Introduction

The purpose of the IUPAC nomenclature system is to establish an international standard of naming compounds to facilitate communication. The goal of the system is to give each structure a unique and unambiguous name, and to correlate each name with a unique and unambiguous structure.

I. Fundamental principle

IUPAC nomenclature is based on naming a molecule’s longest chain of carbons connected by single bonds, whether in a continuous chain or in a ring(s). All deviations, either multiple bonds or atoms other than carbon and hydrogen, are indicated by prefixes or suffixes according to a specific set of priorities.

II. Alkanes and Cycloalkanes

Alkanes are the group of saturated hydrocarbons, namely molecules containing carbon and hydrogen connected by single bonds only. These molecules can be in uninterrupted chains (called linear or acyclic), or in rings (called cyclic or alicyclic). The names of alkanes and cycloalkanes are the root names of organic compounds. Starting with the five-carbon alkane, the number of carbons in a chain is indicated by the Greek or Latin prefix. Rings are designated by the prefix “cyclo”. In geometrical symbols for rings, each apex represents a carbon with the number of hydrogens required to fill its valence.

III. Nomenclature of molecules with substituents and functional groups

A. Priorities of substituents and functional groups

Listed here from highest to lowest priority, except for substituents within Group C) have equivalent priority.

Group A. Functional groups indicated by prefix or suffix.

Group B. Functional groups indicated by suffix only.

Group C. Substituents indicated by prefix only.

Common alkyl groups. Replace “ane” ending of alkane names with “yl”. Complex substituents alternate names are given in brackets.

B. Naming substituted alkanes and cycloalkanes. Group C substituents only.

1. Organic compounds with substituents from group C are named following this steps algorithm, as shown on the examples below:

•Step 1. Find the longest continuous carbon chain. Establish the root name for this main chain. In cyclic compounds, the ring is usually considered the main chain, except that it is attached to a longer carbons chain; specify a ring with the prefix “cyclo” before the root name. When there are two longest chains of equal length, use the chain with a bigger substituents number.

•Step 2. Number the chain in the direction so that the first substituent position number is the smaller number. In case the first substituents from both end have the same number, then number so that the second substituent has the smaller number, etc.

•Step 3. Establish the name and position number of each substituent. A substituent on a nitrogen is assigned with “N” instead of a number; see Section III.D.1. below.

•Step 4. Show the number of identical groups by the prefixes di, tri, tetra, etc.

•Step 5. Arrange the position numbers and names of the substituent groups alphabetically, before the root name. In alphabetizing, neglect prefixes like sec-, tert-, di, tri, etc., but include iso and cyclo. Always incorporate a position number for each substituent, without paying attention to superfluity.

Examples

C. Naming molecules with functional groups from group B—suffix only

1. Alkenes—Follow the same steps as for alkanes, except:

a. Number the chain of carbons that includes the C=C so that the C=C has the lower position number, so far as it has a higher priority than any substituents;

b. Change “ane” to “ene” and assign a position number to the first carbon of the C=C;

c. Denote geometrical isomers with a cis-, trans- or E-, Z- prefix.

2. Alkynes — Follow the same steps as for alkanes, except:

a. Number the chain of carbons that includes the C≡C so that the functional group has the lower position number;

b. Change “ane” to “yne” and appoint a position number to the first carbon of the C≡C.

Note: The Group B functional groups (alkene and alkyne) are considered to have equal priority: in a molecule with both a double and a triple bond, whichever is closer to the end of the chain defines the direction of numbering. In the case where each would have the same position number, the double bond takes the lower number. In the name, “ene” comes before “yne” because of alphabetization. See examples.

Notes:

1. An “e” is dropped if the letter following it is a vowel: “pent-3-en-1-yne” , not “3-pent-3-ene-1-yne”.

2. An “a” is added if inclusion of di, tri, etc., would put two consonants consecutively: “buta-1,3-diene”, not “but-1,3-diene”.

D. Naming molecules containing functional groups from group A — Prefix or Suffix

In naming molecules with one or more of functional groups in Group A, the group of highest priority is indicated by suffix; the others are indicated by prefix, with priority equivalent to any other substituents. The table in Section III. A. establishes the priorities; they are discussed below in order of increasing priority.

Now that functional groups and substituents from Groups A, B, and C have been described, a modified steps kit for naming organic compounds can be applied to all simple structures:

•Step 1. Find the highest priority functional group. Define and name the longest continuous carbon chain that includes this group.

•Step 2. Number the chain so that the highest priority functional group is assigned the lower number.

•Step 3. If the carbon chain includes multiple bonds (Group B), replace “ane” with “ene” for an alkene or “yne” for an alkyne. Denote the position of the multiple bond with the number of the first carbon of the multiple bond.

•Step 4. If the molecule includes Group A functional groups, replace the last “e” with the suffix of the highest priority functional group, and include its position number.

•Step 5. Indicate all Group C substituents, and Group A functional groups of lower priority, with a prefix. Place the prefixes, with appropriate position numbers, in alphabetical order before the root name.

1. Amines: prefix: amino-; suffix: -amine—substituents on nitrogen denoted by “N”

2. Alcohols: prefix: hydroxy-; suffix: -ol

3. Ketones: prefix: oxo-; suffix: -one

4. Aldehydes: prefix: oxo-, or formyl- (O=CH-); suffix: -al (abbreviation: —CHO).

An aldehyde can only be on carbon 1, so the “1” is generally omitted from the name.

Special case: When the chain cannot include the carbon of the -CHO, the suffix “carbaldehyde” is used:

5. Carboxylic Acids: prefix: carboxy-; suffix: -oic acid (abbreviation: —COOH). A carboxylic acid can only be on carbon 1, so the “1” is generally omitted from the name.

Note: Chemists traditionally use, and IUPAC accepts, the names “formic acid” and “acetic acid” in place of “methanoic acid” and “ethanoic acid”.

Special case: When the chain numbering cannot include the carbon of the -COOH, the suffix “carboxylic acid” is used.

E. Naming carboxylic acid derivatives

The six common groups obtained from carboxylic acids are salts, anhydrides, esters, acyl halides, amides, and nitriles. Salts and esters are most important.

1. Salts of Carboxylic Acids

Salts are named with cation first, followed by the anion name of the carboxylic acid, where “ic acid” is replaced by “ate” :

2. Esters

Esters are titled as “organic salts” that is to say, the alkyl name comes first, followed by the name of the carboxylate anion. (common abbreviation: —COOR)

IV. Nomenclature of Aromatic Compounds

“Aromatic” compounds are those derived from benzene and similar ring systems. As with aliphatic nomenclature reported in the previous chapters, the process is: defining the root name of the parent ring; determining priority, name, and position number of substituents. Then, assembling the name in alphabetical order. Functional group priorities are the same in aliphatic and aromatic nomenclature.

A. Common Parent Ring Systems

B. Monosubstituted benzenes

1. Most substituents keep their designation after “benzene” word:

2. Some common substituents change the root name of rings. IUPAC accepts these as root names, listed here in decreasing priority:

C. Disubstituted benzenes

1. There are only three positions:

2. Naming disubstituted benzenes. Priorities determine root name and substituents:

D. Polysubstituted benzenes

E. Aromatic ketones

A special group of aromatic compounds are ketones where the carbonyl group is attached to at least one benzene ring. These compounds are named as “phenones”, the prefix depending on a size and nature of the group on the other side of the carbonyl group. There are common examples: