Organic nomenclature and isomerism

What you'll learn

How to recognise and use molecular, displayed, structural and skeletal formulae.
How to name organic molecules using systematic IUPAC rules.
How to spot structural isomers: chain, position and functional group isomers.
How to decide whether an alkene can show E/Z stereoisomerism.

The starting point: organic molecules

Organic chemistry is the study of compounds containing carbon. At A-Level, you mainly meet molecules built from carbon chains with hydrogen and sometimes atoms such as oxygen, nitrogen, sulfur, chlorine, bromine or iodine.

A hydrocarbon is an organic compound containing carbon and hydrogen only. For example, methane, ethene and benzene are hydrocarbons; ethanol is not, because it contains oxygen.

Carbon is special because each carbon atom normally forms four covalent bonds, so it can make chains, branches and rings. This is why naming needs clear rules.

Homologous series and functional groups

A homologous series is a family of organic compounds with the same functional group, the same general formula, and similar chemical reactions. Adjacent members differ by CH₂.

Definition

Functional group

A functional group is the atom or group of atoms in an organic molecule that gives the molecule its characteristic chemical reactions.

For example, all alcohols contain the -OH functional group, so methanol, ethanol and propan-1-ol react in broadly similar ways.

Common families you should recognise include:

Family	Functional feature	Example name
Alkane	C-C single bonds only	propane
Alkene	C=C double bond	propene
Haloalkane	C-F, C-Cl, C-Br or C-I	1-bromopropane
Alcohol	-OH	propan-1-ol
Aldehyde	-CHO at the end of a chain	propanal
Ketone	C=O within a chain	propanone
Carboxylic acid	-COOH	propanoic acid
Ester	-COO-	ethyl ethanoate

Key Idea

Why functional groups matter

The carbon chain affects properties such as boiling point, but the functional group usually controls the main chemical reactions.

Formulae: different levels of detail

Organic molecules can be shown in several ways. You need to move confidently between them.

Definition

Types of formula

A molecular formula gives the actual number of each atom in one molecule. A displayed formula shows every atom and every bond. A structural formula shows how atoms are connected, often in a shortened form. A skeletal formula shows the carbon skeleton using lines; carbon atoms and most carbon-bonded hydrogens are not written.

The same molecule, propan-2-ol, can be represented in several equivalent ways:

Four formula representations of propan-2-ol

Reading skeletal formulae

In a skeletal formula:

Each line end or vertex represents a carbon atom.
Hydrogen atoms bonded to carbon are omitted.
Heteroatoms such as O, N, Cl and Br are shown.
Hydrogens bonded to heteroatoms are shown, for example the H in -OH.

Example

Converting a skeletal formula into a molecular formula

Suppose a skeletal formula is a four-carbon zig-zag chain with an -OH group on carbon 2.

Count the carbon atoms: the two line ends plus the two vertices give 4 carbon atoms.
Identify the functional group: the -OH group contributes 1 oxygen atom and 1 hydrogen atom.
Add enough hydrogens so each carbon has four bonds: the structure is CH₃CH(OH)CH₂CH₃.
Count atoms from the structural formula: carbon = 4, hydrogen = 10, oxygen = 1, so the molecular formula is C₄H₁₀O.

Naming organic compounds: the IUPAC idea

The systematic naming system used at A-Level is based on IUPAC nomenclature. The aim is that one clear name describes one structure.

The basic naming method

Use this sequence:

Find the longest carbon chain containing the main functional group and any C=C double bond.
Choose the stem from the number of carbons: meth-, eth-, prop-, but-, pent-, hex-, hept-, oct-.
Choose the main suffix: -ane, -ene, -ol, -al, -one, -oic acid, and so on.
Number the chain to give the main functional group or double bond the lowest possible number.
Add side groups as prefixes, such as methyl-, ethyl-, fluoro-, chloro-, bromo- or iodo-.
Use numbers called locants to show positions.

Tip

Naming order

Think: chain length → family suffix → numbering → branches/prefixes. Do not start naming from the nearest branch unless the main functional group has already been considered.

Parent chain stems

The most common stems are:

Carbon atoms	Stem
1	meth-
2	eth-
3	prop-
4	but-
5	pent-
6	hex-

For alkanes, the general formula is $C_nH_{2n+2}$ . For acyclic alkenes with one C=C double bond, the general formula is $C_nH_{2n}$ .

Common Mistake

General formulae have limits

The alkene formula $C_nH_{2n}$ applies to open-chain molecules with one C=C double bond. Cycloalkanes also have formula $C_nH_{2n}$ , so the formula alone does not prove a compound is an alkene.

Naming a branched compound

Example

Naming a branched alcohol

Name CH₃CH(CH₃)CH₂CH₂OH.

