Functional Groups and IUPAC Nomenclature

Organic chemistry is the chemistry of compounds containing carbon, usually in combination with hydrogen and a small number of other elements such as oxygen, nitrogen, and the halogens. Carbon's unique ability to form four strong covalent bonds — to itself and to other elements — gives rise to millions of compounds. To bring order to this enormous family, chemists classify molecules into homologous series based on shared functional groups, and name them using the International Union of Pure and Applied Chemistry (IUPAC) system.

This lesson covers the OCR A-Level Chemistry A (H432) specification point 4.1.1: basic concepts of organic chemistry — nomenclature, formulae, functional groups, and the classification of organic compounds.

1. What is a Functional Group?

A functional group is an atom or group of atoms in a molecule that is responsible for the characteristic chemical reactions of that molecule. Molecules sharing the same functional group show similar chemical behaviour, allowing us to predict reactions across a whole series.

Key Definition — Functional group: A group of atoms responsible for the characteristic reactions of a compound.

Key Definition — Homologous series: A series of organic compounds having the same functional group but with each successive member differing by CH₂.

The Key Functional Groups at A-Level (OCR Spec 4.1.1)

Homologous series	Functional group	Prefix / Suffix	Example	Formula
Alkane	C–C single bonds only	-ane	Ethane	C₂H₆
Alkene	C=C double bond	-ene	Ethene	C₂H₄
Haloalkane	C–X (X = F, Cl, Br, I)	fluoro-/chloro-/bromo-/iodo-	Chloroethane	C₂H₅Cl
Alcohol	–OH	-ol / hydroxy-	Ethanol	C₂H₅OH
Aldehyde	–CHO (terminal)	-al	Ethanal	CH₃CHO
Ketone	C=O (non-terminal)	-one	Propanone	CH₃COCH₃
Carboxylic acid	–COOH	-oic acid	Ethanoic acid	CH₃COOH
Ester	–COO–	-yl -oate	Methyl ethanoate	CH₃COOCH₃
Nitrile	–C≡N	-nitrile	Ethanenitrile	CH₃CN
Amine	–NH₂	-amine / amino-	Methylamine	CH₃NH₂
Amide	–CONH₂	-amide	Ethanamide	CH₃CONH₂
Arene	benzene ring	phenyl-/benzene	Methylbenzene	C₆H₅CH₃

graph TD
  A[Organic compound] --> B{Contains C=C?}
  B -->|Yes| C[Alkene]
  B -->|No| D{Contains C=O?}
  D -->|No, only C-H and C-C| E[Alkane]
  D -->|Yes| F{Bonded to what?}
  F -->|OH| G[Carboxylic acid COOH]
  F -->|H at end| H[Aldehyde CHO]
  F -->|C each side| I[Ketone C=O]
  F -->|OR| J[Ester COOR]
  A --> K{Contains -OH alone?}
  K -->|Yes| L[Alcohol]
  A --> M{Contains halogen?}
  M -->|Yes| N[Haloalkane]
  A --> O{Contains N?}
  O -->|NH2 only| P[Amine]
  O -->|CONH2| Q[Amide]
  O -->|C triple bond N| R[Nitrile]

2. IUPAC Nomenclature — The Rules

IUPAC names have three parts joined together:

[Prefix/Substituent][Stem][Suffix]

Stem tells you the longest continuous carbon chain.
Prefix tells you any branches or substituents.
Suffix tells you the principal functional group.

2.1 Stems for Carbon Chains (1–10 carbons)

Carbons	Stem	Carbons	Stem
1	meth-	6	hex-
2	eth-	7	hept-
3	prop-	8	oct-
4	but-	9	non-
5	pent-	10	dec-

2.2 The Six-Step Naming Procedure

Identify the longest continuous carbon chain containing the principal functional group. This gives the stem.
Identify the principal functional group — this determines the suffix and takes priority in numbering.
Number the chain from the end nearest the principal functional group so that it receives the lowest possible locant.
Identify substituents (branches, halogens, other groups) and name them alphabetically.
Assign locants to each substituent, using the lowest set of numbers overall.
Combine into a single name: substituent locants and names, then stem, then suffix, with hyphens between numbers and letters and commas between adjacent numbers.

2.3 Worked Example 1 — Naming a Branched Alkane

Name the molecule:

      CH3
       |
CH3 - CH - CH2 - CH2 - CH3

Longest chain = 5 carbons → pentane.
No principal functional group other than the alkane → suffix -ane.
Numbering: from the left end, the CH₃ branch is on C2; from the right end it would be on C4. Choose C2.
Substituent: a methyl group at C2.
Name: 2-methylpentane.

2.4 Worked Example 2 — Multiple Substituents

Name:

        Cl   CH3
         |    |
CH3 - CH - CH - CH2 - CH3

Longest chain = 5 → pentane.
Substituents: Cl and CH₃.
Number from the end nearest the first substituent. Left-to-right: Cl at 2, CH₃ at 3 → locants {2,3}. Right-to-left: CH₃ at 3, Cl at 4 → locants {3,4}. Choose {2,3}.
Alphabetise: chloro before methyl.
Name: 2-chloro-3-methylpentane.

2.5 Worked Example 3 — Alcohol

Name:

CH3 - CH2 - CH(OH) - CH3

Longest chain containing –OH = 4 → butane stem.
Principal group is –OH → suffix -ol.
Number from the end that gives –OH the lowest locant: C2.
Name: butan-2-ol.

2.6 Worked Example 4 — Multiple of Same Group

2,3-dichlorobutane contains two chlorine atoms on adjacent carbons of a four-carbon chain. Use di-, tri-, tetra- prefixes for multiple identical substituents, but do not count them when alphabetising.

3. Priority of Functional Groups

When more than one functional group is present, only one becomes the suffix (the principal group). The others become prefixes.

Priority order (highest → lowest) at A-Level:

carboxylic acid > ester > amide > nitrile > aldehyde > ketone > alcohol > amine > alkene > haloalkane

For example, 3-hydroxybutanoic acid: the carboxylic acid is principal (suffix -oic acid), and the hydroxyl becomes the prefix hydroxy-.

Exam Tips

Always draw a skeletal structure quickly before naming — it reveals the longest chain and substituents at a glance.
For the lowest locant rule, compare the full set of locants, not just the first one.
Alphabetise substituents, ignoring di-/tri-/tetra- prefixes but including iso-, sec-, and tert- (OCR follows standard IUPAC here).
For alcohols, aldehydes, and carboxylic acids the functional group must be at the end of the chain for –al and –oic acid names; for –ol the carbon bearing –OH must be in the main chain.
Use hyphens between numbers and letters, commas between numbers. Example: 2,3-dimethylbutane.

Common Exam Mistakes

Forgetting to choose the longest chain containing the principal functional group — not just the longest chain overall.
Giving locants that do not correspond to the lowest set.
Writing 1-methylbutane instead of 2-methylbutane (the methyl must be on an interior carbon for it to be a branch).
Confusing aldehydes (–CHO, terminal) with ketones (C=O, interior).
Using "propane-2-ol" instead of "propan-2-ol" — the final 'e' of the stem is dropped before a suffix beginning with a vowel.
Not applying alphabetical order when listing substituents.

Reference: OCR A-Level Chemistry A (H432), Module 4 — Core Organic Chemistry, section 4.1.1: nomenclature of organic compounds, including the identification of functional groups and the application of IUPAC rules to alkanes, alkenes, haloalkanes, alcohols, aldehydes, ketones, carboxylic acids, and esters.

Lesson 1: Functional Groups and IUPAC Nomenclature