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Isomerism

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Chemistry

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!

  • 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.
  • You'll also learn about stereoisomerism, uncovering the two types of stereoisomers: geometrical (cis/trans) and optical isomerism.
  • We'll also cover how we use E/Z nomenclature.

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.

Structural isomers have the same molecular formula but their atoms are arranged in a different order.

Isomers, butane and methylpropane are structural isomers, StudySmarterButane 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?

Isomerism, a false isomer of butane, StudySmarterIs 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!

  1. Go to Organic Compounds to learn more about the central carbon.
  2. We use the prefix n- to refer to unbranched chains of carbon atoms. For example, unbranched butane is n-butane.

Next up, positional isomers!

Positional isomerism

Positional isomerism refers to molecules that have the same functional group in a different position on the same carbon chain.

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.

Isomerism, Position isomers propan-1-ol and propan-2-ol, StudySmarterPropan-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.

Isomerism, Position isomerism can occur in benzene ring structures e.g. benzyl chloride, StudySmarterPositional isomers can occur in molecules that contain benzene rings too! StudySmarter

Last but not least, 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, molecules with the structural formula C3H6O could be propanal, propanone, or the alcohol 2-propen-1-ol.

Isomerism, Functional isomers of propanal, StudySmarterSome 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?

Try to find the three chain isomers of pentane, C5H12

You might draw the straight-chain molecule first:

CH3-CH2-CH2-CH2-CH3

Next, draw the two branched-chain isomers. Watch out for 'false' isomers! It might help to make some models.

CH3

|

CH3-CH2-CH-CH3

CH3

|

CH3-C-CH3

|

CH3

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

Draw structural isomers for the molecular formula C4H8Cl2

There are four carbons. Some isomers will have all four carbons in a straight chain. Others will be branched-chain isomers with three carbons in a straight chain, and a branch from the middle carbon.

Also, consider the position of the chlorine atoms. For example, the two Cl atoms could both be attached to the first and second carbons. They could also be attached to different carbons such as C1 and C3.

Below you can see all the structural isomers with the molecular formula C4H8Cl2. How many of them did you identify?

Isomerism, Isomers of C4H8Cl2, StudySmarterStructural isomers for C4H8Cl2, Chegg

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.

A stereoisomer is a molecule with the same order of atoms, which has a different spatial arrangement of atoms.

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.

Learn about planar molecules and molecular shapes in Shapes of Molecules.

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.

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

E and Z stand for the German words entgegen and zusammen: they mean 'opposite' and 'together', respectively.

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.

Isomerism, E-1,2-dichloroethene and Z-1,2-dichloroethene, StudySmarterGeometric 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

Cis/Trans naming system

E-Z isomers are also known as cis-trans isomers. Cis and Trans are the Greek words for 'on this side' and 'across'. So, an E-isomer would be a trans-isomer and a Z-isomer would be a cis-isomer. In the above example, E-but-2-ene would be trans-but-2-ene.

The cis-trans system works well to identify simple geometric isomers like trans-but-2-ene. However, cis-trans can be ambiguous when it comes to other structures. Consider the molecules below. Can you identify the cis and trans isomers? It is tough to tell! Fortunately, we have the E-Z system to help us name more complex isomers.

Isomerism, E/Z isomers, StudySmarterWhich one is the trans-isomer and which is the cis-isomer?

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.

Isomerism, (E)-3-chloro-4-methyl-3-hexene, StudySmarterWe 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.

If we use the cis-trans naming system on the above isomer, it would be a cis-isomer. So you see, E-isomers don't always correspond to trans-isomers!

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.

What is optical isomerism?

Optical isomerism is another type of stereoisomerism. Remember, stereoisomers have the same order of atoms, but a different arrangement in space. Optical isomerism happens when molecules are non-superimposable mirror images of each other.

