Question Video: Identifying and Explaining the Difference in Boiling Point Between Isomers Chemistry

(a) Is the boiling point of 2-methylpentane lower or higher than the boiling point of hexane? (b) Is the boiling point of hexane lower or higher than the boiling point of 2,3-dimethylbutane? (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. [B] Branched-chain alkanes can pack more tightly than straight-chain alkanes, and this increases the boiling point due to greater inermolecular forces

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Video Transcript

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.

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