Nomenclature of Organic Compounds

The Need for a Systematic Naming System

Organic chemistry deals with millions of compounds. To identify each one clearly, a systematic method of naming has been developed. This is the IUPAC (International Union of Pure and Applied Chemistry) system of nomenclature. In this system, names are correlated with structure so that anyone reading or hearing the name can deduce the structure from it.

Before IUPAC nomenclature, organic compounds were assigned names based on their origin or certain properties. For example, citric acid is named because it is found in citrus fruits, and the acid found in red ants is named formic acid — the Latin word for ant is formica. These traditional names are called trivial or common names.

Some common names are still used today. For instance, Buckminsterfullerene is a common name given to the newly discovered $C_{60}$ cluster (a form of carbon), noting its structural similarity to the geodesic domes popularised by architect R. Buckminster Fuller. Common names are useful and often indispensable, especially when the alternative systematic names are lengthy and complicated.

Common or Trivial Names of Some Organic Compounds

The following table lists some organic compounds with their common names:

The IUPAC System: Basic Principles

The IUPAC system of nomenclature is based on a set of rules that allow any organic compound to be named systematically. The name is built from several components that describe the structure of the molecule.

The fundamental principle is that the name of an organic compound consists of three main parts:

1. Word root — indicates the number of carbon atoms in the parent chain
2. Suffix — indicates the principal functional group (primary suffix) and the nature of carbon-carbon bonds (secondary suffix)
3. Prefix — indicates substituents or side chains

Word Roots

The word root indicates the number of carbon atoms in the longest continuous chain. For chains of 1 to 10 carbon atoms, the roots are:

For chains longer than 10 carbon atoms, Greek numerical prefixes are used: undec- (11), dodec- (12), tridec- (13), and so on.

Primary Suffix: Saturation and Unsaturation

The primary suffix indicates whether the carbon chain is saturated (single bonds only) or unsaturated (containing double or triple bonds).

The primary suffix is added to the word root. For example:

• $CH_4$: meth- + -ane = methane
• $C_2H_4$: eth- + -ene = ethene
• $C_2H_2$: eth- + -yne = ethyne

Secondary Suffix: Functional Groups

The secondary suffix indicates the principal functional group present in the compound. It is added after the primary suffix. Some common functional groups and their suffixes are:

When a secondary suffix is added, the final 'e' of the primary suffix is dropped if the secondary suffix begins with a vowel. For example:

• $CH_3CH_2OH$: eth- + -ane + -ol = ethanol (the 'e' of ethane is dropped before -ol)
• $CH_3CHO$: eth- + -ane + -al = ethanal (the 'e' is dropped before -al)
• $CH_3COCH_3$: prop- + -ane + -one = propanone (the 'e' is dropped before -one)

Prefixes: Substituents

Prefixes are used to indicate substituents (atoms or groups that replace hydrogen atoms in the parent chain). Common substituents and their prefixes are:

Alkyl groups are derived from alkanes by removing one hydrogen atom. Their names are formed by replacing the -ane suffix of the alkane with -yl:

The Complete IUPAC Name

The complete IUPAC name of an organic compound is constructed as:

Prefix + Word root + Primary suffix + Secondary suffix

For example:

• $CH_3CH_2CH_2Cl$: chloro- + prop- + -ane = chloropropane
• $CH_3CH_2CH_2OH$: prop- + -ane + -ol = propan-1-ol (the position of the functional group is indicated by a number)

Naming of Alkanes

For straight-chain alkanes, the name is simply the word root followed by -ane:

• $CH_4$: methane
• $C_2H_6$: ethane
• $C_3H_8$: propane
• $C_4H_{10}$: butane
• $C_5H_{12}$: pentane

For branched-chain alkanes, the following rules are followed:

Rule 1: Identify the longest continuous chain of carbon atoms. This is the parent chain.

Rule 2: Number the carbon atoms in the parent chain from the end that gives the lowest numbers to the substituents.

Rule 3: Name the substituents as prefixes, with their positions indicated by numbers.

Rule 4: If the same substituent appears more than once, use the prefixes di-, tri-, tetra-, etc., and indicate each position with a number.

Rule 5: If different substituents are present, list them in alphabetical order (ignoring the prefixes di-, tri-, etc., for alphabetization).

For example:

• $(CH_3)_2CHCH_3$ (isobutane): The longest continuous chain has 3 carbons (propane), with one methyl group on the middle carbon (position 2). Name: 2-methylpropane.
• $(CH_3)_4C$ (neopentane): The longest continuous chain has 3 carbons (propane), with two methyl groups on the middle carbon. Name: 2,2-dimethylpropane.

Naming of Alkenes and Alkynes

For alkenes and alkynes, the suffix -ene or -yne is used. The position of the double or triple bond is indicated by the number of the first carbon atom involved in the multiple bond.

Rules:

1. Select the longest chain that contains the double or triple bond.
2. Number the chain from the end nearest the multiple bond.
3. Indicate the position of the multiple bond by the lower-numbered carbon atom involved.

For example:

• $CH_2=CHCH_3$: prop- + -ene = propene (the double bond is between C1 and C2, so no number is needed for a three-carbon chain)
• $CH_3CH=CHCH_3$: but- + -ene = but-2-ene (the double bond is between C2 and C3)
• $CH_3C\equiv CCH_3$: but- + -yne = but-2-yne

When both double and triple bonds are present, the suffix -enyne is used. The chain is numbered to give the lowest numbers to the multiple bonds, with double bond getting preference over triple bond if there is a tie.

Naming of Compounds with Functional Groups …