What is the name of the alkene with the molecular formula C₃H₆? Propane, Butane, Ethene, Propene, or Butene.
We’re asked to name the compound C₃H₆. And the question already tells us that it’s an alkene, which gives us a clue. So, let’s remind ourselves what we know about naming alkenes. The name of an alkene is made up of two parts: a prefix and a suffix. The prefix denotes how many carbons are in the molecule. And the suffix for all alkenes is “ene”.
Simply using this information about the suffix means that we can rule out two of the potential answers straightaway. Both propane and butane end in ane. This makes them alkanes. So, we can rule those out. Now, we just have to work out what the correct prefix is. Remember that the prefix denotes how many carbons are in the compound.
The prefix for one carbon is “meth”. An example would be methane. Two carbons is “eth”, for example, ethanol, an alcohol which contains two carbons. For three carbons, the prefix is “prop”. An example would be propane. And four carbons would be “but”, for example, in butane. So now, let’s work out which of the answers is correct. The remaining three answers all end in “ene”. So, we know that the suffix is correct.
Our molecular formula is C₃H₆. So, we need a prefix which denotes three carbons. Three carbons is “prop”. So, if it begins with “prop” and ends in “ene”, we’re left with propene. Ethene is an alkene containing two carbons. And butene is an alkene containing four carbons. So, our answer for C₃H₆ is propene. Out of interest, here is the structure of propene. It’s an alkene because it contains a carbon–carbon double bond. And we can see that it does indeed contain three carbons and six hydrogens.