Isomerism: structural, cis‑trans, enantiomerism

Q: What is the definition of isomerism in chemistry?

Isomers are compounds that have the same molecular formula but different structures (different arrangements of atoms).

Q: What is structural isomerism?

Structural isomerism occurs when molecules have the same molecular formula but atoms are bonded in a different order/sequence .

Q: What is chain isomerism?

Chain isomerism is a type of structural isomerism where molecules have the same functional group but differ in the arrangement of the carbon skeleton (straight vs. branched).

Q: What is positional isomerism?

Positional isomerism occurs when molecules have the same carbon skeleton and functional group, but the position of the functional group (or multiple bond) along the chain differs.

Q: What is functional group isomerism?

Functional group isomerism occurs when molecules with the same molecular formula contain different functional groups , belonging to different homologous series.

Q: What are the two essential conditions required for a molecule to exhibit cis-trans isomerism?

1. Restricted rotation (such as around a rigid \(\text{C=C}\) double bond). 2. Two different groups attached to each carbon atom of the double bond.

Q: How are cis and trans isomers distinguished structurally in alkenes?

In the cis isomer, identical (or high-priority) groups are on the same side of the double bond, whereas in the trans isomer, they are on opposite sides .

Q: What is a chiral carbon (chiral centre)?

A chiral carbon (often marked \(\text{C}^*\)) is an \(\text{sp}^3\) hybridised carbon atom bonded to four different atoms or groups .

Q: What are enantiomers?

Enantiomers are stereoisomers that are non-superimposable mirror images of each other.

Q: How do the properties of a pair of enantiomers compare?

They have identical physical and achiral chemical properties , but differ in their optical activity and their interactions with chiral environments/biological receptors .

Practice

Chemistry · Senior Secondary (HKDSE) · 6 min read · Updated: 21 Aug 2026

Isomerism: Same Formula, Different Molecules!

Hello! Welcome to the fascinating world of isomerism. Ever played with LEGO blocks? Imagine you have a specific set of blocks – say, 2 red, 4 blue, and 1 yellow. You could connect them in many different ways to build completely different-looking models. That's the main idea behind isomerism in chemistry!

In this chapter, you'll learn how molecules with the exact same chemical formula can have completely different structures and properties. It’s a key concept in organic chemistry because it explains the incredible diversity of carbon compounds. We will explore three main types:

1. Structural Isomerism
2. Cis-trans Isomerism
3. Enantiomerism

Don't worry if these names sound complicated now. We'll break them down step-by-step with simple examples and analogies. Let's get started!

What is Isomerism?

The Core Definition

Isomers are compounds that have the same molecular formula but different structures.

Quick Review: Formula vs. Structure

- A molecular formula just tells you the number and type of atoms in a molecule. For example, \(\text{C}_4\text{H}_{10}\) tells us there are 4 carbon atoms and 10 hydrogen atoms.
- A structural formula shows how those atoms are connected to each other. It's like a blueprint of the molecule.

So, isomers are like different blueprints you can draw using the same set of parts.

Key Takeaway

Same Parts, Different Arrangement. That's the heart of isomerism. The molecular formula gives you the parts list, and the isomers are the different ways you can build with them.

1. Structural Isomerism

Different Connections, Different Molecule

Structural isomerism occurs when the atoms are connected in a completely different order. The bonding sequence itself is different.

Think of it like arranging carriages on a train. If you have an engine (E), a dining car (D), and two passenger cars (P), you could arrange them as E-P-P-D or E-P-D-P. The carriages are the same, but their connection order is different.

Under the HKDSE syllabus, you need to master three subtypes of structural isomerism:

Type 1: Chain Isomerism

Here, the isomers have the same functional group, but the carbon skeleton (the carbon chain backbone) is branched differently.

Example: \(\text{C}_4\text{H}_{10}\)
1. Butane: A straight chain of four carbons: \(\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_3\)
2. 2-Methylpropane: A branched three-carbon chain: \(\text{CH}_3\text{CH}(\text{CH}_3)\text{CH}_3\)

Type 2: Positional Isomerism

Here, the carbon skeleton and the functional group remain the same, but the position of the functional group (or multiple bond / substituent) differs along the chain.

