Video: Identifying a Pair of Isomers from a Set of Pairs

Which of the following pairs contain different isomers of the same compound? [A] CH₃CH₂CH₂Cl and ClCH₂CH₂CH₃ [B] FCH₂CH₂Cl and ClCH₂CH₂F [C] CH₃CH₂CH₂OH and CH₃CH(OH)CH₃ [D] CH₃CH₂COCH₃ and CH₃CH₂CHO [E] CH₃CH₂CH₂CH₃ and CH₃CH₂CH₃

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Video Transcript

Which of the following pairs contain different isomers of the same compound? A) CH₃CH₂CH₂Cl and ClCH₂CH₂CH₃. B) FCH₂CH₂Cl and ClCH₂CH₂F. C) CH₃CH₂CH₂OH and CH₃CH(OH)CH₃. D) CH₃CH₂COCH₃ and CH₃CH₂CHO. E) CH₃CH₂CH₂CH₃ and CH₃CH₂CH₃.

Firstly, we need to define what is meant by the term isomers, which is the keyword found in this question. Structural isomers are molecules that have the same molecular formula, that is the same number and type of atoms in the molecule, but have the atoms connected in different ways. Isomers are not to be confused with isotopes. Isotopes are atoms of the same element containing different numbers of neutrons in their nuclei.

There are three types of structural isomer, chain, position, and functional group. In chain isomers, we see the same functional group but a different arrangement of the carbon atoms. Here, we have butane and methylpropane both sharing the same molecular formula C₄H₁₀. Therefore, they are isomers. And in this case, they are chain isomers because the carbon atoms are arranged in different ways.

In position isomers, we see the same number and type of atom and the same carbon skeleton. But in fact, we see some atoms attached to different carbon atoms. In the example here, we see chlorine attached to a different carbon in each molecule. These are two different position isomers of chlorobutane.

In functional group isomerism, we see molecules sharing the same molecular formula but containing different functional groups. Here, we have an alkene on the left and a cyclic alkane on the right. Both share the same molecular formula. They are functional group isomers.

For each pair of molecules, the first thing we need to ask ourselves is, do they share the same molecular formula? Only then can they in fact be possible isomers. In A, we see two molecules with the molecular formula C₃H₇Cl. They could be possible isomers. In B, we see a pair of molecules with the molecular formula C₂H₄ClF. These are also possible isomers.

In C, we see a pair of molecules sharing the molecular formula C₃H₈O. These are also possible isomers. In D, we have two molecules with different molecular formulae. One contains less carbon atoms than the other. One contains four and one contains three. Therefore, they could not be possible isomers. Lastly, in E, we have a pair of molecules with different molecular formulae. They cannot be isomers.

In this way, we have eliminated D and E from our inquiry. Now, we need to look more closely at the structural arrangement for each pair. If we draw the displayed formulae for our three remaining pairs, we can see if the atoms are in fact arranged differently.

Pair A looks tempting. It looks as if we may have a position isomer involving the chlorine atom in a different position on a different carbon atom. However, if we flip the molecule over through 180 degrees, much like flipping a coin, we discover that we in fact have the same molecule. This is like a coin with two heads sides. One side is exactly the same as the other. They’re just 180 degrees apart. Therefore, answer A is not correct.

For the pair of molecules in response B, the same argument applies. Even if we drew the chlorine or fluorine atoms in what appears to be a different place, due to bond rotation around the carbon carbon single bonds, the molecules are in fact the same structure. We have now eliminated responses A, B, D, and E.

The pair of molecules in C share the same functional group. They are both alcohols. In propan-1-ol, the hydroxy group is attached to carbon number one in the carbon chain. In propan-2-ol, the hydroxy group is attached to carbon number two in the carbon chain. They are, therefore, structurally different. These two molecules share the same molecular formula. And they have a different structure. This is the correct answer

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