Lesson Video: Structural Isomerism | Nagwa Lesson Video: Structural Isomerism | Nagwa

Lesson Video: Structural Isomerism Chemistry • Third Year of Secondary School

In this video, we will learn how to define a structural isomer and explain why structural isomers display different physical properties.

18:05

Video Transcript

In this video, we will learn what a structural isomer is and why compounds that are structural isomers of each other display different physical properties.

What are structural isomers? Organic compounds all have a carbon-based skeleton or backbone, for example, this five-carbon chain. So carbon is the basis of organic compounds bonded to other common elements such as hydrogen, oxygen, sulfur, and nitrogen. Since carbon can form four bonds, carbon atoms can combine together in a variety of ways, forming many compounds. For example, this five-carbon molecule has 12 hydrogens. This is the structural formula of this compound, and this is its molecular formula. The name of this compound is pentane or n-pentane, the n indicating that it is a straight-chain alkane. “-ane” indicates that there are only single bonds between the carbon atoms, so the molecule is fully saturated. And “pent-” indicates that there are five carbon atoms.

We said that because carbon atoms can form four bonds, they can arrange themselves in a variety of ways, forming many compounds. Because of this ability, compounds with the same molecular formula can form different structures. This carbon molecule also has the molecular formula C5H12. It has five carbons and 12 hydrogens. But the structure or arrangement of the carbons and hydrogens is different to the first molecule. Here, the carbon chain is branched. We have a branched-chain alkane. Its IUPAC name is 2-methylbutane, “-ane” because all the bonds between the carbons are saturated, “but-” because there are four carbons in the main chain, and “2-methyl” because there is a methyl group on carbon number two.

Note that the structure of 2-methylbutane can be represented on paper in several different ways, with the methyl group at the bottom or shifted one to the right or on the top in this position or on the top in this position. The base chain can even be drawn in sort of a bent or curved structure. All of these variance represent the same compound 2-methylbutane.

Remember that in reality single bonds can rotate and chain-like molecules can bend to a certain degree. We need to be careful when looking at structural formulas and identify whether they represent the same structural formula just represented differently or in a different orientation. For simplicity, let’s write 2-methylbutane like this. Now there is a third structural formula for the molecular formula C5H12. And that is this molecule, whose IUPAC name is 2,2-dimethylpropane, “-ane” because all carbon bonds are saturated, “prop-” because they are three carbons in the base chain, and “2,2-dimethyl” because there are two methyl groups on the second carbon atom. This molecule is also a branched-chain alkane.

These three compounds or molecules have the same number of atoms of each element, five carbons and 12 hydrogens. But their atoms are connected to each other in different ways. We say they are structural isomers of each other, in other words, molecules or compounds with the same molecular formula but a different bonding arrangement of atoms. The key here is that the atoms are bonded in a different arrangement. This gives structural isomers. The term isomer on its own has a very similar definition: molecules with the same molecular formula but a different arrangement of atoms in space. We won’t go into the subtle differences between these definitions here. But we can say that with an increasing number of carbon atoms, there is an increasing number of structural isomers.

Let’s look at the types of structural isomerism. There are three types of structural isomers: chain isomers, positional isomers, and functional isomers. Chain isomers are molecules with the same molecular formula but different arrangements of the carbon chain. The three isomers we saw earlier are all chain isomers. We had five carbons in our example arranged in a straight chain, which we can represent like this. We had a four-chain carbon with a methyl side group. Let’s represent it like this for simplicity to show the branch. And we had a three-carbon chain structural isomer with two methyl groups, which we can simplify like this, showing two branches in the chain.

The second type of structural isomer, positional isomers, are molecules with the same molecular formula and functional groups. But the functional groups have a different position in the carbon chain. Here is an example. This is but-2-ene. “-ene” tells us there’s a carbon-carbon double bond, “but-” telling us there are four carbons in the chain, and the “2” indicating that the double bond starts on carbon number two. This positional isomer is but-1-ene whose structure differs from but-2-ene by the position of the functional group. The carbon-carbon double bond starts on carbon number one.

