Last updated: 19 August 2026
Organic chemistry becomes much easier once you stop seeing each molecule as a completely new collection of atoms. Most structures can be read in two parts: a carbon framework and one or more features attached to that framework that strongly influence how the molecule behaves.
Those features are the functional groups. Once you can recognise C=C, –OH, C=O, –CO₂H, –C≡N and the other important groups, you can identify the family to which a compound belongs and begin constructing its systematic name. These same skills will be used repeatedly throughout A-Level Organic Chemistry.
Try these five questions before reading the chapter. Look carefully at each structure and use the bonding pattern rather than relying only on the atoms you can see.
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Ways of representing organic molecules
The same molecule can be represented in several different ways. Each representation answers a slightly different question. Some tell us only which atoms are present, while others reveal how those atoms are connected or arranged in three-dimensional space.
You therefore need to recognise what information a particular formula contains before using it to identify or name an organic compound.
| Representation | What it shows | Main limitation |
|---|---|---|
| Empirical formula | The simplest whole-number ratio of the atoms present | Does not give the actual molecular composition or connectivity |
| Molecular formula | The actual number of each type of atom in one molecule | Does not necessarily show how the atoms are connected |
| Structural formula | How atoms or groups are connected in a condensed form | Not every individual bond is necessarily drawn |
| Displayed formula | The atoms and the bonds joining them | Does not always communicate the full three-dimensional arrangement |
| Skeletal formula | The carbon framework with important functional groups shown explicitly | Carbon symbols and most H atoms bonded to carbon are omitted |
| Stereochemical formula | The three-dimensional orientation of selected bonds | Used only when spatial arrangement needs to be communicated |
Empirical and molecular formulae
The empirical formula gives the simplest whole-number ratio of atoms in a compound, while the molecular formula gives the actual number of each type of atom in one molecule.
For example, a molecule with molecular formula C₃H₆O₃ has the empirical formula CH₂O because all three subscripts can be divided by three.
A molecular formula gives composition, but not necessarily connectivity.
This distinction matters enormously in organic chemistry because the same molecular formula can correspond to different arrangements of atoms. A structural formula or skeletal formula is therefore needed if we want to know exactly how the atoms are joined.
Structural and skeletal representations of the same molecule
Ethanol has molecular formula C₂H₆O and structural formula CH₃CH₂OH. Its skeletal representation contains the same two-carbon chain and terminal oxygen atom, but most C and H labels are omitted.
How to read a skeletal formula
Skeletal formulae are especially important at A-Level because larger organic molecules become difficult to read if every carbon and hydrogen atom is written. The skeletal system removes information that can be inferred automatically while keeping the important bonding pattern visible.
Carbon atoms are hidden at line ends and vertices
- Each unlabelled line end represents one carbon atom.
- Each unlabelled vertex represents one carbon atom.
- A line represents a covalent bond, not a carbon atom.
- Hydrogen atoms bonded directly to carbon are normally omitted.
- Enough H atoms are implied to give each neutral carbon four covalent bonds.
- Atoms such as O, N, Cl and Br are written explicitly.
- Double and triple bonds are always shown.
Count ends and corners, not line segments.
The skeletal structure of butane contains three line segments but four carbon atoms. Two carbons are at the ends and two are at the internal vertices.
Inferring hydrogen atoms
Once you know where the carbon atoms are, infer enough hydrogen atoms to give each neutral carbon four bonds.
A terminal carbon joined to one other carbon by a single bond is normally CH₃. A carbon joined by single bonds to two neighbouring carbon atoms is normally CH₂ unless something else is attached to it.
Multiple bonds change the number of implied hydrogens. A carbon already involved in a C=C double bond has two bonds accounted for by the double bond and therefore requires fewer additional bonds.
Incorrect idea: A skeletal structure containing four line segments must contain four carbon atoms.
Correct reasoning: Count unlabelled line ends and vertices. The lines themselves represent bonds.
