Nomenclature and Functional Groups

This lesson covers the systematic IUPAC naming of organic compounds, the identification of functional groups, homologous series, and the different ways of representing organic molecules. A thorough understanding of nomenclature is essential for every area of organic chemistry at A-Level.

IUPAC Naming Rules

The International Union of Pure and Applied Chemistry (IUPAC) system provides a universal method for naming organic compounds. The name of an organic molecule consists of three parts:

Part	What it tells you	Examples
Prefix	Side chains / substituents	methyl-, ethyl-, chloro-, bromo-
Root	Length of longest carbon chain	meth- (1C), eth- (2C), prop- (3C), but- (4C), pent- (5C), hex- (6C), hept- (7C), oct- (8C)
Suffix	Principal functional group	-ane, -ene, -ol, -al, -one, -oic acid, -amine, -amide, -nitrile

Step-by-step naming procedure

Identify the longest continuous carbon chain that contains the principal functional group. This gives the root name.
Number the carbon atoms starting from the end nearest to the principal functional group, giving it the lowest possible locant (position number).
Identify all substituents (branches and other groups attached to the main chain). Name each substituent and give it the locant of the carbon to which it is attached.
List substituents alphabetically as prefixes. Use di-, tri-, tetra- for multiple identical substituents (these multiplying prefixes do not count for alphabetical ordering).
Assemble the name: locants-prefixes + root + suffix.

Worked Example 1

Name the compound: CH₃CH(CH₃)CH₂CH₂OH

Longest chain containing the -OH group: 4 carbons (butane backbone).
The -OH is on carbon 1 (number from the end nearest to it).
A methyl group is on carbon 3.
Name: 3-methylbutan-1-ol

Worked Example 2

Name the compound: CH₃CHClCH₂CH₃

Longest chain: 4 carbons (butane backbone).
A chloro substituent is on carbon 2.
Name: 2-chlorobutane

Worked Example 3

Name: CH₃C(CH₃)₂CH₂COOH

Longest chain containing -COOH: 4 carbons (butanoic acid backbone, with -COOH as C-1).
Two methyl groups on carbon 3.
Name: 3,3-dimethylbutanoic acid

Exam Tip: Always number from the end that gives the principal functional group the lowest locant. If there is a tie, give the lowest locant to the first point of difference (substituent positions).

Homologous Series

A homologous series is a family of organic compounds that:

share the same general formula
have the same functional group
show a gradual trend in physical properties (e.g. boiling point increases with chain length)
differ from one member to the next by -CH₂- (14 g mol⁻¹)
undergo similar chemical reactions

Homologous Series	General Formula	Functional Group	Example
Alkanes	CₙH₂ₙ₊₂	None (C–C and C–H only)	CH₄ (methane)
Alkenes	CₙH₂ₙ	C=C double bond	C₂H₄ (ethene)
Alcohols	CₙH₂ₙ₊₁OH	–OH (hydroxyl)	C₂H₅OH (ethanol)
Halogenoalkanes	CₙH₂ₙ₊₁X	–X (halogen)	CH₃Cl (chloromethane)
Aldehydes	CₙH₂ₙO	–CHO (carbonyl at chain end)	CH₃CHO (ethanal)
Ketones	CₙH₂ₙO	C=O (carbonyl within chain)	CH₃COCH₃ (propanone)
Carboxylic acids	CₙH₂ₙO₂	–COOH (carboxyl)	CH₃COOH (ethanoic acid)
Esters	CₙH₂ₙO₂	–COO–	CH₃COOCH₃ (methyl ethanoate)
Amines	CₙH₂ₙ₊₃N	–NH₂ (primary)	CH₃NH₂ (methylamine)
Amides	CₙH₂ₙ₊₁NO	–CONH₂	CH₃CONH₂ (ethanamide)
Nitriles	CₙH₂ₙ₋₁N	–C≡N	CH₃CN (ethanenitrile)

Functional Group Identification

A functional group is an atom or group of atoms that determines the characteristic chemical reactions of a molecule. Being able to identify functional groups from structural, displayed, or skeletal formulae is a core exam skill.