Another example of positional isomerism is seen in cyclic or ring-like carbon structures. The structure is 1,2-dibromobenzene. But we can put the bromine on a different carbon atom to give a different positional isomer. This is 1,3-dibromobenzene, and this is 1,4-dibromobenzene. The second bromine functional group is in a different position in each of the positional isomers of dibromobenzene. Let’s put our first example of positional isomers back. We had but-2-ene with the functional group on the second carbon and but-1-ene with the functional group on the first carbon.

Let’s look at the last type of structural isomerism, functional isomerism. Functional isomers, also known as functional group isomers, are molecules or compounds with the same molecular formula but different functional groups. Here is an example of two functional isomers. The first molecule is propanal, the L indicating that there is an aldehyde functional group on this three-carbon chain indicated by “prop-.” And the second molecule is propanone, the “-one” suffix indicating that there is a keytone functional group. And again, we have a three-carbon chain. Both these functional isomers have a molecular formula C3H6O. But their atoms are arranged in a different way, giving different functional groups.

Let’s simplify these last structures. So far, we’ve seen that chain isomers have the same molecular formula but a different arrangement of carbon atoms in the chain. Positional isomers have the same molecular formula. But the functional group has a different position in the carbon chain. And functional isomers have the same molecular formula as each other but a different functional group to each other.

It is interesting to note that the subtle differences in the chain arrangement between chain isomers in the position of the functional group between positional isomers and in the type of functional group between functional isomers result in these isomers having different physical properties. For example, n-pentane has a boiling point of 36.1 degree Celsius, whereas 2-methylbutane has a boiling point of 27.9 degrees Celsius. And this isomer 2,2-dimethylpropane has a way different boiling point of only 9.5 degrees Celsius. Again, different physical properties between isomers occur as a result of different structures. Now it’s time to practice.

This is a three-part question. Part (a) is the boiling point of 2-methylpentane lower or higher than the boiling point of hexane? Part (b) is the boiling point of hexane lower or higher than the boiling point of 2,3-dimethylbutane? Part (c) which of the following correctly explains this difference in boiling points? (A) Straight-chain alkanes can pack more tightly than branched-chain alkanes, and this increases the boiling point due to greater intermolecular forces. Or (B) branched-chain alkanes can pack more tightly than straight-chain alkanes, and this increases the boiling point due to greater inermolecular forces

We are asked to compare the boiling point of two substances, 2-methylpentane and hexane. We do not know the boiling point values of these two compounds. But since we are given the two compound names, we can start by drawing their structures. 2-Methylpentane has five carbons in the base chain, and all the bonds between these carbons are fully saturated or single bonds. And there is a methyl group on carbon number two. So if we number our carbons one, two, three, four, and five, we can place our methyl group on the second carbon and then fill in the rest of the hydrogens. This is the structural formula of 2-methylpentane. And its molecular formula is C6, because there are six carbons, H14, for the 14 hydrogen atoms. It is a branched-chain alkane, with the methyl group forming the branch.

Now let’s investigate hexane in a bit more depth. Hexane is a six-carbon chain, and the bonds between the carbons are fully saturated or single bonds. Here are the six carbons. There are no other side groups or functional groups, so we can fill in all the hydrogens. We know that carbon forms four bonds, and because all the carbon-carbon bonds are saturated, we can fill in the hydrogens until each carbon has a total of four bonds. And this gives us a molecular formula for hexane of C6H14. Hexane is a straight-chain alkane because it has no side branches.

Notice that 2-methylpentane and hexane have the same molecular formula. They both have six carbon atoms and 14 hydrogen atoms. However, their structures are different. We say they have different structural formulas. Compounds with the same molecular formula but different structural formulas are called isomers. These two compounds are structural isomers. They are molecules or compounds with the same molecular formula but a different bonding arrangement of atoms.