Example 1 (Alcohols, \(\text{C}_3\text{H}_8\text{O}\)):
1. Propan-1-ol: The -OH group is on carbon 1: \(\text{CH}_3\text{CH}_2\text{CH}_2\text{OH}\)
2. Propan-2-ol: The -OH group is on carbon 2: \(\text{CH}_3\text{CH}(\text{OH})\text{CH}_3\)

Example 2 (Alkenes, \(\text{C}_4\text{H}_8\)):
1. But-1-ene: Double bond is between C1 and C2: \(\text{CH}_2=\text{CH}-\text{CH}_2-\text{CH}_3\)
2. But-2-ene: Double bond is between C2 and C3: \(\text{CH}_3-\text{CH}=\text{CH}-\text{CH}_3\)

Type 3: Functional Group Isomerism

The atoms are arranged to form completely different functional groups, belonging to different homologous series.

Example: \(\text{C}_2\text{H}_6\text{O}\)
1. Ethanol: An alcohol with an -OH group: \(\text{CH}_3\text{CH}_2\text{OH}\)
2. Methoxymethane: An ether with a C-O-C bridge: \(\text{CH}_3\text{OCH}_3\)

Ethanol is a liquid with a relatively high boiling point due to hydrogen bonding, while methoxymethane is a gas at room temperature. They possess completely different chemical and physical properties!

Common Mistake to Avoid!

Just bending a carbon chain doesn't create a new isomer! A four-carbon chain drawn in a straight line, a 'Z' shape, or a 'U' shape is still butane. To be a different isomer, you must break a bond and reconnect it elsewhere.

2. Stereoisomerism: Isomers in 3D Space

Now we move to stereoisomerism. Stereoisomers have the same molecular formula AND the same atom connectivity (the same bonding sequence). What differs is their arrangement in 3D space!

Cis-Trans Isomerism (Geometrical Isomerism)

This type of stereoisomerism occurs when there is restricted rotation in a molecule, typically around a carbon-carbon double bond (C=C) in acyclic alkenes.

The Two Conditions for Cis-Trans Isomerism:

1. Restricted Rotation: There must be a \(\text{C=C}\) double bond (rigid \(\pi\)-bond preventing free rotation).
2. Two Different Groups on Each Carbon: Each carbon atom of the double bond must be bonded to two different atoms or groups.

Example: But-2-ene (\(\text{C}_4\text{H}_8\))

In but-2-ene, each double-bonded carbon is bonded to \(-\text{H}\) and \(-\text{CH}_3\):

1. cis-But-2-ene: The two \(-\text{CH}_3\) groups are on the same side of the double bond.
2. trans-But-2-ene: The two \(-\text{CH}_3\) groups are on opposite sides of the double bond.

Note: But-1-ene (\(\text{CH}_2=\text{CHCH}_2\text{CH}_3\)) does NOT show cis-trans isomerism because the terminal carbon has two identical \(-\text{H}\) atoms attached.

Real-World Connection: Cis and Trans Fats

Natural unsaturated fatty acids typically exist in the cis form, creating a bend or "kink" in the chain. Industrial partial hydrogenation can produce trans fats with straight chains, which pack tightly and contribute to cardiovascular disease. This illustrates how 3D geometry directly influences biological function!

3. Enantiomerism (Optical Isomerism)

Enantiomerism is another form of stereoisomerism. Enantiomers are like your left and right hands: mirror images of each other, but non-superimposable.

The Key Ingredient: The Chiral Carbon

Enantiomerism occurs in molecules containing a chiral carbon (chiral centre), which is a carbon atom bonded to four different atoms or groups (often denoted with an asterisk, \(\text{C}^*\)).

