Isomerism

Isomers are molecules with the same molecular formula but different arrangements of atoms. This lesson covers structural isomerism and stereoisomerism (E/Z and optical), which are fundamental to understanding organic reactivity and biological activity at A-Level.

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Types of Isomerism — Overview

Isomerism
├── Structural Isomerism
│   ├── Chain isomerism
│   ├── Position isomerism
│   └── Functional group isomerism
└── Stereoisomerism
    ├── E/Z (geometric) isomerism
    └── Optical isomerism

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Structural Isomerism

Structural isomers have the same molecular formula but different structural formulae — the atoms are connected in a different order.

Chain Isomerism

The carbon skeleton is arranged differently (different chain lengths and branching patterns).

Example: C₅H₁₂ has three chain isomers:

• Pentane: CH₃CH₂CH₂CH₂CH₃
• 2-Methylbutane: CH₃CH(CH₃)CH₂CH₃
• 2,2-Dimethylpropane: C(CH₃)₄

Chain isomers have similar chemical properties (same functional group) but slightly different physical properties. Branched isomers have lower boiling points because they are more compact and have weaker London dispersion forces.

Position Isomerism

The same functional group is attached to different positions on the same carbon chain.

Example: C₃H₇OH has two position isomers:

• Propan-1-ol: CH₃CH₂CH₂OH
• Propan-2-ol: CH₃CH(OH)CH₃

Position isomers have similar chemical properties but may react at different rates. For example, propan-1-ol is a primary alcohol (oxidised to an aldehyde then a carboxylic acid), while propan-2-ol is a secondary alcohol (oxidised to a ketone only).

Functional Group Isomerism

The atoms are arranged to give different functional groups entirely.

Example: C₂H₆O can be:

• Ethanol: CH₃CH₂OH (an alcohol)
• Methoxymethane: CH₃OCH₃ (an ether)

These have very different chemical and physical properties. Ethanol can form hydrogen bonds with water and is miscible; methoxymethane has a much lower boiling point.

Example: C₃H₆O can be:

• Propanal: CH₃CH₂CHO (an aldehyde)
• Propanone: CH₃COCH₃ (a ketone)

Exam Tip: When asked to draw structural isomers, work systematically. Start with the longest straight chain, then shorten the chain by one carbon and attach it as a branch. Continue shortening and branching. Then consider different positions for functional groups and finally different functional groups.

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E/Z (Geometric) Isomerism

E/Z isomerism is a type of stereoisomerism that arises when there is:

1. A C=C double bond (which prevents free rotation), AND
2. Two different groups on each carbon of the double bond.

Because the C=C double bond contains a pi bond (formed by sideways overlap of p-orbitals), rotation around the bond is restricted. This means groups can be locked on the same side or opposite sides.

Cahn-Ingold-Prelog (CIP) Priority Rules

To assign E or Z, use the CIP priority rules:

1. At each carbon of the C=C, identify the two atoms directly bonded to it.
2. The atom with the higher atomic number has higher priority.
3. If the atoms are the same, move outward along the chain until a point of difference is found.

• Z (zusammen, German for "together"): the two higher-priority groups are on the same side of the double bond.
• E (entgegen, German for "opposite"): the two higher-priority groups are on opposite sides of the double bond.

Worked Example 1 — But-2-ene

CH₃CH=CHCH₃

At C-2: groups are CH₃ (higher priority — C > H) and H. At C-3: groups are CH₃ (higher priority) and H.

• (Z)-but-2-ene: both CH₃ groups on the same side.
• (E)-but-2-ene: CH₃ groups on opposite sides.

Worked Example 2 — 1-Bromo-1-chloropropene

BrClC=CHCH₃

At C-1: Br (atomic number 35, higher priority) and Cl (atomic number 17). At C-2: CH₃ (C, atomic number 6, higher priority) and H (atomic number 1).

• (Z): Br and CH₃ on the same side.
• (E): Br and CH₃ on opposite sides.