Key functional groups to recognise

Hydroxyl (–OH): Found in alcohols and carboxylic acids. In alcohols it is bonded to a saturated carbon; in carboxylic acids it is part of –COOH.
Carbonyl (C=O): In aldehydes it is at the end of the chain (–CHO); in ketones it is within the chain (R–CO–R').
Carboxyl (–COOH): Combination of C=O and –OH on the same carbon. Acidic — donates H⁺.
Ester linkage (–COO–): Formed from a carboxylic acid and an alcohol. The name takes the form alkyl alkanoate.
Amino (–NH₂): Basic nitrogen group found in amines and amino acids.
Amide (–CONH₂): Formed from a carboxylic acid and an amine. Contains a C=O bonded to nitrogen.
Nitrile (–C≡N): Triple bond between carbon and nitrogen. Carbon of C≡N is counted in the longest chain.
Halogen (–X): F, Cl, Br, or I attached to a carbon. Named as fluoro-, chloro-, bromo-, iodo- prefixes.
C=C double bond: Found in alkenes. Site of addition reactions.

Common Misconception: Students sometimes confuse aldehydes and ketones. Remember: an aldehyde always has the C=O at the end of the chain (there is always at least one H on the carbonyl carbon), while a ketone has the C=O within the chain (bonded to two carbon groups).

Ways of Representing Organic Molecules

Molecular formula

Shows the actual number of each type of atom in one molecule but gives no information about structure. Example: C₄H₁₀O

Empirical formula

The simplest whole-number ratio of atoms. Example: C₂H₅O (for C₄H₁₀O₂)

Structural formula

Shows the minimal detail needed to make the structure unambiguous, written as condensed text. Example: CH₃CH(OH)CH₃ for propan-2-ol

Displayed formula

Shows every bond and every atom drawn out. All bonds are shown as lines between atom symbols. This is the most detailed 2D representation.

Skeletal formula

Shows only the carbon skeleton as a zig-zag line. Carbon atoms are at each vertex and at the end of each line. Hydrogen atoms bonded to carbon are not shown — they are implied. Heteroatoms (O, N, halogen, etc.) and their attached H atoms are shown explicitly.

Worked Example 4

Butan-2-ol:

Molecular formula: C₄H₁₀O
Structural formula: CH₃CH(OH)CH₂CH₃
In a skeletal formula: a four-carbon zig-zag with an OH group shown on the second carbon

Exam Tip: In skeletal formulae, count the number of vertices and line-ends to determine the number of carbon atoms. Each vertex or end-point represents a carbon, and hydrogen atoms are added to satisfy carbon's valency of 4.

Naming Compounds with Multiple Functional Groups

When a molecule contains more than one functional group, the principal functional group determines the suffix. The order of priority (highest to lowest) for common A-Level groups is:

–COOH (carboxylic acid) → -oic acid
–COO– (ester) → -oate
–CONH₂ (amide) → -amide
–C≡N (nitrile) → -nitrile
–CHO (aldehyde) → -al
–C=O (ketone) → -one
–OH (alcohol) → -ol
–NH₂ (amine) → -amine
C=C (alkene) → -ene
C–C (alkane) → -ane

Lower-priority groups are named as prefixes (e.g. hydroxy- for –OH, oxo- for C=O, amino- for –NH₂).

Worked Example 5

HOCH₂CH₂COOH — this contains both –OH and –COOH. Since –COOH has higher priority, the suffix is -oic acid and the –OH is named as a prefix:

Longest chain: 3 carbons (propanoic acid backbone).
–OH on carbon 3.
Name: 3-hydroxypropanoic acid

Summary

IUPAC names consist of prefix (substituents) + root (chain length) + suffix (principal functional group).
A homologous series shares a general formula, functional group, and similar reactions.
Molecular, structural, displayed, and skeletal formulae each provide different levels of detail.
In skeletal formulae, C and H atoms on the carbon skeleton are implied.
When multiple functional groups are present, the highest-priority group determines the suffix.
Mastering nomenclature is foundational — every mechanism, synthesis, and analysis question relies on it.