Organic nomenclature

Naming organic compounds is very useful in order to communicate without ambiguity between chemists. However, it can be difficult. This article presents the basics principles of organic nomenclature, then a method for naming organic compounds, some examples of application of this method, as well as some information for naming cyclic compounds.

Basic principles

The name of an organic molecule is composed of the prefix name of the secondary substituents, followed by the root of the molecule, the double and triple bonds (if any), and the suffix name of the primary substituent. Each substituent is preceded by its locator and its multiplier if it is present more than once.

example 1

If the carbon skeleton is ramified, then each ramification is treated as a functional group, and is included to the name of the molecule between brackets.

example 2

A method for naming organic compounds

Step 1

Identify the primary substituent on your main chain, using the following table (the functional groups are classified from the highest priority to the lowest priority). Name this primary substituent according to the "suffix" column.

All the other substituents are secondary substituents. Name them according to the "prefix" column.

Functional groupPrefixSuffix
Carboxylic acidcarboxy--oic acid
EsterR-oxycarbonylR-oate
Acyl halidehalocarbonyl-oyl halide
Amidecarbamoyl--amide
Nitrilecyano--nitrile
Aldehydeformyl--al
Ketoneoxo--one
Alcoholhydroxy--ol
Thiolmercapto--thiol
Amineamino--amine
Imineimido--imine
Alkyl halidehalo--ane
Etheralkoxy--ane
Nitronitro--ane

Step 2

Find the hydrocarbon chain that, in order of precedence :

  • bears the biggest number of primary substituents;
  • bears the biggest number of triple bonds;
  • bears the biggest number of double bonds;
  • is the longest.

This chain constitutes the main chain of the molecule. Find its prefix in the following table :

Number of carbon atomsPrefix
1meth-
2eth-
3prop-
4but-
5pent-
6hex-
7hept-
8oct-
9non-
10dec-
11undec-
12dodec-

Step 3

Number your substituents. These numbers are called the "locators" of your substituents. Start numbering from the end of the chain that will give the lowest locator possible for the primary functional group.

Number your double and triple bonds. Since a double or triple bond is located between two carbon atoms, it will have the smallest number of the two.

Step 4

If some substituents are present more than once on the chain, give them a multiplier according to the following table :

NumberMultiplier
1nothing
2di-
3tri-
4tetra-
5penta-
6hexa-

Step 5

Name the branches of your molecule as if they were separate molecules, and put their names into brackets. Their name must finish by "yl".

Step 6

Glue together, in order : the secondary functional groups sorted by alphabetical order, the root, the double bonds, triple bonds and the primary functional group. Use the following rules :

  • Put commas between numbers;
  • Put hyphens between numbers and letters.

Application of the method to simple molecules

Example 1

  • Step 1 : The primary substituent is an aldehyde. Its suffix name is "-al". The secondary substituants are methyl groups. Their prefix name is "methyl".
  • Step 2 : Here, the main chain is the longest chain which bears the aldehyde group and the two double bonds. It has eight carbon atoms, so its prefix will be "octa".
  • Step 3 : We start numbering the main chain from the carbon atom of the aldehyde (carbon number 1). Thus, the double bonds will have the numbers 2 and 6, and the two methyl groups will have the numbers 3 and 7.
  • Step 4 : The two double bonds will have the multiplier "di-", as well as the two methyl groups.
  • Step 5 : There is no branch in this molecule, so we skip this step.
  • Step 6 : The full name of the molecule is composed by the names of the secondary fonctional groups ("3,7-dimethyl"), followed by the root ("octa"), followed by the double bonds ("diene") followed by the suffix of the primary functional group ("al"). We do not write the "e" of the "ene" suffix, in order not to have two consecutive vowels.

The name of the molecule is thus : "3,7-dimethylocta-2,6-dienal".

