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
Isomerism is the name for the existence of isomers: molecules with the same molecular formula but different chemical structures.
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.
Structural isomers have the same molecular formula but different structural formulae.
Fig. 1: Butane (left) and methylpropane (right). These two molecules have the same molecular formula but different structural formulae.Vaia Originals
Check out Organic Compounds to learn more about the different types of formulae used in chemistry.
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.
Go to IUPAC Nomenclature to learn more about carbon chains and to find out how we name isomers.
Next up, positional isomers!
Positional isomerism
Positional isomerism refers to molecules that have the same molecular formula and functional group, but the functional group is found in a different position on the carbon chain.
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
Functional group isomers have the same molecular formula, but have different functional groups. In other words, they belong to different homologous series.
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.Vaia Originals
Visit Functional Groups to discover the different functional groups you'll meet in organic chemistry.
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?
Try to find the three chain isomers of pentane, C5H12.
Draw the straight-chain molecule first. Then, draw the two branched-chain isomers - but watch out for 'false' isomers!
Fig. 4: The chain isomers of pentane.Vaia Originals
Here, we have one isomer with a root carbon chain that is four atoms long, and a single side chain attached to carbon 2. If we attached the side chain to carbon 3, we would end up with a 'false' isomer - these are the same molecules if you number the carbon chain from the other direction. We also have a second isomer. This isomer has a root carbon chain that is three atoms long, as well as two side chains, both attached to carbon 2.
Well done! You've drawn the chain isomers of pentane. Let us try another example.
Draw structural isomers for the molecular formula C4H8Cl2.
This formula represents a few more isomers than the first example that we looked at, C5H12, because it contains a greater variety of atoms. We must consider not only chain isomerism, but also functional group and positional isomerism too.
First, chain isomerism. This formula has four carbon atoms, giving us multiple possible arrangements of our carbon chain. Some of its isomers will have all four carbons in a straight chain. Others will be branched isomers with three carbons in a straight chain, and a side chain joined to carbon 2.
Next, consider functional group isomerism. All molecules represented by this formula must have a C-Cl bond, and so are all some kind of chloroalkane. That means that for this formula, we actually don't get any functional group isomers.
Finally, positional isomerism. Consider the possible positions of the chlorine atoms. For example, the two Cl atoms could both be attached to the same carbon atom, but they could instead be attached to different carbon atoms. If you combine that with the two different carbon chain arrangements we found just a second ago, you'll see that C4H8Cl2 has a lot of possible structural isomers! You can see them all below. How many did you manage to identify?
Fig. 5: Structural isomers for C4H8Cl2.Image credits: Chegg
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.
Stereoisomers are molecules with the same structural and molecular formulae, but different spatial arrangements of atoms.
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 a type of stereoisomerism that occurs in molecules with restricted rotation around a C=C double bonds. Geometric isomers are known as E-Z isomers.
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.Vaia 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:
- Take one of the carbon atoms involved in the C=C double bond.
- Look at the two groups attached to this carbon atom.
- 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.
- Repeat with the second carbon atom involved in the C=C double bond.
- 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.
Fig. 7: How to use the E-Z naming system.Image credits: kpu.pressbooks.pub
E and Z stand for the German words entgegen and zusammen. They mean 'opposite' and 'together', respectively.
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
E-Z isomers are also known as cis-trans isomers. Cis and trans are the Greek words for 'on this side' and 'across'. For simple molecules, the cis-trans system matches up to the E-Z system - the Z-isomer is known as the cis-isomer, and the E-isomer is known as the trans-isomer. However, this only works when the C=C double bond is attached to just two different groups. If the C=C bond is attached to three or four groups, the system breaks down, and we must use E-Z notation instead.
Fig. 9: Examples of geometric isomers using both E-Z and cis-trans notation. Vaia Originals
You'll come across geometric isomerism again when you encounter Alkenes.
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 isomers are molecules that have the same structural and molecular formulae, but are non-superimposable mirror images of each other.
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.
To understand optical isomerism, we must first understand the meaning of chirality, or 'handedness'. What do we mean by this? Well, take a look at your two hands. They have the same elements: four fingers, a thumb, and a palm. Even the distances between each finger are the same on each hand! If you clap your hands together so that your thumbs are opposite each other, you will see all the other fingers match up. Your hands are mirror images of each other.
Now, place one hand on top of the other. Can you get your fingers and thumbs to match up like before? It is impossible, no matter how many ways you turn and twist your hands. We call this chirality (kai-ral-uh-tee). Chiral means that an object or molecule cannot be superimposed on its mirror image. It comes from the Greek word for 'hand', cheir.
Your hands are great examples of non-superimposable mirror images.
Image credits: Pexels
What do hands have to do with optical isomerism? Well, just like our left and right hands, pairs of optical isomers are mirror-image molecules. They contain the same atoms and groups, joined in the same order. However, if we overlay them, one on top of the other, their structures don't match up. The isomers are non-superimposable and so show chirality.
The chirality of optical isomers is caused by something called a chiral center. This is a carbon atom bonded to four different atoms or groups. The groups are bonded to the central carbon atom in different ways, leading to two possible mirror-image arrangements - in other words, two isomers. No matter how hard you try, you can't get one isomer to perfectly match the second.
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.Vaia OriginalsOptical isomers are identical in some regards but different in others. You can find out more about optical isomers and all the terms that we have mentioned here, such as enantiomers and chirality, in the article Optical Isomerism. There, you'll also discover what we mean by a racemic mixture.
Isomerism - Key takeaways
- Isomerism is the name for the existence of isomers: molecules with the same molecular formula but different chemical structures.
- Structural isomers have the same molecular formula but different structural formulae.
- There are three types of structural isomerism:
- Chain isomers have different arrangements of their carbon chains.
- Positional isomers have the same functional group, but it is positioned at different points on the carbon chain.
- Functional group isomers contain different functional groups.
- Stereoisomers have the same molecular and structural formulae, but different spatial arrangements of atoms.
- There are two types of stereoisomerism:
- Geometric isomers differ in their arrangement of groups around a C=C double bond.
- Optical isomers differ in their arrangement of four unique groups around a central carbon atom. As a result, they form non-superimposable mirror-image molecules.
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