To understand optical isomerism, we must first understand the meaning of chirality or 'handedness'. What do we mean by this? 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 non-superimposable mirror images of each other, Pexels

What does this have to do with isomers? Take a look at the following two molecules. Notice that they are mirror images of each other. They contain the same atoms in the same order. If we were to overlay them, one on top of the other, does each atom match up?

No, they are not superimposable. That means these molecules show chirality. In other words, they are optical isomers of each other. Would you like to know more? Visit Optical Isomerism!

Isomerism, Optical isomers, StudySmarterOptical isomers are non-superimposable mirror images of each other, kpu pressbooks

Isomerism - Key takeaways

  • Structural isomers have the same molecular formula but different spatial arrangements of atoms.
  • Chain isomers have the same molecular formula, but different carbon chains. For example, butane and methylpropane.
  • Positional isomerism refers to molecules that have the same functional group in a different position on the same carbon chain. For example, propan-1-ol and propan-2-ol.
  • Functional isomers have the same molecular formula but have different functional groups i.e. they belong to different homologous series.
  • A stereoisomer is a molecule with the same order of atoms which has a different spatial arrangement of atoms.
  • Restricted rotation about carbon-carbon double bonds causes geometric E-Z isomerism.
  • 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.
  • To name geometric E-Z isomers we use Cahn-Ingold-Prelog rules:
    • Step one: Treat each end of the C=C bond separately.
    • Step two: Give priority to the atom with the highest atomic number on each end of the molecule.
    • 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.
  • Optical isomers are non-superimposable mirror images of each other.

Isomerism

Geometric isomerism happens in molecules that have restricted rotation about C=C bonds. Geometric isomers can be either E-isomers or Z-isomers. E-isomers have the highest priority groups across the double bonds from each other. While Z-isomers have the highest priority groups both above or both below the double bond.

Optical isomerism is a type of stereoisomerism. Optical isomerism happens when molecules have the same order of atoms but are non-superimposable mirror images of each other. That is, they show chirality.

Sometimes in chemistry molecules 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, which gives them different physical or chemical properties.

Linkage isomerism is a type of structural isomerism. Linkage isomers have a ligand with multiple atoms connected to the central ion. The ligands must be ambidentate - only connected in one place. For example, the NO2- ion is an ambidentate ligand. It can only attach to the central ion through the nitrogen atom or the oxygen atom.

Optical isomers are non-superimposable mirror images of each other. Complexes whose mirror image is not superimposable are optical isomers. We can also identify complexes that show optical isomerism by looking at their plane of symmetry. Optical isomers do not show a plane of symmetry.

Final Isomerism Quiz

Question

Define Isomerism.

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Answer

Isomerism is when molecules have the same molecular formula but differ in their chemical and physical properties.



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Question

What are the two types of isomers?

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Answer

Structural Isomerism and stereoisomerism

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Question

Name the three types of structural isomers.

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Answer

Positional, Functional and Chain.

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Question

How many isomers does butane have? 


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Answer

2

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Question

What are the two types of stereoisomerism?

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Answer

Geometrical and Optical



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Question

What are structural isomers?

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Answer

Structural isomers have the same molecular formula but their atoms are arranged in a different order.

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What are position isomers?

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Answer

Position isomers have the same functional group in a different position on the same carbon chain.

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Question

What is a functional group isomer?

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Answer

Functional group isomers have the same molecular formula but have different functional groups. In other words, they belong to different homologous series.


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What is chain isomerism?

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Answer

Chain isomers have the same molecular formula, but different carbon chains.

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What is stereoisomerism?

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Answer

A stereoisomer is a molecule with the same molecular formula and order of atoms but has a different spatial arrangement of atoms.

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Question

Name the following isomer using Cahn-Ingold-Prelog rules.

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Answer

(E)-3-methyl-2-pentene

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Question

How many structural isomers does C3H2Cl6 have?

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4

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Question

Draw and label the geometric E-Z isomers of pent-2-ene.

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Question

Which of the following molecules are structural isomers of the compound shown below?


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Question

The following are isomers of hexane. Draw the structures of two other branched chain isomers of hexane.


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