Isomerism

While you study chemistry, you may occasionally spot molecules that look different from each other but have the same molecular formula! We call this phenomenon isomerism.

Isomers are molecules with the same molecular formula but different structures. This gives them different physical or chemical properties. You will discover two distinct types of isomerism, structural isomerism and stereoisomerism. Read on to learn more!

What is structural isomerism?

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

Structural isomerism refers to the positions of atoms in molecules when they are relatively different from one another. In other words, a molecule of butane has the same amount of carbon atoms and hydrogen atoms as a molecule of methylpropane, but in a different arrangement.

Butane and 2-methylpropane have the same molecular formula but a different arrangement of atoms, chemguide

Types of structural isomerism

Lets us consider 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. They have the same molecular formula, but different carbon chains. Butane has an unbranched chain (or 'straight-chain' ) of carbons. On the other hand, methylpropane has a branched-chain. Consider the following organic molecule. Is it also a chain isomer of butane and methylpropane?

Is this an isomer of butane? Chemguide

While it may look like another chain isomer, the above molecule is simply n-butane rotated about the central carbon. We call this a 'false' isomer. Both molecules have the same carbon chain - no carbon atom is bonded to more than two other carbon atoms. It might be tricky to identify isomers with Lewis structures, but you can build models to help you figure them out!

Next up, positional isomers!

Positional isomerism

For example, propan-1-ol and propan-2-ol are positional isomers. Their carbon chains are the same, but the -OH group is attached to a different carbon in each case.

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

You may discover instances where both chain isomerism and positional isomerism are present. For example, the molecules isopentanol and pentan-3-ol show both chain and positional isomerism.

Isopentanol and pentan-3-ol show both chain and positional isomerism, StudySmarter

Positional isomers also occur on benzene rings. For instance, the molecular formula C₇H₇Cl has four isomers depending on the position of the chlorine atom.

Positional isomers can occur in molecules that contain benzene rings too! StudySmarter

Last but not least, let us consider functional group isomers!

Functional group isomerism

For example, molecules with the structural formula C₃H₆O could be propanal, propanone, or the alcohol 2-propen-1-ol.

Some isomers with the molecular formula C₃H₆O, StudySmarter

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

How to draw structural isomers

When you draw structural isomers, you might come across structures that look different on paper but are actually false isomers. A clever trick to know whether you have drawn a true isomer is to name the structure using IUPAC rules. 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 structural isomers of pentane. Let us try another example.

What is stereoisomerism?

You have seen how structural isomers have a different arrangement of atoms. We will now consider the second type of isomerism - stereoisomerism.

In stereoisomerism, molecules have the same order of atoms but different spatial arrangements. How is that possible? Consider that atoms bonded to a C=C double bond are planar (all on the same plane). C=C double bonds are rigid - the atoms bonded to them cannot rotate about them. However, the atoms can still pivot about any single bonds in the molecule. Restricted rotation about carbon-carbon double bonds causes what we know as E-Z isomers.

Geometric E-Z isomerism

Geometric isomerism happens in molecules that have restricted rotation about C=C bonds. Geometric isomers can be either E-isomers or Z-isomers. As the name suggests, it is easy to identify E-Z isomers!

E-isomers have the highest priority groups across the double bonds from each other. Z-isomers have the highest priority groups both above or both below the double bond.

How to use the E/Z naming system, kpu.pressbooks.pub

Consider the models shown below. Are they isomers? While they may look different, the models show the same molecule. Remember, atoms can freely rotate about single bonds. So the two structures represent the same molecule- 1,2-dichloroethane.

These two molecules represent the same molecule- 1, 2-dichloroethane, chemguide

Now consider the following two molecules. Are they isomers? You will notice that both molecules have a C=C bond, which restricts rotation. So you cannot simply rotate the bonds to transform one structure into the other. The two structures, E-1,2-dichloroethene and Z-1,2-dichloroethene, are geometric isomers.

Geometric E/Z isomerism only happens when there is restricted rotation about the C=C double bond, chemguide

Other common geometric isomers you may come across are E-but-2-ene and Z-but-2-ene.

Geometric isomers, (E)-but-2-ene and (Z)-but-2-ene, chemguide

As mentioned before, E-isomers have the highest priority groups across the double bonds from each other. Z-isomers have the highest priority groups both above or both below the double bond. How do we figure out which groups have priority in a molecule? We use CIP priority rules!

What are Cahn-Ingold-Prelog (CIP) priority rules?

In 1956, Robert S. Cahn, Sir Christopher K. Ingold, and Vladimir Prelog invented a technique to help chemists name molecules that had chiral centres with four ligands. Today we call their method the Cahn-Ingold-Prelog priority rules. Let us discuss the rules.

• Step one: Treat each end of the C=C bond separately. In this example, one end has a bromine and fluorine atom. The other end has a chlorine and hydrogen atom.
• Step two: Give priority to the atom with the highest atomic number on each end of the molecule. In this example, bromine's atomic number is higher than fluorine's. Also, chlorine has a higher atomic number than hydrogen.
• Step three: The molecule with the high priority atoms across the C=C bond from each other is the E-isomer. The molecule with the high priority atoms both above or both below the C=C bond is the Z-isomer.

We use CIP rules to identify which ligand has priority, StudySmarter

What about when a group of atoms is attached to the C=C bond?

Easy! Focus on the atom directly connected to the C=C bond. Let us use a complicated molecule like the one below as an example.

We can use CIP rules to identify this E-isomer, kpu pressbooks

First, focus on the right-hand side: clearly, Cl has a higher priority than C in the CH₂CH₃ group. Next, consider the left-hand side: both groups have a carbon atom directly attached to the C=C bond. Which one has priority?

In cases like this, we focus on the priorities of the next group of atoms directly attached to the two carbons. In the upper group, we have H, H, C but in the lower group, we have H, H, H. We use the atom with the greatest atomic number in each group. In this example, carbon has a higher atomic number. So the group H₃CH₂C has priority on the left-hand side.

The two groups with priority, H₃CH₂C and Cl are across the C=C bond from each other. This means that this is an E-isomer. The complete name for the compound is (E)-3-chloro-4-methyl-3-hexene.

You have learned about two types of isomers - structural isomers and stereoisomers. You have also mastered how to draw structural isomers and how to name geometric isomers with Cahn-Ingold-Prelog rules.

But wait! There is still one type of isomer we have not yet covered - optical isomers! Before we conclude, let us take a brief look at what they are.