Wedges and dashed bonds
Molecules are three-dimensional, even though ordinary structural formulae are drawn on a flat page. Stereochemical formulae can show whether a bond projects towards or away from the viewer.
- An ordinary line represents a bond approximately in the plane of the page.
- A solid wedge represents a bond projecting towards the viewer.
- A dashed or hashed wedge represents a bond projecting away from the viewer.
You will use this notation more extensively when studying stereoisomerism.
Functional groups in organic compounds
A functional group is an atom or group of atoms responsible for the characteristic chemical reactions of an organic compound.
This does not mean that the rest of the molecule is irrelevant. A longer carbon chain, for example, can affect intermolecular forces and physical properties. However, the functional group is often the best starting point for predicting the types of reactions the molecule will undergo.
Identify the functional group before trying to name the molecule.
Hydrocarbons
| Family | Characteristic feature | Naming feature |
|---|---|---|
| Alkane | Only C–C single bonds within the carbon framework | -ane |
| Alkene | C=C | -ene |
| Arene | Benzene ring | Often uses benzene as the parent name |
Halogenoalkanes, alcohols and phenols
| Family | Characteristic feature | Naming feature |
|---|---|---|
| Halogenoalkane | C–F, C–Cl, C–Br or C–I | fluoro-, chloro-, bromo- or iodo- |
| Alcohol | –OH bonded to an aliphatic carbon | -ol |
| Phenol | –OH bonded directly to a benzene ring | phenol |
Alcohol: –OH is bonded to an aliphatic carbon atom.
Phenol: –OH is bonded directly to a carbon atom of a benzene ring.
Carbonyl compounds: aldehydes and ketones
Aldehydes and ketones both contain the carbonyl group, C=O. You therefore cannot identify the functional class simply by spotting C=O. You must inspect what is bonded to the carbonyl carbon.
Aldehyde: the carbonyl carbon forms part of a terminal –CHO group.
Ketone: the carbonyl carbon is bonded to carbon-containing groups on both sides.
Carboxylic acids and their derivatives
| Family | Characteristic feature | Naming feature |
|---|---|---|
| Carboxylic acid | –CO₂H | -oic acid |
| Acyl chloride | –COCl | -oyl chloride |
| Ester | –COO– linking two carbon-containing groups | alkyl alkanoate |
Nitrogen-containing functional groups
| Family | Characteristic feature | Naming feature |
|---|---|---|
| Amine | N-containing group without an adjacent carbonyl carbon | -amine in simple examples |
| Amide | Carbonyl carbon directly bonded to nitrogen | -amide |
| Nitrile | –C≡N | -nitrile |
| Amino acid | Contains both an amino group and a carboxylic acid group | Contains both functionalities within the systematic name |
Amine: nitrogen is present without being directly bonded to a carbonyl carbon.
Amide: nitrogen is directly bonded to the carbonyl carbon of a C=O group.
Systematic naming of organic compounds
Systematic nomenclature converts structural information into a name. Instead of trying to memorise the complete name of every possible molecule, build the name systematically from its individual parts.
Step 1: identify the principal functional group
The principal functional group normally determines the ending, or suffix, of the name.
- alkane → -ane
- alkene → -ene
- alcohol → -ol
- aldehyde → -al
- ketone → -one
- carboxylic acid → -oic acid
- acyl chloride → -oyl chloride
- amide → -amide
- nitrile → -nitrile
Step 2: choose the parent carbon framework
Choose the appropriate continuous carbon framework containing the principal functional group. The number of carbon atoms determines the parent stem.
| Number of carbon atoms | Stem | Example alkane |
|---|---|---|
| 1 | meth- | methane |
| 2 | eth- | ethane |
| 3 | prop- | propane |
| 4 | but- | butane |
| 5 | pent- | pentane |
| 6 | hex- | hexane |
| 7 | hept- | heptane |
| 8 | oct- | octane |
A carbon chain can bend around corners in a skeletal formula.
