix for the main functional group. In brief: CH3CH(CH3)CH2CH3 is named 2-methylbutane: the longest chain has 4 carbons (butane), and a methyl group is attached to carbon 2.

The IUPAC rules mean that every structure has a name that identifies it, and every name leads to just one structure. Common names such as acetone and acetic acid are still widely used, but the systematic name is needed for most compounds. This article presents the basic rules for naming alkanes, alkenes, alkynes and compounds with one functional group, with tables of the prefixes and worked examples.

What are the root names for carbon chains?

Root names for chains of 1 to 10 carbon atoms
Carbons Root Carbons Root
1meth-6hex-
2eth-7hept-
3prop-8oct-
4but-9non-
5pent-10dec-

The roots for the first four carbons come from older traditional names, and the roots for longer chains come from Greek numbers.

How do you name a branched alkane?

  1. Find the longest continuous chain of carbon atoms. This gives the parent name (such as pentane). If there are two chains of equal length, choose the one with the most substituents.
  2. Number the chain from the end that gives the lowest numbers (locants) to the substituents at the first point of difference.
  3. Name each substituent as an alkyl group (methyl, ethyl, …) or as a halogen prefix (chloro-, bromo-).
  4. Use multiplying prefixes (di-, tri-, tetra-) for identical substituents and give a locant for each one: 2,2-dimethyl.
  5. Put the substituents in alphabetical order, ignoring the multiplying prefixes (di-, tri-) when alphabetizing. In 3-ethyl-2,2-dimethylpentane, ethyl (e) comes before methyl (m), so it is named first.
  6. Write the name as one word, with numbers separated by commas and numbers separated from words by hyphens.

What are the common substituent prefixes?

Common substituent groups
Group Prefix Group Prefix
–CH3methyl–Ffluoro
–CH2CH3ethyl–Clchloro
–CH2CH2CH3propyl–Brbromo
–CH(CH3)2propan-2-yl (isopropyl)–Iiodo
–OHhydroxy–NO2nitro

How do functional groups change the name?

When the compound has a functional group that can be a suffix, the chain must contain it, and the number is chosen to give the group the lowest possible number. The suffix replaces the final -e of the alkane name. For example, CH3CH2OH is ethanol (ethane + ol).

When there is more than one group, the highest-priority group becomes the suffix and the others are prefixes. The usual order of priority, from highest to lowest, is: carboxylic acid, ester, amide, nitrile, aldehyde, ketone, alcohol, amine, alkene, alkyne. Halogens and alkyl groups are always prefixes. For example, HOCH2COOH is hydroxyethanoic acid: the acid has priority, and OH is a “hydroxy” prefix.

Worked examples

Example 1: 2-methylbutane

The structure CH3CH(CH3)CH2CH3 has a longest chain of 4 carbons (butane) and a CH3 branch. Numbering from the left or right, the branch is on carbon 2 (not 3), so the name is 2-methylbutane.

Example 2: 2,2,4-trimethylpentane

(CH3)3CCH2CH(CH3)2 has a longest chain of 5 carbons. There are three methyl groups. Numbering from the end with the two methyl groups gives locants 2,2,4, while numbering from the other end gives 2,4,4. The first set is lower at the first point of difference, so the name is 2,2,4-trimethylpentane. It is “isooctane”, the reference compound for the octane rating of 100.

Example 3: 3-methylhexane

CH3CH2CH(CH3)CH2CH2CH3 has a chain of 6 carbons (hexane) with a methyl on carbon 3 (numbering from the left gives 3, from the right gives 4). The name is 3-methylhexane.

Example 4: compounds with functional groups

Naming compounds with functional groups
Structure Chain Group and position Name
CH3CH(OH)CH2CH34 C–OH on C2Butan-2-ol
CH2=CHCH2CH34 CC=C at C1But-1-ene
CH3CH2COOH3 C–COOH at C1Propanoic acid
CH3COCH2CH34 CC=O at C2Butan-2-one
CH3CHClCH33 C–Cl on C22-Chloropropane

For the aldehyde, ketone or carboxylic acid at the end of a chain, the C=O carbon is always C1 in aldehydes and acids, so no locant is needed.

How do you draw a structure from a name?

Example: 3-ethyl-2-methylpentane

  1. The parent is pentane: C–C–C–C–C, numbered 1 to 5.
  2. Put a methyl group on carbon 2 and an ethyl group on carbon 3.
  3. Add hydrogens to complete four bonds on each carbon: CH3–CH(CH3)–CH(CH2CH3)–CH2–CH3.
  4. Count the carbons: 5 + 1 + 2 = 8, so the molecular formula is C8H18.

How did IUPAC naming develop?

As the number of known compounds increased in the 19th century, the same substance could have several names. A conference in Geneva in 1892 produced the first international system of nomenclature for organic compounds. The International Union of Pure and Applied Chemistry (IUPAC) was founded in 1919 and published rules over the following decades. The recommendations were revised many times, most recently in a major publication in 2013. Some traditional names, such as acetic acid and acetone, are still accepted by IUPAC for general use.

Which mistakes are common?