Video Transcript
How many isomers are there of C7H16?
To answer this question, we will need to determine the number of isomers for the molecular formula C7H16. The molecular formula tells us the exact number and type of atoms of each element in a molecule. A molecule of C7H16 contains seven carbon atoms and 16 hydrogen atoms. Isomers are defined as molecules with the same molecular formula, but a different arrangement of atoms in space. The best way to determine the number of isomers possible for a given molecular formula is to sketch out different arrangements of atoms. We need to ensure that each molecular structure that we draw has exactly seven carbon atoms and 16 hydrogen atoms. Let’s begin by drawing seven carbon atoms connected with only single bonds.
We know that each carbon atom can have a total of four single bonds with other atoms. We also know that the remaining atoms in the molecule are hydrogen atoms. The carbon atoms at each end of the molecule have one single bond. So these two carbon atoms can have an additional three single bonds to hydrogen atoms. The remaining five carbon atoms each have two single bonds. So these carbon atoms can form two additional single bonds with hydrogen atoms. The displayed formula that we just drew has a molecular formula of C7H16. Because this molecule has only single bonds between carbon atoms, it’s classified as an alkane. The name of this alkane is heptane. The stem “hept” indicates the molecule contains seven carbon atoms, and the suffix A-N-E indicates that it’s an alkane.
Now, let’s sketch an isomer that has a carbon chain with six carbon atoms and a branch that contains one carbon atom. Together, the chain and the branch have a total of seven carbon atoms. We can see from the displayed formula that this molecule also has a molecular formula of C7H16. These two molecules are considered structural isomers. They are molecules with the same molecular formula that differ from one another by how the atoms are connected. Isomers also have different names. The isomer we just drew is considered a branched-chain alkane. When naming a branched-chain alkane, we start by finding the longest carbon chain to determine the stem. The longest chain in the molecule contains six carbon atoms. So we’ll need to use the stem “hex.”
Next, we can add the suffix A-N-E. Finally, we need to identify the side chain and locate it on the lowest-number carbon of the chain. The side chain contains one carbon atom. So we’ll need to add the stem “meth” followed by Y-L to the beginning of the name. The side chain is located on carbon number two of the carbon chain. Therefore, the name of this isomer is 2-methylhexane. If we had numbered the carbon atoms in the carbon chain in the opposite direction, the side chain would have been on carbon number five. Therefore, we used the first numbering method to ensure that the side chain was on the lowest-number carbon in the carbon chain.
We could make a similar isomer by moving the methyl group to a different carbon atom in the carbon chain. If the methyl side chain is located on the third carbon atom of the carbon chain instead of the second carbon atom, the isomer is called 3-methylhexane. So far, we’ve drawn a total of three structural isomers, but more isomers are possible. Let’s see how many isomers we can draw that have carbon chains containing five carbon atoms. Two methyl groups could be located on the second and third carbon atoms in the carbon chain, or they could be located on the second and fourth carbon atoms in the carbon chain. The names of these two isomers are 2,3-dimethylpentane and 2,4-dimethylpentane. Di- is used in the names to indicate that there are two methyl side chains.
There are two more isomers that are composed of two methyl groups bonded to a five-carbon chain. If the two methyl groups are bonded to the second carbon atom in the carbon chain, then the name of the isomer is 2,2-dimethylpentane. And if the two methyl groups are bonded to the third carbon atom in the carbon chain, the name of the isomer is 3,3-dimethylpentane. There is one more isomer that has a carbon chain with five carbon atoms. The side chain in this isomer is an ethyl group. And because the ethyl group is bonded to the third carbon atom in the carbon chain, the name of this isomer is 3-ethylpentane.
Finally, we can try to create an isomer that has a carbon chain with four carbon atoms. The total number of carbon atoms in the side chains must be three carbon atoms for the molecule to have a total of seven carbon atoms. In this isomer, two methyl groups are bonded to the second carbon atom in the chain and a third methyl group is bonded to the third carbon atom in the chain. Therefore, the name of the isomer is 2,2,3-trimethylbutane. In the name, tri- indicates that there are three methyl side chains and the stem “but” is used to indicate that there are four carbon atoms in the main carbon chain. Including the first three isomers that we drew, which were heptane, 2-methylhexane, and 3-methylhexane, there are a total of nine isomers for the molecular formula C7H16.
