Video Transcript
Which of the following is not a chain isomer of C6H14? (A) 2-Methylpentane, (B) 2,3- dimethylbutane, (C) 2,2-dimethylpropane, (D) 2,2-dimethylbutane, (E) 3-methylpentane.
In this question, we are given the molecular formula of a hydrocarbon. The molecule is composed of six carbon atoms and 14 hydrogen atoms. We are being asked to determine which of the molecules listed in the answer choices is not a chain isomer of C6H14. Chain isomers are molecules with the same molecular formula but a different arrangement of the carbon chain. To begin, let’s draw the displayed formula of a straight-chain alkane using the molecular formula given in the problem. The molecule contains a chain of six carbon atoms connected by single bonds and 14 hydrogen atoms. The name of this molecule is hexane.
To help us identify which of the molecules listed in the answer choices is not a chain isomer of hexane, it would be helpful to draw displayed formulas for each of the molecules. First of all, all of the molecules in the answer choices are branched- chain alkanes. Each name includes one or more numbers which tells the location of the side chains, followed by the name of the side chain. If there’s more than one of the same type of side chain, a prefix is used. For example, when the prefix di- appears before methyl in the name, it indicates that there are two methyl side chains. Finally, the last part of the name is the name of the carbon chain. If there’s only one type of side chain in the molecule, the name of the side chains and the carbon chain are written together as one word.
Let’s go ahead and clear some space now to begin drawing displayed formulas for the answer choices. In answer choice (A), the carbon chain is pentane and a methyl side chain is located on the second carbon atom in the carbon chain. The stem pent- in pentane indicates that the carbon chain in the molecule contains five carbon atoms. Let’s sketch out the carbon chain by connecting five carbon atoms with single bonds. The stem meth- indicates that the side chain contains one carbon atom. So we need to connect one carbon atom with a single bond to the second carbon atom in the carbon chain. After adding in the hydrogen atoms, we can see that the molecule contains six carbon atoms and 14 hydrogen atoms.
The 2-methylpentane molecule has the same molecular formula as hexane but a different carbon-chain arrangement. Therefore, 2-methylpentane is a chain isomer of C6H14. In answer choice (B) 2,3-dimethylbutane, the stem but- tells us that the carbon chain has four carbon atoms. The prefix di- indicates that there are two methyl groups. One methyl group is located on the second carbon atom in the carbon chain, and the second methyl group is located on the third carbon atom in the carbon chain. The molecule contains six carbon atoms and 14 hydrogen atoms. So it has the same molecular formula as the molecule given in the problem. Therefore, 2,3-dimethylbutane is a chain isomer of C6H14.
In answer choice (C) 2,2-dimethylpropane, the stem prop- tells us that the carbon chain contains three carbon atoms, and the prefix di- indicates there are two methyl side chains. Both methyl side chains are bonded to the second carbon atom in the carbon chain. This molecule contains five carbon atoms and 12 hydrogen atoms, which means its molecular formula is C5H12. 2,2-Dimethylpropane does not have the same molecular formula as the molecule given in the problem. Therefore, it’s not a chain isomer of C6H14.
To sketch structural formulas for the molecules in answer choices (D) and (E), we can use some of the other answer choices to help us. 2,2-Dimethylbutane is very similar to answer choice (B), which is 2,3-dimethylbutane. Instead of the methyl side chains being bonded to the second and third carbon atoms in the carbon chain, in 2,2-dimethylbutane, both metal side chains are bonded to the second carbon atom in the carbon chain. Changing the location of the methyl side chains does not change the molecular formula of the molecule, which is C6H14. Therefore, 2,2-dimethylbutane is yet another chain isomer of C6H14.
As for answer choice (E) 3-methylpentane, this molecule is very similar to answer choice (A). Instead of the methyl side chain being located on the second carbon atom of the five-carbon chain, in 3-methylpentane, the methyl side chain is bonded to the third carbon atom in the five-carbon chain. The molecular formula of 3-methylpentane is C6H14. Therefore, 3-methylpentane is a chain isomer of C6H14.
Which of the following is not a chain isomer of C6H14? 2,2-Dimethylpropane, or answer choice (C).
