Video Transcript
What is the general formula for an alkene with a single carbon-carbon double bond?
An alkene is an unsaturated hydrocarbon that contains at least one carbon-carbon double bond. The question asks us to determine the general formula for an alkene that contains a single carbon-carbon double bond. Since an alkene is a hydrocarbon, we know that the general formula will contain both carbon and hydrogen atoms. It’s our job to determine how many of each.
To do this, let’s take a look at three different alkenes that each contain a single carbon-carbon double bond. The simplest alkene is ethene. Ethene contains two carbon atoms and four hydrogen atoms. The next largest alkene is propene. A molecule of propene contains three carbon atoms and six hydrogen atoms. An alkene containing four carbon atoms is called butene. More specifically, the butene drawn here is but-1-ene. A molecule of butene contains four carbon atoms and eight hydrogen atoms.
Looking at each of the chemical formulas, we can see that there are twice as many hydrogen atoms as carbon atoms in an alkene that contains a single carbon-carbon double bond. So, if an alkene contained 𝑛 number of carbon atoms, then the molecule must contain two times 𝑛 hydrogen atoms. To verify that this is the correct general formula for an alkene with a single carbon-carbon double bond, let’s substitute a value of five for 𝑛. This molecule must contain five carbon atoms, one carbon-carbon double bond, and 10 hydrogen atoms. This is the structure of an alkene, pentene, more specifically pent-1-ene. This means that our general formula is correct.
So the general formula for an alkene with a single carbon-carbon double bond is C𝑛H2𝑛.