Example: Butan-2-ol
Let's look at C2 of butan-2-ol: \(\text{CH}_3-\text{C}^*\text{H}(\text{OH})-\text{CH}_2\text{CH}_3\)
The chiral carbon is bonded to four distinct groups:
- A hydrogen atom (\(-\text{H}\))
- A hydroxyl group (\(-\text{OH}\))
- A methyl group (\(-\text{CH}_3\))
- An ethyl group (\(-\text{CH}_2\text{CH}_3\))

3D Representation: Tetrahedral Wedge-and-Dash

In exams, you must represent enantiomers around the tetrahedral chiral carbon using 3D wedge-dash notation:

- Solid lines: Bonds lying in the plane of the paper.
- Wedge (bold/solid triangle): Bond pointing forwards (out of the page towards you).
- Dash (hashed line): Bond pointing backwards (into the page away from you).

To draw a pair of enantiomers, draw one tetrahedral structure and then draw its reflection across a vertical mirror plane.

Properties of Enantiomers & Optical Activity

Enantiomers share identical physical properties (identical melting points, boiling points, densities, and solubilities in achiral solvents) and identical chemical properties towards achiral reagents.

However, they differ in two critical ways:
1. Optical Activity: A pair of enantiomers rotate the plane of plane-polarized light by the same angle but in opposite directions (one clockwise / dextrorotatory, the other anticlockwise / levorotatory).
2. Biological / Chiral Interactions: Since biological receptors and enzymes are chiral, enantiomers often exhibit completely different physiological activities.

Did You Know? Real-Life Examples

- Carvone: One enantiomer gives the scent of spearmint, while its mirror image smells like caraway seeds.
- Thalidomide: In the 1950s, one enantiomer acted as a sedative for morning sickness, but its mirror image caused severe birth defects. This highlighted the crucial necessity of enantiomer separation in pharmacology.

Chapter Summary: A Quick Guide to Classifying Isomers

Follow this flowchart when analysing two molecules with the same molecular formula:

1. Same molecular formula?
- If NO: Not isomers.
- If YES: Check connectivity.

2. Are atoms bonded in the same sequence?
- If NO: Structural Isomers (Chain, Positional, or Functional Group).
- If YES: Stereoisomers.

3. What kind of Stereoisomers?
- Due to restricted rotation around \(\text{C=C}\) with 2 different groups on each carbon: Cis-Trans Isomers.
- Due to a chiral carbon forming non-superimposable mirror images: Enantiomers (optically active, rotate plane-polarized light in opposite directions).

Quick check

Can you answer these now?

Open each question to check the key ideas from this chapter.

What is the definition of isomerism in chemistry?

Isomers are compounds that have the same molecular formula but different structures (different arrangements of atoms).

What is structural isomerism?

Structural isomerism occurs when molecules have the same molecular formula but atoms are bonded in a different order/sequence.

What is chain isomerism?

Chain isomerism is a type of structural isomerism where molecules have the same functional group but differ in the arrangement of the carbon skeleton (straight vs. branched).

What is positional isomerism?

Positional isomerism occurs when molecules have the same carbon skeleton and functional group, but the position of the functional group (or multiple bond) along the chain differs.

What is functional group isomerism?

Functional group isomerism occurs when molecules with the same molecular formula contain different functional groups, belonging to different homologous series.

What are the two essential conditions required for a molecule to exhibit cis-trans isomerism?

1. Restricted rotation (such as around a rigid \(\text{C=C}\) double bond).
2. Two different groups attached to each carbon atom of the double bond.

How are cis and trans isomers distinguished structurally in alkenes?

In the cis isomer, identical (or high-priority) groups are on the same side of the double bond, whereas in the trans isomer, they are on opposite sides.

What is a chiral carbon (chiral centre)?

A chiral carbon (often marked \(\text{C}^*\)) is an \(\text{sp}^3\) hybridised carbon atom bonded to four different atoms or groups.

What are enantiomers?

Enantiomers are stereoisomers that are non-superimposable mirror images of each other.

How do the properties of a pair of enantiomers compare?

They have identical physical and achiral chemical properties, but differ in their optical activity and their interactions with chiral environments/biological receptors.

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