Isomerism

While you study chemistry, you may occasionally spot molecules that have identical numbers and types of atoms, but look entirely different from each other! We call this phenomenon isomerism.

• In this article about isomerism, you will first learn about structural isomers.
• You'll discover the three types of structural isomerism: chain, positional, and functional group isomerism.
• You'll work out how to draw structural isomers from molecular formulae.
• Furthermore, you'll learn about stereoisomerism, uncovering the two types of stereoisomers: geometrical (E-Z or cis-trans) and optical isomerism.

Isomerism: definition

Chemical isomers are a little like anagrams. Anagrams contain the same number of each type of letter, but these letters are arranged in different ways. This forms two (or more!) unique words that look and sound quite unlike each other. Similarly, isomers contain the same number of each type of atom. However, these atoms are arranged differently, which gives isomers different physical and/or chemical properties.

There are two distinct types of isomerism: structural isomerism and stereoisomerism. Read on to learn more!

Structural isomerism: definition

You may have come across the two compounds butane and methylpropane (shown below). Clearly, they are two different compounds with very different structures. However, butane and methylpropane share the same molecular formula: C₄H₁₀. We call this structural isomerism.

Fig. 1: Butane (left) and methylpropane (right). These two molecules have the same molecular formula but different structural formulae.StudySmarter Originals

We can further divide structural isomerism into three different types: chain, positional, and functional group isomerism. Let's explore them now.

Types of structural isomerism: chain, positional, and functional group

As we mentioned, there are three kinds of structural isomers:

• Chain isomers.
• Positional isomers.
• Functional group isomers.

Chain isomerism

The above example of butane and methylpropane is an instance of two chain isomers. Chain isomers have the same molecular formula, but different arrangements of their carbon chains. For example, butane has an unbranched chain (or straight chain) of carbon atoms, whilst methylpropane has a branched chain. Other chain isomers might have branches of different lengths, or the branches could be joined to the main carbon chain at varying positions.

Next up, positional isomers!

Positional isomerism

For example, propan-1-ol and propan-2-ol are positional isomers. They both have a straight chain made of just three carbon atoms, but the -OH group is attached to a different carbon in each case.

Fig. 2: Propan-1-ol and propan-2-ol are positional isomers because they share a molecular formula but their -OH groups are in different positions. Image credits: chemguide.

To conclude this section, let us consider functional group isomers!

Functional group isomerism

For example, the molecular formula C₃H₆O might refer to propanal (an aldehyde), propanone (a ketone), or various unsaturated alcohols (such as prop-2-en-1-ol).

Fig. 3: Examples of functional group isomerism. These molecules are all functional group isomers of propanal.StudySmarter Originals

Identifying isomers can be tricky. With practice, you can get the hang of it. The following examples can help!

Structural isomerism: examples

When you draw structural isomers, you might come across structures that look different on paper but are actually 'false' isomers: if you wriggle them around a bit, the two 'different' molecules turn out to be one and the same! A clever trick to know whether you have drawn a true isomer or not is to name the structure using IUPAC rules, as we explore in IUPAC Nomenclature. A true isomer will have a unique name.

Can you find all the true isomers in the following examples?

Well done! You've drawn the chain isomers of pentane. Let us try another example.

Stereoisomerism: definition

So far, you've learned about structural isomerism. This involves isomers that have the same molecular formula but different structural formulae. We will now consider the second type of isomerism - stereoisomerism.

Unlike structural isomers, stereoisomers also have the same structural formula. From first appearance, they look like they have the same IUPAC name, too. However, we have ways of distinguishing between different stereoisomers, as you'll learn in just a second.

We can further divide stereoisomerism into two different types: geometric isomerism and optical isomerism. Once again, we'll explore them both in turn.

Types of stereoisomerism: geometric and optical

Let's now look at the finer details of geometric and optical isomerism.

Geometric isomerism

Geometric isomerism is caused by C=C double bonds. This is because, unlike C-C single bonds, C=C double bonds can't rotate. This means that they are held in a fixed arrangement. If the carbon atoms at either end of the bond are joined to different groups, we end up with two different molecules.

Consider a C=C bond. Each carbon is joined to two further groups, which we'll call A and B. If we draw this structure out on paper, we see that there are two possible arrangements of both A and B groups. Either:

• The A groups are both found on the same side of the double bond, directly opposite each other.
• The A groups are found on opposite sides of the double bond, diagonally across from each other.

Fig. 6: An example of geometric isomerism, caused by lack of rotation around a C=C double bond.StudySmarter Originals

To distinguish between the two stereoisomers, we use E-Z notation. We decide which isomer is the E-isomer and which isomer is the Z-isomer by assigning priority:

1. Take one of the carbon atoms involved in the C=C double bond.
2. Look at the two groups attached to this carbon atom.
3. Assign priority to the two groups:
   • Find the first atom in each group and consider which has the higher atomic mass. This group takes higher priority.
   • If both groups contain the same first atom, consider the second atom. As you'd expect, the group whose second atom has the higher atomic mass takes higher priority.
4. Repeat with the second carbon atom involved in the C=C double bond.
5. Consider the relative positions of the higher priority group from each of the two carbon atoms in the C=C bond:
   • If the higher priority groups are on the same side of the double bond, directly opposite each other (as in left-hand example), the isomer is the Z-isomer.
   • If the higher priority groups are on opposite sides of the double bond, diagonally across from each other (as in the right-hand example), the isomer is the E-isomer.

Common geometric isomers you may come across are E-but-2-ene and Z-but-2-ene. Here, both carbons in the C=C bond are joined to a hydrogen atom and a -CH₃ group. The first atom in the -CH₃ group, C, has a higher atomic mass than H, and so the -CH₃ group takes priority over the H atom. Here are our two isomers:

Fig. 8: Geometric isomers of but-2-ene.Image credits: chemguide

We're almost finished with this article. But wait! There is still one type of isomerism we have not yet covered - optical isomerism! Before we conclude, let us take a brief look at what it is.

Optical isomerism

Optical isomerism is another type of stereoisomerism, just like geometric isomerism. But whilst geometric isomers differ in their arrangement of groups around a C=C double bond, optical isomers vary in their arrangement of four different groups around a central carbon atom. This forms two non-superimposable, mirror-image chiral molecules, which we call enantiomers.

Here's an example of optical isomerism, to help you understand what we mean:

Fig. 10: These molecules are optical isomers, known as enantiomers. No matter how you rotate the right-hand molecule (as shown above), it will never perfectly match its left-hand partner.StudySmarter Originals