Example 2

  • Step 1 : The primary substituent is a carboxylic acid. Its suffix name is "-oic acid". The secondary substituants are three amines groups, which prefix are "amino-" and one alcohol, which prefix is "hydroxy-".
  • Step 2 : Here, the main chain is the chain which bears the carboxylic acid. It has six carbon atoms, so its prefix will be "hexa".
  • Step 3 : We start numbering the main chain from the carbon atom of the carboxylic acid (carbon number 1). Thus, we have one amine on position 2, and another amine, bearing a secondary chain, on position 6 (we will give a name to this secondary chain at step 5).
  • Step 4 : No functional group is present more than once on the primary chain, so we don't need a multiplier.
  • Step 5 : We repeat the steps 2, 3 and 4 for the chain beared by the amine on position 6 of the main chain. We number this chain starting from the atom bound to the primary chain. Thus, we have an amine in position 4 ("4-amino") and an alcohol in position 2 ("2-hydroxy"). This secondary chain has four carbon atoms, so it will have the suffix "butyl". The chain is named "4-amino-2-hydroxybutyl".
  • Step 6 : The full name of the molecule is composed by the names of the secondary fonctional groups ("2-amino" and "6-((4-amino-2-hydroxybutyl)amino)"), followed by the root ("hexan"), followed the suffix of the primary functional group ("-oic acid").

The name of the molecule is thus "2-amino-6-((4-amino-2-hydroxybutyl)amino)hexanoic acid".

Example 3

  • Step 1 : The primary substituent is a ketone functional group. Its suffix name is "-one".
  • Step 2 : Here, the main chain is the chain colored in black (the secondary chain being colored in red). It has eight carbon atoms, so its prefix will be "octa".
  • Step 3 : We start numbering the main chain from the carbon atom at the left of the ketone (carbon number 1). Thus, we have one chlorine atom in position 3("3-chloro"), and two double bonds in positions 4 and 6 ("4,6-diene"), and a secondary chain in position 6 (we will give a name to this secondary chain at step 5).
  • Step 4 : No functional group is present more than once on the primary chain, so we don't need a multiplier.
  • Step 5 : We repeat the steps 2, 3 and 4 for the chain which is on position 6 of the main chain. We number this chain starting from the atom bound to the primary chain. This chain has two carbon atoms ("ethyl"), and we have two alcohols in positions 1 and 2, so the name of the secondary will be "1,2-dihydroxyethyl".
  • Step 6 : The full name of the molecule is composed by the names of the secondary fonctional groups ("3-chloro", "1,2-dihydroxyethyl"), followed by the root ("octa"), followed by the name of the double bonds ("4,6-dien"), followed the suffix of the primary functional group ("2-one").

The name of the molecule is thus "3-chloro-6-(1,2-dihydroxyethyl)octa-4,6-dien-2-one".

Trivial names of substituents

Some substituents have trivial names, that are used more frequently than their systematic name :

Trivial nameAbbreviationFormulaSystematic name
isopropyli-Pr1-methylethyl
isobutyli-Bu2-methylpropyl
sec-butyls-Bu1-methylpropyl
tert-butylt-Bu1,1-dimethylethyl
phenylPhcyclohexa-1,3,5-triene
benzylBnphenylmethyl
benzoylBzbenzenecarbonyl
acetylAcmethyloxidocarbon
mesylMsmethanesulfonyl
tosylTstoluenesulfonyl
triflylTftrifluoromethanesulfonyl
tritylTrtriphenylmethyl

Nomenclature of cyclic compounds : two special cases

Disubstitued benzenes

Usually, a benzene bearing multiple substituents is named by prefixing each substituent by its locator.

Example 1 benzenes

However, if a benzene bears exactly two substituents, instead of using the locators of the substituents, its name can be prefixed by "ortho-" if the substituents are next to each other, "meta-" if the substituents are beared by carbon atoms separated by one other carbon atom, or "para-" if the substituents are on the opposite ends of the cycle.

Example 2 benzenes

Heterocycles

Most heterocycles (cycles containing one more non-carbon atoms in the cycle skeleton) have a non-systematic name. The document below contains the names of some common heterocycles.

Common heterocycles

comments powered by Disqus