Video: Applying Knowledge of Isomers

For statements I and II, state for each if they are true or false. I) CH₃CH₂—O—CH₃ and CH₃CH₂CH₂—OH are isomers. II) CH₃CH₂—O—CH₃ and CH₃CH₂CH₂—OH have the same molecular formula but different structures. If both are true, state if II is a correct explanation for I.

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Video Transcript

For statements one and two, state for each if they are true or false. One, CH3CH2—O—CH3 and CH3CH2CH2—OH are isomers. Two, CH3CH2—O—CH3 and CH3CH2CH2—O—H have the same molecular formula but different structures. If both are true, state if two is a correct explanation for one.

Isomers are molecules or ions that have the same chemical formula. But they have different arrangements of atoms, so they have different structures. Let’s take a look at these two molecules as an example. On the one on the left, we have four carbons that are all attached in a chain. On the one on the right, we have three carbons in a chain. But one of them is branching off the central carbon. So these two molecules have different structures. However, both of them have four carbons and 10 hydrogens. So they both have the molecular formula C4H10. Because they have the same molecular formula but different structures, these two molecules are isomers.

These three molecules all share the molecular formula C4H8. As you can see, they all vary in structure. The first two have different positions of the double bond, and the bottom one, we have all of the carbons attached in a circle. When, again, because they share a molecular formula, but they have different structures, these three molecules are also isomers. This kind of isomer is called a structural isomer. Structural isomers are the most common kind of isomer that you’ll run into, but there are other types of isomers as well. There are cis/trans isomers that vary due to their position of atoms around the double bond. Then there’s also stereo isomers, which are molecules that are mirror images of each other, similar to your right and your left hand.

Now that we know a bit about isomers, let’s return to the question. Statement one is asking us if these two molecules are isomers. CH3CH2—O—CH3 has this structure and if you’re curious, it’s called ethyl methyl ether. CH3CH2CH2—O—H has this structure, and it’s called propanol. These molecules certainly have different structures. So to determine if they’re isomers, we need to figure out if they have the same molecular formula. The molecule on the left has three carbons, eight hydrogens, and one oxygen. So its molecular formula is C3H8O. The molecule on the right also has three carbons, eight hydrogens, and an oxygen. So it also has the molecular formula C3H8O. Since these two molecules have the same molecular formula but different structures, they are isomers. So statement one is true.

Statement two is asking us if these two molecules have the same molecular formula but different structures. We’ve already determined that they do, so statement two is true as well. Since both statement one and two are true, we need to state if two is a correct explanation for one. Statement two says that our molecules have the same molecular formula but different structures, while statement one says that our molecules are isomers, which we’ve defined as chemical species with the same molecular formula but different structures. Statement two matches our definition for isomers exactly. So it’s true that statement two is a correct explanation for one.

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