As you learn about chemistry, you might come across molecules that appear different but have the same formula. This is called isomerism. Isomers are molecules that share a formula but have diverse structures, which results in distinct physical or chemical characteristics. There are two main types of isomerism: structural isomerism and stereoisomerism. Keep reading to find out more! Learn about isomerism and how molecules with the same formula can look different but have different properties.

What is structural isomerism?

Butane and 2-methylpropane have the same molecular formula, C4H10, but different arrangements of atoms. This type of difference between molecules is known as structural isomerism. Structural isomers have the same molecular formula but different arrangements of atoms. For example, butane has a straight or continuous chain of four carbon atoms, while 2-methylpropane has a branched chain with a carbon atom in the center of the Lewis structure bonded to three other carbon atoms. This difference in structure leads to different physical and chemical properties for the two compounds.

Types of structural isomerism

When it comes to structural isomers, there are three main types: chain isomers, positional isomers, and functional group isomers. Chain isomers have the same molecular formula but different arrangements of the carbon chain. For example, butane and 2-methylpropane are chain isomers. Positional isomers have the same molecular formula and the same carbon chain, but the position of the functional group is different. For example, 1-propanol and 2-propanol are positional isomers. Both have the molecular formula C3H8O, but the hydroxyl group (-OH) is attached to a different carbon atom in each compound.

Functional group isomers have the same molecular formula but different functional groups. For example, ethanol and dimethyl ether (CH3OCH3) are functional group isomers. Both have the molecular formula C2H6O, but ethanol has an -OH group while dimethyl ether has an ether (-O-) functional group. These three types of structural isomers demonstrate how small changes in the arrangement of atoms can lead to different chemical and physical properties.

Chain isomerism

No, the above molecule is not a chain isomer of butane and methylpropane. It is simply n-butane rotated about the central carbon, which is known as a 'false' isomer. Both molecules have the same carbon chain - no carbon atom is bonded to more than two other carbon atoms.

Positional isomers, on the other hand, are structural isomers that can be viewed as differing only on the location of a functional group, substituent, or some other feature on a "parent" structure.

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.

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.

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

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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.

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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, C5H12You might draw the straight-chain molecule first:CH3-CH2-CH2-CH2-CH3Next, 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?

Structural 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

To determine the priority of groups in a molecule for the E-Z naming system, we use the Cahn-Ingold-Prelog (CIP) priority rules. These rules assign priority to substituents on a double bond based on their atomic number. The higher the atomic number, the higher the priority.

To apply the CIP priority rules, we first assign a priority number (1-4) to each substituent on the double bond based on its atomic number. Then, we determine whether the high-priority groups are on the same side of the double bond (Z) or opposite sides (E).

For example, let's consider the molecule 2-bromo-1-chloro-1-fluoroethene:

• The bromine atom has the highest atomic number and is assigned priority 1.
• The chlorine atom is assigned priority 2 because it has a higher atomic number than fluorine.
• The fluorine atom is assigned priority 3.
• The ethene carbon closest to the bromine is assigned priority 4, and the other carbon is assigned priority 5.

Since the bromine and chlorine atoms are on the same side of the double bond, this molecule is a Z-isomer.

The CIP priority rules can be used to determine the E-Z isomerism of more complex molecules as well, such as those with multiple double bonds or functional groups.

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

While the above explanation refers to the E-Z isomerism of double bonds, the Cahn-Ingold-Prelog priority rules are used to determine the absolute configuration of chiral molecules. Chiral molecules are molecules that are non-superimposable on their mirror image, and they have at least one chiral center, also known as an asymmetric carbon.

The Cahn-Ingold-Prelog priority rules assign a priority number (1-4) to each substituent on a chiral center based on its atomic number. The higher the atomic number, the higher the priority.

To apply the rules, we first identify the chiral center in the molecule. Then, we assign priority numbers to the four substituents on the chiral center based on their atomic numbers. The substituent with the highest atomic number is assigned priority 1, the next highest is assigned priority 2, and so on.

Next, we orient the molecule so that the lowest priority substituent is pointing away from us (into the page or screen). Then, we look at the remaining three substituents and determine their orientation in space. If the priority 2 and 3 substituents are oriented in a clockwise direction, the molecule has an R configuration (from the Latin rectus, meaning right). If they are oriented counterclockwise, the molecule has an S configuration (from the Latin sinister, meaning left).

For example, let's consider the molecule 2-chlorobutane:

• The carbon with the chlorine atom is the chiral center.
• The chlorine atom is assigned priority 1 because it has the highest atomic number.
• The carbon atom attached to the chiral center is assigned priority 2 because it is attached to two other carbon atoms (atomic number 6), while the hydrogen atom is assigned priority 4.
• The other two carbon atoms attached to the chiral center are equivalent in terms of atomic number, so we move to the next set of atoms out from the chiral center. The carbon atom attached to the priority 2 atom is assigned priority 3 because it is attached to two other carbon atoms (atomic number 6), while the other carbon atom is attached to a hydrogen atom (priority 4).

Since the priority 2, 3, and 4 substituents are oriented counterclockwise, this molecule has an S configuration.

The Cahn-Ingold-Prelog priority rules can be used to determine the absolute configuration of more complex chiral molecules as well.

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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.

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.

To add to the above explanation, optical isomerism arises from the presence of a chiral center in a molecule. A chiral center is an atom with four different substituents attached to it, creating a tetrahedral arrangement of atoms. This chiral center gives the molecule its handedness or chirality.

When a molecule has a chiral center, it can exist in two mirror-image forms, called enantiomers. Enantiomers have identical physical and chemical properties except for their interaction with plane-polarized light. One enantiomer will rotate plane-polarized light clockwise, while the other enantiomer will rotate it counterclockwise. This property is called optical activity, and it can be measured using a polarimeter.

Enantiomers have the same physical and chemical properties except for their interaction with plane-polarized light, chemguide

Many biological molecules, such as amino acids, sugars, and enzymes, exist as enantiomers. In fact, living organisms typically use only one enantiomer of a chiral molecule, while the other enantiomer may be inactive or even toxic. This is why the synthesis of chiral drugs and other molecules is an important area of research in organic chemistry.

In summary, optical isomerism arises from the presence of a chiral center in a molecule, creating two non-superimposable mirror-image forms called enantiomers. These enantiomers have identical physical and chemical properties except for their interaction with plane-polarized light, and they play an important role in biological processes and drug development.

Isomerism - Key takeaways

To add further, there are two types of stereoisomers: geometric isomers and optical isomers. Geometric isomers, also known as cis-trans isomers, arise from restricted rotation around a carbon-carbon double bond. In a cis isomer, the two highest priority groups are on the same side of the double bond, while in a trans isomer, they are on opposite sides. Optical isomers, also known as enantiomers, arise from the presence of a chiral center in a molecule. Enantiomers are non-superimposable mirror images of each other and have identical physical and chemical properties except for their interaction with plane-polarized light. One enantiomer will rotate plane-polarized light clockwise, while the other will rotate it counterclockwise. This property is called optical activity, and it can be measured using a polarimeter. It is important to note that while structural isomers have different physical and chemical properties due to their different structures, stereoisomers have identical physical and chemical properties except for their spatial arrangement of atoms. This can make them difficult to separate and distinguish from each other, but it also makes them useful in fields such as drug development, where the properties of one enantiomer may differ significantly from the other.

Isomerism

What is geometrical 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.

What is optical isomerism?

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.

What is isomerism in chemistry?

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.

What is linkage isomerism?

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.

Which complexes show optical isomerism?

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.

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