Choose the longest chain containing the -OH group. Starting at the -OH end gives HOCH₂CH₂CH(CH₃)CH₃, so the parent chain has 4 carbons: butan-.
Number from the -OH end because the alcohol group needs the lowest possible locant. The -OH group is on carbon 1, so the suffix is butan-1-ol.
Locate the branch. The extra CH₃ group is attached to carbon 3, so it is a 3-methyl substituent.
Combine the parts with correct punctuation: 3-methylbutan-1-ol.

Common Mistake

Numbering from the wrong end

A branch does not usually outrank the main functional group. In alcohols and alkenes, number the chain to give the -OH group or C=C double bond the lowest possible locant.

Isomerism: same formula, different arrangement

Definition

Isomers

Isomers are compounds with the same molecular formula but different arrangements of atoms.

Isomerism is a big reason organic chemistry has so many compounds. A molecular formula such as C₄H₁₀O does not tell you whether the compound is an alcohol or an ether, or whether the carbon chain is straight or branched.

Types of isomerism with examples

Structural isomerism

Definition

Structural isomers

Structural isomers have the same molecular formula but different structural formulae, meaning their atoms are connected in different ways.

There are three structural isomerism types you should know well.

Chain isomerism

Chain isomers have different carbon skeletons. For example, butane and 2-methylpropane both have molecular formula C₄H₁₀, but one is a straight chain and one is branched.

Position isomerism

Position isomers have the same carbon skeleton and functional group, but the functional group, branch or double bond is in a different position. For example, but-1-ene and but-2-ene both have formula C₄H₈.

Functional group isomerism

Functional group isomers have the same molecular formula but different functional groups. For example, ethanol and methoxymethane both have formula C₂H₆O, but ethanol is an alcohol and methoxymethane is an ether.

Example

Classifying structural isomers

Classify the isomerism shown by each pair: butan-1-ol and butan-2-ol; butane and 2-methylpropane; ethanol and methoxymethane.

Compare butan-1-ol and butan-2-ol. Both are alcohols with four carbons, but the -OH group moves from carbon 1 to carbon 2, so they are position isomers.
Compare butane and 2-methylpropane. Both have molecular formula C₄H₁₀, but the carbon skeleton changes from straight to branched, so they are chain isomers.
Compare ethanol and methoxymethane. Both have molecular formula C₂H₆O, but ethanol contains -OH while methoxymethane contains C-O-C, so they are functional group isomers.

Stereoisomerism and E/Z isomerism

Definition

Stereoisomers

Stereoisomers have the same structural formula, but a different arrangement of atoms in space.

At this stage, the key type is E/Z isomerism in alkenes. A C=C double bond has restricted rotation, so groups attached to the double bond can be locked in different positions.

For E/Z isomerism to exist:

There must be a C=C double bond.
Each carbon atom in the C=C must be bonded to two different groups.

If either carbon in the C=C has two identical groups, E/Z isomerism is not possible.

Assigning E and Z

To decide whether an alkene is E or Z:

Look at one carbon of the C=C and decide which attached group has the higher priority.
Do the same for the other carbon of the C=C.
If the higher-priority groups are on the same side, the isomer is Z.
If the higher-priority groups are on opposite sides, the isomer is E.

At A-Level, priority is usually decided using atomic number: higher atomic number means higher priority. For example, Br has higher priority than Cl, and Cl has higher priority than C or H.

Tip

Remembering E and Z

Z comes from the German zusammen, meaning together: the higher-priority groups are on the same side. E means they are on opposite sides.

Example

Assigning E/Z to an alkene

An alkene has Br and H attached to the left-hand carbon of the C=C, and CH₃ and H attached to the right-hand carbon. Br and CH₃ are drawn on the same side of the double bond.

Check that E/Z isomerism is possible. The left carbon has Br and H, and the right carbon has CH₃ and H, so each C=C carbon has two different groups.
Assign priority on the left carbon. Br has a higher atomic number than H, so Br is the higher-priority group.
Assign priority on the right carbon. The carbon in CH₃ has a higher atomic number than H, so CH₃ is the higher-priority group.
Compare the higher-priority groups. Br and CH₃ are on the same side, so the molecule is the Z isomer.

Common Mistake

Using cis/trans when it is not enough

Cis/trans works only in simple cases where the same group appears on both carbons of the C=C. E/Z is more general and is the safer A-Level method.

Exam technique

In the exam

For naming questions, identify the longest chain containing the main functional group before numbering anything.
For isomerism questions, first check the molecular formula is the same, then decide whether connectivity changes or only spatial arrangement changes.
For E/Z questions, prove that each C=C carbon has two different groups before assigning E or Z.

Self review

Check yourself

What is the difference between a displayed formula and a structural formula?
Name CH₃CH₂CH(CH₃)CH₂CH₃ systematically.
Why can but-2-ene show E/Z isomerism, but but-1-ene cannot?

