Isomerism

Specification Reference Organic chemistry, Introduction to organic chemistry 3.3.1.3

Quick Notes

Isomers are compounds with the same molecular formula but different structures.
Two main types of isomerism:

Structural isomerism – different connectivity of atoms where molecules have the same molecular formula and a different structural formula.
Stereoisomerism – same connectivity but different arrangement of atoms in space. Molecules have the same molecular and the same structural formula, however the atoms are arranged differently in space.

Types of structural isomerism:

Chain isomerism – different carbon chain arrangement.
Position isomerism – same functional group in a different position.
Functional group isomerism – different functional groups with the same atoms.

Stereoisomerism:

E-Z isomerism – occurs in alkenes due to restricted rotation around the C=C bond.

CIP priority rules are used to determine whether an alkene is E (highest priority groups on opposite sides of C=C) or Z (highest priority groups on same side of C=C).

Full Notes

Structural isomers have the same molecular formula but different arrangements of atoms. There are three types of structural isomerism you should know - chain isomerism, position isomerism and functional group isomerism.

1. Chain Isomerism

Carbon chain is arranged differently.

Example: C₄H₁₀ (butane, methylpropane)

AQA A-Level Chemistry diagram showing chain isomers: butane vs methylpropane

Butane: Straight chain, 2-Methylpropane: One branch.

2. Position Isomerism

Same functional group, different position on the chain.

Example: C₅H₁₁Br (Bromopentane isomers)

AQA A-Level Chemistry diagram showing position isomers 1-bromopentane, 2-bromopentane and 3-bromopentane

1-Bromopentane: Br on carbon 1.

2-Bromopentane: Br on carbon 2.

3-Bromopentane: Br on carbon 3.

3. Functional Group Isomerism

Different functional groups, same molecular formula.

Example: C₃H₆O (Aldehyde and Ketone Isomers)

AQA A-Level Chemistry diagram showing propanal (aldehyde) vs propanone (ketone) functional group isomers

Propanal (Aldehyde, −CHO)

Propanone (Ketone, −CO− group)

Stereoisomerism

Stereoisomers have the same structural formula but atoms are arranged differently in space. There are different types of stereoisomer, however at this stage you need to be confident with stereoisomerism in alkenes.

E-Z Stereoisomerism

The C=C double bond in an alkene is unable to freely rotate, it has restricted rotation.

This means atoms and groups bonded to the carbons in a double bond are ‘locked’ into position and there are two possible ways they can be arranged.

If the two atoms or groups bonded to each carbon are different, stereoisomerism exists.

AQA A-Level Chemistry figure showing restricted rotation around the C=C bond leading to stereoisomers

For Example: in 1-bromo-2-chloroethene the Br and Cl groups bonded to the C=C double bond can have two different relative positions.

AQA A-Level Chemistry diagram showing E and Z arrangements for 1-bromo-2-chloroethene

To name these kinds of stereoisomers we use E and Z notation.

The highest priority groups on each carbon determine the name using Cahn–Ingold–Prelog (CIP) rules (see below).

AQA A-Level Chemistry illustration comparing E and Z isomers with priority groups on opposite or same sides

If highest priority groups are on opposite sides = E Isomer (opposite sides)

If highest priority groups are on the same side = Z Isomer (same side)

Example: But-2-ene

AQA A-Level Chemistry diagram showing E-but-2-ene and Z-but-2-ene

E-But-2-ene: CH₃ and H on opposite sides of C=C.

Z-But-2-ene: CH₃ and H on the same side of C=C.

Cahn–Ingold–Prelog (CIP) Priority Rules

Step 1: Look at the atoms directly attached to the C=C bond.

Step 2: Assign higher priority to the atom with the higher atomic number. If the highest priority atoms bonded to each carbon are the same (for example C) you then assign the highest priority of the groups bonded to that atom (for example CH₂CH₃ group has higher priority than CH₃ group).

Step 3:
If the higher priority groups are on the same side, it is Z.
If the higher priority groups are on opposite sides, it is E.

Summary Table: Types of Isomerism

Type	Description	Example
Structural isomerism	Different connectivity of atoms where molecules have the same molecular formula and a different structural formula.	—
Chain isomerism	Different carbon chain arrangement.	C₄H₁₀ (butane, methylpropane)
Position isomerism	Same functional group in a different position.	C₅H₁₁Br (1-, 2-, 3-bromopentane)
Functional group isomerism	Different functional groups with the same atoms.	C₃H₆O (propanal / propanone)
Stereoisomerism	Same connectivity but different arrangement of atoms in space. Molecules have the same molecular and the same structural formula, however the atoms are arranged differently in space.	—
E-Z isomerism	Occurs in alkenes due to restricted rotation around the C=C bond. CIP priority rules determine whether an alkene is E (highest priority groups on opposite sides of C=C) or Z (highest priority groups on same side of C=C).	But-2-ene (E and Z)