Video Transcript
What is the general formula for the group of the compounds to which the following structural formula is related?
This question is asking us to determine the general formula by using the provided structural formula. Firstly, the molecule shown is classified as a hydrocarbon because it is composed of only hydrogen and carbon atoms. To write the name of this hydrocarbon, we’d use the prefix but- to indicate the presence of four carbon atoms and we’d use the suffix Y-N-E to indicate that a carbon-to-carbon triple bond is present. So the name of this hydrocarbon is butyne. Hydrocarbons, which contain at least one carbon-to-carbon triple bond, belong to a group of compounds called alkynes.
Before we begin identifying the general formula of alkynes, let’s first determine the molecular formula of butyne. A molecular formula is a type of chemical formula that expresses the exact number and type of atoms of each element in a molecule. We already know that the butyne molecule contains four carbon atoms. The molecule also contains three plus three, or six, hydrogen atoms. Therefore, the molecular formula of butyne is C4H6.
Now, to determine the general formula of alkynes, it would be helpful to take a look at a few other simple alkynes. Let’s take a look at the structural formulas of acetylene, also known as ethyne, and propyne. The molecular formula of acetylene is C2H2, and the molecular formula of propyne is C3H4. The general formula C𝑛H2𝑛−2 can be used to predict the molecular formulas of alkynes, like the three shown here. In this formula, 𝑛 represents the number of carbon atoms. For example, in acetylene, we can see that 𝑛 equals two. The number of hydrogen atoms can be found using the expression 2𝑛−2. By substituting 2 in for 𝑛, we get two times two minus two, and the result is two hydrogen atoms.
In propyne, there are three carbon atoms, so 𝑛 equals three. So the number of hydrogen atoms is two times three minus two, or four. Finally, in butyne, there are four carbon atoms, so 𝑛 equals four. Therefore, the number of hydrogen atoms is two times four minus two, or six. In conclusion, the general formula for the group of the compounds to which the structural formula is related is C𝑛H2𝑛−2.
