Nomenclature of Organic Compounds (OCR A-Level Chemistry A): Revision Notes

Nomenclature of Organic Compounds

Introduction to IUPAC nomenclature

With millions of different organic compounds in existence, chemists need a universal system to name and communicate about them clearly. The International Union of Pure and Applied Chemistry (IUPAC) provides this standardised naming system. IUPAC was established in 1919 by chemists from industry and education who recognised the need for consistent nomenclature across the globe. This system ensures that any chemist, anywhere in the world, can identify a compound from its systematic name.

Classification of hydrocarbons

Hydrocarbons are organic compounds containing only carbon and hydrogen atoms. They can be classified into three main categories based on how their carbon atoms are arranged:

Aliphatic hydrocarbons have carbon atoms joined together in unbranched (straight) or branched chains. These chains do not form closed ring structures.

Alicyclic hydrocarbons contain carbon atoms joined together in ring (cyclic) structures. Despite being arranged in a ring, these compounds behave similarly to aliphatic compounds.

Aromatic hydrocarbons contain one or more benzene rings in their structure. A benzene ring consists of six carbon atoms arranged in a hexagonal ring with alternating single and double bonds.

Homologous series

Within aliphatic hydrocarbons, there are three important homologous series you need to know. A homologous series is a family of compounds that share the same functional group and general formula, with each successive member differing by a $\ce{CH2}$ unit.

Alkanes are saturated hydrocarbons containing only single carbon-to-carbon bonds. They have the general formula $\ce{C_nH_{2n+2}}$.

Alkenes are unsaturated hydrocarbons containing at least one carbon-to-carbon double bond ($\ce{C=C}$). They have the general formula $\ce{C_nH_{2n}}$ (for compounds with one double bond).

Alkynes are unsaturated hydrocarbons containing at least one carbon-to-carbon triple bond ($\ce{C≡C}$). They have the general formula $\ce{C_nH_{2n-2}}$ (for compounds with one triple bond).

Understanding stem, prefix, and suffix

IUPAC names are constructed using three key components:

The stem indicates the number of carbon atoms in the longest continuous chain in the molecule. This longest chain is called the parent chain. For example, 'meth-' means one carbon, 'eth-' means two carbons, 'prop-' means three carbons, and so on.

A prefix is added before the stem to indicate the presence and position of side chains (alkyl groups) or certain functional groups. Multiple prefixes may be used when there are several substituents. Prefixes are listed in alphabetical order.

A suffix is added after the stem to indicate the main functional group present in the molecule. The suffix determines the compound's chemical class (alkane, alkene, alcohol, etc.).

The first ten alkanes and their alkyl groups

You must be able to name the first ten alkanes and recognise their corresponding alkyl groups. An alkyl group is formed when one hydrogen atom is removed from an alkane, creating a substituent that can attach to a parent chain.

The table shows the relationship between the number of carbon atoms, the stem name, and both the parent alkane and its alkyl group. Notice that alkyl groups have one fewer hydrogen atom than the corresponding alkane (e.g., methane $\ce{CH4}$ becomes methyl $\ce{CH3}$).

Naming aliphatic alkanes

When naming alkanes, follow this systematic four-step process:

Step 1: Identify the suffix

All alkanes use the suffix -ane. This immediately tells you the compound contains only single carbon-to-carbon bonds.

Step 2: Identify the parent chain

Find the longest continuous chain of carbon atoms in the molecule. This becomes your parent chain. Count the carbon atoms in this chain to determine the stem name. If you find two or more chains of equal length, choose the chain with the most branches (substituents) attached.

Step 3: Identify and name side chains

Look for any side chains (alkyl groups) attached to the parent chain. These are branches coming off the main carbon chain. Name each alkyl group using the appropriate name from the table (methyl, ethyl, propyl, etc.).

Step 4: Number the substituents

Number the carbon atoms in the parent chain starting from the end that gives the lowest possible numbers for the positions where substituents are attached. Write these numbers before the names of the alkyl groups. When naming the compound, list alkyl groups in alphabetical order, preceded by their position numbers. If the same alkyl group appears more than once, use prefixes: di- (two), tri- (three), or tetra- (four). These multiplying prefixes are ignored when alphabetising.

Naming alicyclic alkanes

Cyclic alkanes follow the same basic rules as aliphatic alkanes, with one key addition: the prefix cyclo- is added in front of the stem to indicate the carbon atoms are arranged in a ring structure.

For example, this five-membered ring alkane has five carbon atoms (stem: pent-) arranged in a ring (prefix: cyclo-). The name is cyclopentane.

If substituents are present on the ring, number around the ring to give the lowest possible position numbers, just as with branched alkanes.

Naming alkenes

Alkenes follow similar rules to alkanes but with important differences:

Step 1: Use the suffix -ene instead of -ane to indicate the presence of a carbon-carbon double bond.

Step 2: Identify the longest continuous chain containing the double bond. This chain determines the stem.

Step 3: For alkenes with four or more carbon atoms in the longest chain, you must indicate the position of the double bond. Number the parent chain from the end that gives the double bond the lowest position number. Place this number before the suffix (-ene).

Functional groups and their nomenclature

Functional groups are specific groups of atoms that determine the chemical properties and reactions of organic compounds. Each functional group has characteristic prefixes and suffixes for naming.

The table shows common functional groups you'll encounter at A-Level:

• Alkenes ($\ce{C=C}$): suffix -ene
• Alcohols ($\ce{-OH}$): prefix hydroxy- or suffix -ol
• Haloalkanes ($\ce{-Cl}$, $\ce{-Br}$, $\ce{-I}$): prefix chloro-, bromo-, or iodo-
• Aldehydes ($\ce{-CHO}$): suffix -al
• Ketones ($\ce{-C(=O)C-}$): suffix -one
• Carboxylic acids ($\ce{-COOH}$): suffix -oic acid
• Esters ($\ce{-COO-}$): suffix -oate
• Acyl chlorides ($\ce{-COCl}$): suffix -oyl chloride
• Amines ($\ce{-NH2}$): prefix amino- or suffix -amine
• Nitriles ($\ce{-CN}$): suffix -nitrile

Naming compounds containing functional groups

The principles for naming compounds with functional groups are similar to those for alkanes:

Step 1: Identify the longest unbranched chain of carbon atoms. The stem comes from the corresponding alkane name.

Step 2: Identify all functional groups and alkyl side chains present. Choose the appropriate prefixes or suffixes for them.

Step 3: Number the carbon atoms in the longest unbranched chain. Start from the end that gives functional groups the lowest position numbers. Functional groups take priority over alkyl groups when numbering.

Important naming conventions

Priority rules

When multiple functional groups are present, there is a priority order. The highest priority functional group determines the suffix, while lower priority groups become prefixes. Generally, the priority order is: carboxylic acids > aldehydes > ketones > alcohols > amines > alkenes > halogens.

Numbering conventions

Always number to give the functional group (if present) or the first substituent the lowest possible number. When you have a choice between multiple valid numbering systems, choose the one that gives the lowest set of numbers overall.

Alphabetical ordering

When listing multiple prefixes, arrange them alphabetically. Ignore multiplying prefixes (di-, tri-, tetra-) when determining alphabetical order. For example, in 'diethyl' and 'methyl', you alphabetise based on 'e' and 'm', not 'd' and 'm'.