In this explainer, we will learn how to define a structural isomer and explain why structural isomers display different physical properties.
There are a lot of different organic compounds that are made mostly of carbon. Many of these compounds contain chains of multiple carbon atoms bonded together such as butane, a common fuel used in lighters, which has the molecular formula . The displayed structure of butane is shown below.
There is another molecule that we could draw that has the same molecular formula as butane. If we draw the structure with a branch in the chain, we will get the following structure, a molecule called 2-methylpropane.
These two molecules, butane and 2-methylpropane, both have the molecular formula , but they are different molecules since the atoms are arranged differently.
Whenever we have two or more molecules that have different arrangements of atoms but the same molecular formula, we call those molecules isomers of each other. To be specific, these isomers are examples of structural isomerism. Structural isomers vary in how the atoms are connected to each other, so they will have different structural formulas.
Definition: Isomers
They are molecules with the same molecular formula but a different arrangement of the atoms in space.
Definition: Structural Isomers
They are molecules with the same molecular formula that differ from one another by how atoms are connected.
There are just two structural isomers of , but the number of structural isomers increases quickly the more atoms there are. There is not any quick way to determine the number of structural isomers there will be for a given molecular formula besides trying out different arrangements of atoms.
When trying to determine the number of structural isomers, be careful not to include identical molecules! This may seem obvious, but remember that single bonds can rotate. These rotated bonds can give the appearance of different molecules that are, in fact, the same. Below, we have two structures represented by the displayed formula on the top and a 3D model on the bottom. Both are butane, but they are not isomers of each other.
Also, do not forget that a molecule that is rotated can look quite a bit different. The following four molecules are not isomers of each other. They are, in fact, the same molecule, 2-methylbutane.
Example 1: Determining the Number of Isomers given the Molecular Formula
How many isomers are there of ?
Answer
Isomers are molecules with the same molecular formula but different arrangements of atoms. We need to determine the number of isomers there is for the molecular formula . There is not any trick to finding the number of isomers there is for a given compound; we simply have to try connecting atoms and seeing how many structures we end up with!
Perhaps the most obvious isomer is one where we connect all the atoms together in a straight chain, giving us the molecule heptane.
We could also try creating isomers with branches off of the main chain in a number of different locations. Let’s try making structures with a main chain of six carbon atoms with a methyl () group attached in various parts of the chain. Below, we have 2-methylhexane on the left and 3-methylhexane on the right.
We could try putting the methyl group in different locations other than the two above, but those will not create additional isomers since we could rotate the molecule to give us 2-methylhexane and 3-methylhexane again.
We can also try to attach an ethyl () group to a five-carbon chain. It may seem that there are several isomers like this, but these would simply be twisted versions of some of the above isomers! The only isomer like this is 3-ethylpentane.
Next, we can make structures that have a main carbon chain of five carbon atoms and two methyl () groups attached to the main chain. There are four isomers like this we can draw; their structures are shown below. Starting in the top left and working clockwise, these molecules are 3,3-dimethylpentane, 2,4-dimethylpentane, 2,2-dimethylpentane, and 2,3-dimethylpentane.
Finally, we can draw isomers that have three methyl () groups attached to a four-carbon chain. This gives us 2,2,3-trimethylbutane.
Any other isomers we could try to draw will be rotated or twisted versions of isomers we have already drawn. So, in total, there are 9 isomers with the molecular formula .
There are three different types of structural isomerism in molecules: chain, positional, and functional isomerism.
Chain isomers will differ in the arrangement of the carbon chain; they will have branches in the chain in different locations. The isomers of that we have identified already are examples of chain isomers.
Definition: Chain Isomers
They are molecules with the same molecular formula but a different arrangement of the carbon chain.
Example 2: Identifying the Molecule That Is Not a Chain Isomer
Which of the following is not a chain isomer of ?
- 2-Methylpentane
- 2,3-Dimethylbutane
- 2,2-Dimethylpropane
- 2,2-Dimethylbutane
- 3-Methylpentane
Answer
Chain isomers are molecules that have the same molecular formula, but the carbon atoms that make up the carbon chain of the molecule are arranged differently. For example, the molecular formula given in this question () could be the molecular formula for the molecule hexane, where the carbon atoms are arranged in a straight chain (on the left below). The same number of carbon and hydrogen atoms could also be arranged with a branch in the chain, like the molecule on the right below, which is called 2-methylpentane.
Both these molecules have the molecular formula , but the carbon chain is arranged differently making them chain isomers. This means the correct answer, the molecule that is not a chain isomer of , will not have the molecular formula .
It would be helpful to see the structures of each molecule in the options to determine the correct answer. The structure of option A, 2-methylpentane, is above. We have already identified it as a chain isomer of , so this option can be eliminated.
Option B, 2,3-dimethylbutane, is shown below. It has the molecular formula since the molecular formula is the same and only the carbon chain is arranged differently. This is also a chain isomer of .