Do not automatically choose the longest horizontal-looking line on the page. Follow connected carbon atoms through vertices and branches while ensuring that the principal functional group is included in the parent framework.
Step 3: number the parent framework
Number from the end that gives the principal functional group the lowest appropriate number.
A position number used within an organic name is called a locant.
The –OH group is nearer one end of the chain. Numbering from that end places –OH at carbon 2 rather than carbon 4.
Step 4: identify substituents
Carbon groups attached to the parent framework can be named as alkyl substituents.
- –CH₃ → methyl-
- –CH₂CH₃ → ethyl-
Halogen atoms are also represented by prefixes such as fluoro-, chloro-, bromo- and iodo-. Each substituent receives a locant showing where it is attached.
Step 5: assemble the complete name
Combine the substituent locants and prefixes with the parent name and functional-group suffix. Different substituent names are normally placed in alphabetical order.
Hyphens separate numbers from words, while commas separate consecutive numbers.
Functional group → parent framework → numbering → substituents → complete name
Worked examples of organic nomenclature
Example 1: a branched alcohol
The parent chain containing –OH has five carbon atoms, giving pent-. Numbering from the nearer end places –OH at carbon 2. A methyl substituent occurs at carbon 3. The compound is therefore 3-methylpentan-2-ol.
Incorrect: 3-methylpentan-4-ol
Correct: 3-methylpentan-2-ol, because the principal –OH group must receive the lower locant.
Example 2: locating a C=C bond
The parent chain contains five carbon atoms. Numbering from the nearer end places the C=C bond beginning at carbon 2.
Example 3: naming a ketone
The parent chain contains four carbon atoms. The carbonyl carbon lies within the chain, so the molecule is a ketone. Numbering from the nearer end places the carbonyl carbon at carbon 2.
Example 4: naming a halogenoalkane
The parent chain contains four carbon atoms. Numbering from the nearer end gives the bromo substituent locant 2.
Example 5: naming an ester
Ester names are built differently from the names of simple carbon chains. Identify the group attached to the single-bonded oxygen first. Here it is an ethyl group.
The carbonyl-containing part contains two carbon atoms and is therefore ethanoate.
Do not treat all the carbon atoms of an ester as one continuous parent chain.
Primary, secondary and tertiary compounds
Primary, secondary and tertiary classification describes the degree of substitution around a particular carbon atom.
For alcohols and halogenoalkanes, first identify the carbon atom carrying the functional group. Then count how many other carbon atoms are directly bonded to that carbon.
| Classification | Carbon neighbours of the relevant carbon | Meaning |
|---|---|---|
| Primary, 1° | 1 | The carbon carrying the functional group is directly bonded to one carbon atom |
| Secondary, 2° | 2 | The carbon carrying the functional group is directly bonded to two carbon atoms |
| Tertiary, 3° | 3 | The carbon carrying the functional group is directly bonded to three carbon atoms |
The same idea applies to halogenoalkanes
For a halogenoalkane, identify the carbon directly bonded to the halogen and count its carbon neighbours.
- one carbon neighbour → primary halogenoalkane;
- two carbon neighbours → secondary halogenoalkane;
- three carbon neighbours → tertiary halogenoalkane.
Incorrect approach: Count all the carbon atoms in the molecule.
Correct approach: Count only the carbon atoms directly attached to the carbon carrying the relevant functional group.
What is a quaternary carbon?
A quaternary carbon atom is bonded directly to four other carbon atoms.
A carbon atom of this type cannot also carry an –OH group because it already has four covalent bonds. For this reason, alcohols are classified as primary, secondary or tertiary rather than quaternary.