Let’s simplify their structures. Drawing organic structures in a simplified form like this will help us compare or understand their relative boiling points. Since we are asked about the boiling points, we can assume that these two substances are in the liquid phase. Liquid molecules have a fair amount of motion and movement, and single bonds can rotate. Long chains can also bend. However, hexane molecules are often likely to be packed together like this. Although this is a simplistic diagram, we can see that because hexane is a straight-chain alkane molecule, the molecules can pack relatively closely, even though they have movement, because they’re in the liquid phase.

The Van der Waals forces of attraction between these nonpolar molecules, in this case London dispersion forces, is relatively high because the molecules are close together. However, in liquid 2-methylpentane, the molecules cannot pack as closely together because of their branched structure. And as a result, the Van der Waals forces of attraction between these nonpolar molecules is relatively weaker.

The close packing in hexane, because of its structure and the associated stronger Van der Waals forces of attraction, should therefore result in a relatively high boiling point. Because more energy is required to overcome these forces of attraction and separate the molecules during boiling. But because of the looser packing in 2-methylpentane due to its branch structure and its relatively weaker Van der Waals forces of attraction between molecules, we can deduce that there is a relatively low boiling point. Because less energy would be required to separate the molecules during boiling and overcome the Van der Waals forces of attraction.

So we can conclude that the boiling point of 2-methylpentane is probably lower than the boiling point of hexane. And in reality, this is the case. Hexane’s boiling point is 69 degrees Celsius and its structural isomer 2-methylpentane has a boiling point of 60 degrees Celsius.

Part (b) is the boiling point of hexane lower or higher than the boiling point of 2,3-dimethylbutane?

Again, we are asked to compare boiling point, this time between hexane and 2,3-dimethylbutane. The structural formulas of these compounds are drawn here. Hexane is a six-carbon chain. It is a straight-chain alkane. 2,3-Dimethylbutane has four carbons in its base chain, with 2 methyl groups, one on carbon number two and one on carbon number three. It is a branched-chain alkane with two branches. Again, these two compounds are structural isomers of each other. They both have six carbon atoms and 14 hydrogen atoms. But the atoms are arranged in a different way. The bonding is different. We say they are chain isomers of each other because although they have the same molecular formula, their chain arrangement is different.

Let’s simplify their structures and have a look at how they’d pack in a liquid phase. Remember, there is constant movement. But hexane’s molecules, because of their straight-chain nature, will arrange themselves closer to each other. And as a result, the intermolecular Van der Waals forces are stronger. Molecules of 2,3-dimethylbutane pack or arrange themselves less closely. And therefore, their intermolecular forces of attraction are weaker. More energy is required to overcome the attractive forces between hexane molecules. And so, hexane will have a higher boiling point. And their boiling points are for hexane 69 degrees Celsius and its isomer 2,3-dimethylbutane, 58 degrees Celsius.

Part (c) which of the following correctly explains this difference in boiling points? (A) Straight-chain alkanes can pack more tightly than branched-chain alkanes, and this increases the boiling point due to greater intermolecular forces. Or (B) branched-chain alkanes can pack more tightly than straight-chain alkanes, and this increases the boiling point due to greater inermolecular forces.

We have already seen in two examples that straight-chain alkanes can get closer together than branched-chain alkanes, even when they have the same number of carbon and hydrogen atoms, in other words when they are isomers of each other. The closer the molecules can get, the greater the intermolecular forces of attraction and the higher the boiling point.

Let’s summarize what we’ve learnt. We learnt that structural isomers are organic molecules with the same molecular formula, but with a different bonding arrangement of atoms. We saw that there are three types of structural isomers: chain isomers, where the arrangement of the carbon chain differs; positional isomers, where the position of a functional group on the carbon chain differs; and functional isomers, where the functional group differs. And that because of their differences in structure, structural isomers have different physical properties, for example, boiling point.

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