Organic nomenclature and isomerism

What you'll learn

How to recognise and use molecular, displayed, structural and skeletal formulae.
How to name organic molecules using systematic IUPAC rules.
How to spot structural isomers: chain, position and functional group isomers.
How to decide whether an alkene can show E/Z stereoisomerism.

The starting point: organic molecules

A hydrocarbon is an organic compound containing carbon and hydrogen only. For example, methane, ethene and benzene are hydrocarbons; ethanol is not, because it contains oxygen.

Carbon is special because each carbon atom normally forms four covalent bonds, so it can make chains, branches and rings. This is why naming needs clear rules.

Homologous series and functional groups

A homologous series is a family of organic compounds with the same functional group, the same general formula, and similar chemical reactions. Adjacent members differ by CH₂.

Definition

Functional group

A functional group is the atom or group of atoms in an organic molecule that gives the molecule its characteristic chemical reactions.

For example, all alcohols contain the -OH functional group, so methanol, ethanol and propan-1-ol react in broadly similar ways.

Common families you should recognise include:

Family	Functional feature	Example name
Alkane	C-C single bonds only	propane
Alkene	C=C double bond	propene
Haloalkane	C-F, C-Cl, C-Br or C-I	1-bromopropane
Alcohol	-OH	propan-1-ol
Aldehyde	-CHO at the end of a chain	propanal
Ketone	C=O within a chain	propanone
Carboxylic acid	-COOH	propanoic acid
Ester	-COO-	ethyl ethanoate

Key Idea

Why functional groups matter

The carbon chain affects properties such as boiling point, but the functional group usually controls the main chemical reactions.

Formulae: different levels of detail

Organic molecules can be shown in several ways. You need to move confidently between them.

Definition

Types of formula

The same molecule, propan-2-ol, can be represented in several equivalent ways:

Four formula representations of propan-2-ol

Reading skeletal formulae

In a skeletal formula:

Each line end or vertex represents a carbon atom.
Hydrogen atoms bonded to carbon are omitted.
Heteroatoms such as O, N, Cl and Br are shown.
Hydrogens bonded to heteroatoms are shown, for example the H in -OH.

Example

Converting a skeletal formula into a molecular formula

Suppose a skeletal formula is a four-carbon zig-zag chain with an -OH group on carbon 2.

Count the carbon atoms: the two line ends plus the two vertices give 4 carbon atoms.
Identify the functional group: the -OH group contributes 1 oxygen atom and 1 hydrogen atom.
Add enough hydrogens so each carbon has four bonds: the structure is CH₃CH(OH)CH₂CH₃.
Count atoms from the structural formula: carbon = 4, hydrogen = 10, oxygen = 1, so the molecular formula is C₄H₁₀O.

Naming organic compounds: the IUPAC idea

The systematic naming system used at A-Level is based on IUPAC nomenclature. The aim is that one clear name describes one structure.

The basic naming method

Use this sequence:

Find the longest carbon chain containing the main functional group and any C=C double bond.
Choose the stem from the number of carbons: meth-, eth-, prop-, but-, pent-, hex-, hept-, oct-.
Choose the main suffix: -ane, -ene, -ol, -al, -one, -oic acid, and so on.
Number the chain to give the main functional group or double bond the lowest possible number.
Add side groups as prefixes, such as methyl-, ethyl-, fluoro-, chloro-, bromo- or iodo-.
Use numbers called locants to show positions.

Tip

Naming order

Think: chain length → family suffix → numbering → branches/prefixes. Do not start naming from the nearest branch unless the main functional group has already been considered.

Parent chain stems

The most common stems are:

Carbon atoms	Stem
1	meth-
2	eth-
3	prop-
4	but-
5	pent-
6	hex-

For alkanes, the general formula is $C_nH_{2n+2}$ . For acyclic alkenes with one C=C double bond, the general formula is $C_nH_{2n}$ .

Common Mistake

General formulae have limits

The alkene formula $C_nH_{2n}$ applies to open-chain molecules with one C=C double bond. Cycloalkanes also have formula $C_nH_{2n}$ , so the formula alone does not prove a compound is an alkene.

Naming a branched compound

Example

Naming a branched alcohol

Name CH₃CH(CH₃)CH₂CH₂OH.

Choose the longest chain containing the -OH group. Starting at the -OH end gives HOCH₂CH₂CH(CH₃)CH₃, so the parent chain has 4 carbons: butan-.
Number from the -OH end because the alcohol group needs the lowest possible locant. The -OH group is on carbon 1, so the suffix is butan-1-ol.
Locate the branch. The extra CH₃ group is attached to carbon 3, so it is a 3-methyl substituent.
Combine the parts with correct punctuation: 3-methylbutan-1-ol.