Option C, 2,2-dimethylpropane, is shown below. It has the molecular formula . This means that it is not a chain isomer of since it has a different molecular formula.
Option D, 2,2-dimethylbutane, is shown below. It has the molecular formula , so it is also a chain isomer.
Option E, 3-methylpentane, is shown below. It has the molecular formula .
So, the only molecule that is not a chain isomer of is option C, 2,2-dimethylpropane.
Positional isomers will have the same carbon chain, but the position of an atom, double bond, triple bond, or other functional groups on the chain will be different. Below are two different molecules that have the same molecular formula (). On the left is 1-butanol and on the right is 2-butanol. They only differ by where the alcohol group is attached to the carbon chain, so they are positional isomers of each other.
Definition: Positional Isomers
They are molecules with the same molecular formula and functional groups, but the functional groups have different positions on the carbon chain.
Molecules can also have a combination of positional and chain isomerism. Here are two more isomers with the same molecular formula as the previous molecules (). On the left, we have 2-methyl-1-propanol, and on the right, we have 2-methyl-2-propanol.
Cyclic molecules, such as benzene, can also have positional isomers. The molecules below all have the formula and are identical except for the positions of the chlorine atoms attached to the benzene ring. The molecule on the left is 1,2-dichlorobenzene, the molecule in the middle is 1,3-dichlorobenzene, and the molecule on the right is 1,4-dichlorobenzene.
Example 3: Identifying the Molecule That Is a Positional Isomer
Which of the following molecules is a positional isomer of ?
(A)
(B)
(C)
(D)
(E)
Answer
Positional isomers are molecules that have the same molecular formula, but a functional group in the molecule is at a different position on the carbon chain. In this question, we are tasked with finding a positional isomer for . The at the end of this formula suggests that this molecule contains the alcohol or functional group. There are a number of isomers that correspond to this molecular formula; one possibility is the following molecule, 1-butanol.
A positional isomer of this molecule will have the same molecular formula () and will also contain an alcohol or group somewhere on the carbon chain.
Looking through the options, option A is an ether, option C contains a ketone, and option E contains an aldehyde, so these options are not correct since they do not contain the alcohol functional group. Only options B and D contain the alcohol functional group.
To determine whether option B or D is the correct answer, we can turn to the molecular formula of each molecule. The molecular formula of option B is , while the molecular formula of option D is . So, only option B has the correct molecular formula and the correct functional group. The positional isomer of is option B.
The final type of structural isomerism is functional group isomerism. In positional isomerism, the identity of the functional group stays the same. Functional group isomers on the other hand will have completely different functional groups attached to the carbon chain.
Definition: Functional Group Isomers
They are molecules with the same molecular formula but different functional groups.
The following two molecules both have the molecular formula . The molecule on the left is ethanol, and the molecule on the right is dimethyl ether. Though these two molecules have the same molecular formula, they have completely different functional groups! Ethanol contains the alcohol or functional group, while the dimethyl ether is an ether.
Example 4: Identifying the Molecule That Is a Not a Functional Group Isomer
Which of the following structures is not a functional group isomer of ?
(A)
(B)
(C)
(D)
(E)
Answer
Functional group isomers are molecules with the same molecular formula but different functional groups. The options that are functional group isomers will therefore have the same molecular formula () but different functional groups. The answer we are looking for, which is not a functional group isomer of , will have a different molecular formula.
Option A is the molecule propanoic acid, which contains the carboxylic acid functional group. It has the molecular formula , so it is an isomer.
Option B is the molecule 3-hydroxypropanal, which contains two functional groups: an alcohol group and an aldehyde. The molecular formula of this molecule is also , so it is also a functional group isomer.
Option C is 2-propenoic acid, which contains the carboxylic acid functional group and a double bond. The molecular formula is , so it is not a functional group isomer of the other molecules.
Option D is propene-1,1-diol, which contains two alcohol groups and a double bond. Its molecular formula is , so it is an isomer of the other molecules.
Option E is methyl ethanoate, which contains the ester functional group. Its molecular formula is as well, so it is also an isomer of the molecules.
Though all the molecules in the question have different functional groups, all of them shared the same molecular formula () except for option C. This means that option C is not a functional group isomer of the other molecules.
It is probably obvious for these functional group isomers that even though they share a molecular formula, they have very different properties since they belong to entirely different families of compounds! The substances in the above question will have different melting and boiling points, and some of them will be better solvents, while others will be more acidic due to the presence of the carboxylic acid functional group.
In general, structural isomers will often have different properties. This could be due to the presence of different kinds of intermolecular forces, the shape of the molecule, or the presence of different functional groups.
Key Points
- Structural isomers are molecules with the same molecular formula but different structural formulas.
- There are three types of structural isomers: chain, positional, and functional group.
- Chain isomers differ by the arrangement of the carbon chain.
- Positional isomers differ by the position of a functional group on the carbon chain.
- Functional isomers have different functional groups.
- Structural isomers often have different physical and chemical properties.