Common Mistakes
| Common mistake | Why it is wrong | Better approach |
|---|---|---|
| Counting line segments instead of carbon positions in a skeletal formula | A line represents a bond rather than a carbon atom. | Count every unlabelled line end and vertex. |
| Calling every compound containing –OH an alcohol | –OH directly attached to benzene gives a phenol. | Inspect the carbon atom to which –OH is bonded. |
| Calling every compound containing C=O a ketone | Aldehydes also contain C=O. | Check what is bonded directly to the carbonyl carbon. |
| Calling every nitrogen-containing compound an amine | Nitrogen can also occur in amides, nitriles and other functional groups. | Examine the complete bonding environment around nitrogen. |
| Choosing the visually straight chain as the parent | Carbon chains can bend around vertices. | Trace the appropriate continuous carbon framework through every corner. |
| Numbering from the end nearest a branch instead of the principal functional group | The principal functional group has priority when the numbering direction is chosen. | Give the principal functional group the lowest appropriate locant first. |
| Treating an ester as one uninterrupted carbon chain | An ester contains an alkyl part attached through oxygen and an acid-derived alkanoate part. | Name the alkyl group first, followed by the alkanoate. |
| Using the total number of carbons to classify an alcohol as 1°, 2° or 3° | The classification concerns one particular carbon atom. | Count the carbon neighbours of the carbon carrying –OH. |
What you should be able to do
Represent organic molecules
I can distinguish empirical, molecular, structural, displayed, skeletal and stereochemical formulae.
Read skeletal structures
I can identify carbon atoms at unlabelled line ends and vertices and infer omitted hydrogen atoms.
Recognise functional groups
I can identify the important hydrocarbon, oxygen-containing, halogen-containing and nitrogen-containing functional groups.
Distinguish similar functional groups
I can distinguish alcohols from phenols, aldehydes from ketones and amines from amides using their bonding patterns.
Name organic compounds
I can choose the parent carbon framework, number it correctly, identify substituents and assemble a systematic name.
Classify substituted carbon atoms
I can identify primary, secondary and tertiary alcohols or halogenoalkanes by counting carbon neighbours.
These ten questions test whether you can apply the chapter to unfamiliar structures, functional-group identification and systematic naming.
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Frequently asked questions
Questions students often ask about functional groups and organic nomenclature
These questions clarify some of the details that commonly cause difficulty when moving between structures and systematic names.
Do I need to draw every hydrogen atom in a skeletal formula?
No. Hydrogen atoms bonded directly to carbon are normally omitted. Infer enough hydrogen atoms to give each neutral carbon four covalent bonds.
Atoms such as O, N and halogens are shown explicitly, and hydrogen atoms associated with important functional groups may also need to be shown.
Why is phenol not simply treated as an alcohol?
In an alcohol, –OH is attached to an aliphatic carbon atom. In phenol, –OH is bonded directly to a carbon atom of the benzene ring.
This difference in bonding environment leads to different chemical behaviour, so the two functional classes are distinguished.
How can I distinguish an aldehyde from a ketone quickly?
Look at the carbonyl carbon. An aldehyde contains a terminal –CHO group. In a ketone, the carbonyl carbon is bonded to carbon-containing groups on both sides.
Do I always choose the longest carbon chain I can see?
The parent framework must contain the principal functional group. You should then identify the appropriate continuous carbon framework rather than simply choosing the longest straight-looking line on the page.
Remember that carbon chains can bend around corners in skeletal formulae.
Why is an ester name written in two parts?
An ester contains the –COO– linkage between two carbon-containing parts. The carbon group bonded to the single-bonded oxygen is named first as an alkyl group.
The carbonyl-containing part is named second as an alkanoate. For example, CH₃COOCH₂CH₃ is ethyl ethanoate.
Functional groups provide the organising language of organic chemistry. Once you can read structural and skeletal formulae, you can identify the part of a molecule responsible for much of its characteristic chemistry. Systematic nomenclature then translates the same structural information into a name using the parent carbon framework, functional group, substituents and locants. These skills provide the foundation for understanding the reactions of each organic family in the chapters that follow.