Common Mistake

Numbering from the wrong end

A branch does not usually outrank the main functional group. In alcohols and alkenes, number the chain to give the -OH group or C=C double bond the lowest possible locant.

Isomerism: same formula, different arrangement

Definition

Isomers

Isomers are compounds with the same molecular formula but different arrangements of atoms.

Types of isomerism with examples

Structural isomerism

Definition

Structural isomers

Structural isomers have the same molecular formula but different structural formulae, meaning their atoms are connected in different ways.

There are three structural isomerism types you should know well.

Chain isomerism

Chain isomers have different carbon skeletons. For example, butane and 2-methylpropane both have molecular formula C₄H₁₀, but one is a straight chain and one is branched.

Position isomerism

Functional group isomerism

Example

Classifying structural isomers

Classify the isomerism shown by each pair: butan-1-ol and butan-2-ol; butane and 2-methylpropane; ethanol and methoxymethane.

Compare butan-1-ol and butan-2-ol. Both are alcohols with four carbons, but the -OH group moves from carbon 1 to carbon 2, so they are position isomers.
Compare butane and 2-methylpropane. Both have molecular formula C₄H₁₀, but the carbon skeleton changes from straight to branched, so they are chain isomers.
Compare ethanol and methoxymethane. Both have molecular formula C₂H₆O, but ethanol contains -OH while methoxymethane contains C-O-C, so they are functional group isomers.

Stereoisomerism and E/Z isomerism

Definition

Stereoisomers

Stereoisomers have the same structural formula, but a different arrangement of atoms in space.

At this stage, the key type is E/Z isomerism in alkenes. A C=C double bond has restricted rotation, so groups attached to the double bond can be locked in different positions.

For E/Z isomerism to exist:

There must be a C=C double bond.
Each carbon atom in the C=C must be bonded to two different groups.

If either carbon in the C=C has two identical groups, E/Z isomerism is not possible.

Assigning E and Z

To decide whether an alkene is E or Z:

Look at one carbon of the C=C and decide which attached group has the higher priority.
Do the same for the other carbon of the C=C.
If the higher-priority groups are on the same side, the isomer is Z.
If the higher-priority groups are on opposite sides, the isomer is E.

At A-Level, priority is usually decided using atomic number: higher atomic number means higher priority. For example, Br has higher priority than Cl, and Cl has higher priority than C or H.

Tip

Remembering E and Z

Z comes from the German zusammen, meaning together: the higher-priority groups are on the same side. E means they are on opposite sides.

Example

Assigning E/Z to an alkene

An alkene has Br and H attached to the left-hand carbon of the C=C, and CH₃ and H attached to the right-hand carbon. Br and CH₃ are drawn on the same side of the double bond.

Check that E/Z isomerism is possible. The left carbon has Br and H, and the right carbon has CH₃ and H, so each C=C carbon has two different groups.
Assign priority on the left carbon. Br has a higher atomic number than H, so Br is the higher-priority group.
Assign priority on the right carbon. The carbon in CH₃ has a higher atomic number than H, so CH₃ is the higher-priority group.
Compare the higher-priority groups. Br and CH₃ are on the same side, so the molecule is the Z isomer.

Common Mistake

Using cis/trans when it is not enough

Cis/trans works only in simple cases where the same group appears on both carbons of the C=C. E/Z is more general and is the safer A-Level method.

Exam technique

In the exam

For naming questions, identify the longest chain containing the main functional group before numbering anything.
For isomerism questions, first check the molecular formula is the same, then decide whether connectivity changes or only spatial arrangement changes.
For E/Z questions, prove that each C=C carbon has two different groups before assigning E or Z.

Self review

Check yourself

What is the difference between a displayed formula and a structural formula?
Name CH₃CH₂CH(CH₃)CH₂CH₃ systematically.
Why can but-2-ene show E/Z isomerism, but but-1-ene cannot?

Organic nomenclature and isomerism

What you'll learn

The starting point: organic molecules

Homologous series and functional groups

Formulae: different levels of detail

Reading skeletal formulae

Naming organic compounds: the IUPAC idea

The basic naming method

Parent chain stems

Naming a branched compound

Isomerism: same formula, different arrangement

Structural isomerism

Chain isomerism

Position isomerism

Functional group isomerism

Stereoisomerism and E/Z isomerism

Assigning E and Z

Organic nomenclature and isomerism Revision Guide

Organic nomenclature and isomerism

What you'll learn

The starting point: organic molecules

Homologous series and functional groups

Formulae: different levels of detail

Reading skeletal formulae

Naming organic compounds: the IUPAC idea

The basic naming method

Parent chain stems

Naming a branched compound

Isomerism: same formula, different arrangement

Structural isomerism

Chain isomerism

Position isomerism

Functional group isomerism

Stereoisomerism and E/Z isomerism

Assigning E and Z

Organic nomenclature and isomerism Revision Guide