Which of the following is a saturated hydrocarbon? A) C₂H₂, B) C₂H₆, C) CH₃OH, D) CBr₄, or E) HCOOH.
A hydrocarbon is a compound that contains only carbon and hydrogen. An example would be methane, which has the chemical formula CH₄. Methane is an extremely simple hydrocarbon. With this information, we can rule out answer choices C, D, and E because they are compounds that don’t just contain carbon and hydrogen.
When a hydrocarbon is saturated, that means that it only contains single bonds. When a hydrocarbon contains only single bonds, it has the maximum amount of hydrogens possible, given the amount of carbon. So these molecules are saturated with hydrogens, hence the name. If a hydrocarbon has a double or triple bond, it will be missing some of the hydrogens possible. So we call that unsaturated.
If you’ve ever looked at the nutrition label for food, you’ll see that there’s unsaturated and saturated dietary fats. The meaning here is the same. Saturated fats contain only single bonds and unsaturated fats have double bonds between some of the carbons. So we need to figure out between A and B which compound only has single bonds.
To determine which molecule is saturated, we’ll either have to look at the structure to see which one only contains single bonds. Or we can follow this general rule for saturated hydrocarbons, which says that saturated hydrocarbons will have the general formula C𝑛H₂𝑛+2.
Our first answer choice is C₂H₂, which is an organic compound called ethyne. [Ethyne] clearly breaks our C𝑛H₂𝑛+2 rule for saturated hydrocarbons and in this case would be two. So we clearly don’t have the correct number of hydrogens to have a saturated hydrocarbon. And if we look at [ethyne] structure, it has a triple bond, not only single bonds.
Our last answer choice is called ethane. From ethane structure, we can see that it contains only single bonds. And if we look at a rule of thumb for saturated hydrocarbons, here 𝑛 is still two. And two times two plus two equals six. So C₂H₆ is a formula for a saturated hydrocarbon. Either way, answer choice B, C₂H₆ or ethane, is a saturated hydrocarbon.
It should be mentioned though that sometimes hydrocarbons form compounds that are in rings. And for these compounds, the rule of thumb that I gave is violated even though these compounds may contain only single bonds. So they’re saturated. This rule of thumb only applies to hydrocarbons that are not in a ring, which will be the majority of hydrocarbons that you run into at this level.