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Question Video: Identifying the Incorrectly Named Structural Isomer of a Haloalkane Chemistry

Considering correct nomenclature and isomerism, which of the following is not a correctly named structural isomer of C₃H₆Cl₂? [A] 2,2-Dichloropropane [B] 1,1-Dichloropropane [C] 1,2-Dichloropropane [D] 1,3-Dichloropropane [E] 2,3-Dichloropropane

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Video Transcript

Considering correct nomenclature and isomerism, which of the following is not a correctly named structural isomer of C3H6Cl2? (A) 2,2-Dichloropropane, (B) 1,1-dichloropropane, (C) 1,2-dichloropropane, (D) 1,3-dichloropropane, (E) 2,3-dichloropropane.

First of all, structural isomers are molecules with the same molecular formula that differ from one another by how atoms are connected. A structural isomer of C3H6Cl2 must have the same molecular formula, which means the molecule must contain exactly three carbon atoms, six hydrogen atoms, and two chlorine atoms. To determine which of the molecules in the answer choices is a structural isomer of C3H6Cl2 that is named incorrectly, we first need to draw the displayed formulas so that we can see how the atoms are connected and then we can check that the IUPAC name is correct.

The five molecules are haloalkanes, which is a type of organic molecule that contains only carbon, hydrogen, and one or more halogens. In this case, the halogen is chlorine. The names of haloalkanes contain three main parts: the stem, which is the name of the parent alkane, the prefixes, which tell the number and type of halogen atoms present, and the indexes, which tell us which carbon atom or atoms in the carbon chain the halogen atoms are bonded to. Now, we notice that all of the names have the same stem, which is propane. Prop- indicates the presence of three carbon atoms, and the suffix -ane tells us that the bonds between the carbon atoms are all single bonds.

Let’s make some space to sketch a carbon chain composed of three carbon atoms connected with single bonds for all of the molecules. For now, let’s exclude the hydrogen atoms from our sketches. Now, we’ve probably also noticed that all of the names include the prefix dichloro-, which indicates that there are two chlorine atoms bonded to the carbon chain. What differs among the names are the indexes, which tell us to which carbon atoms the chlorine atoms are bonded. According to IUPAC nomenclature, the carbon chain should be numbered so that the chlorine atoms get the lowest possible numbers. We will need to check that this is true for each molecule.

For molecule (A), let’s number the carbon chain from left to right and draw two chlorine atoms singly bonded to the second carbon atom. If we had numbered the carbon chain from right to left, the two chlorine atoms would still be located on the second carbon atom. So the name of the molecule would not change. Therefore, this molecule is named correctly. Let’s go ahead and add in the hydrogen atoms. After adding in enough hydrogen atoms so that each carbon atom is surrounded by a total of four bonds, we can see that this molecule has a molecular formula of C3H6Cl2. Therefore, answer choice (A) is a correctly named structural isomer of C3H6Cl2. And we can eliminate it because we are looking for the incorrectly named structural isomer.

For molecule (B), after numbering the carbon chain from left to right, we can draw two chlorine atoms singly bonded to the first carbon atom. Now, if we number the carbon chain from right to left, these chlorine atoms would be located on carbon number three. We know that the chlorine atoms must be located on the lowest possible carbon numbers. Therefore, based on what we have drawn, the carbon chain should be numbered from left to right and the correct IUPAC name is 1,1-dichloropropane. After adding in the correct number of hydrogen atoms, we can see that the molecular formula of this molecule is C3H6Cl2. So this molecule is a correctly named structural isomer and can be eliminated.

As before, let’s number the carbon chain for molecule (C) from left to right. Then, we can draw two chlorine atoms singly bonded to the first and second carbon atom. If we number the carbon chain from right to left instead, the chlorine atoms would be located on the second and third carbon atoms of the chain instead of the first and second. Therefore, the correct numbering is from left to right and the correct IUPAC name is 1,2-dichloropropane. After adding in the correct number of hydrogen atoms, we can see that the molecular formula of this molecule is C3H6Cl2. So we can eliminate molecule (C) because it is a structural isomer and it has a correct IUPAC name.

After numbering the carbon chain for molecule (D) from left to right, we can draw two chlorine atoms singly bonded to the first and third carbon atom. And if we number the chain from right to left instead, the chlorine atoms are still located on the first and third carbon atom. Therefore, 1,3-dichloropropane is the correct IUPAC name for this molecule. Finally, after adding in the correct number of hydrogen atoms, we see that the molecule has the molecular formula C3H6Cl2. So molecule (D) is a structural isomer with a correct IUPAC name and can be eliminated.

If we number the carbon chain for molecule (E) from left to right, then we would need to draw two chlorine atoms singly bonded to the second and third carbon atom. If we now numbered the carbon chain from right to left, we’d notice that the chlorine atoms are now located on the first and second carbon atom. Because this numbering gives the chlorine atoms the lowest possible carbon numbers, the correct IUPAC name should be 1,2-dichloropropane, not 2,3-dichloropropane. After adding in the correct number of hydrogen atoms, we see that the molecular formula of this molecule is C3H6Cl2. So molecule (E) is a structural isomer; however, it was not named correctly.

In conclusion, the incorrectly named structural isomer of C3H6Cl2 is 2,3-dichloropropane, or answer choice (E).

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