The Alkane Formula
Before we can answer the question of what is the alkane formula, we have to ask ourselves what is an alkane. An alkane is a simple hydrocarbon containing carbon and hydrogen single bonded to each other, with a carbon backbone. Any molecule with this structure is going to have the formula CnHn+2, where n is any integer. Let’s look at some examples of this:
Please note, that this formula doesn’t tell us anything about the structure of the alkane, it only tells us that is fully unsaturated. For example, both pentane and 2,2-dimethylpropane will both be fully unsaturated and have the same molecular formula, but completely different structures and physical properties therefore we have to be very careful not to make assumptions about the structure of an alkane just based on its formula.
It’s a nice trick to know that any time you see a molecule with the formula CnHn+2, you know it is a fully saturated hydrocarbon, meaning that there are no double or triple bonds only single bonds. The degree of unsaturation, or “some of double bonds and rings” (SODAR) can be expressed using a similar formula. Here is a great post discussing the SODAR formula.
A quick rule of thumb to be able to determine if a molecule is saturated or not is just to ask if there are more than double the number of hydrogens as compared to carbon in the molecule. For example, if you have a hydrocarbon with the formula C5H12, we know that there are more than double the number of hydrogen therefore this is saturated. If we have a structure with the formula C5H10, there are only double the number of hydrogens therefore this is unsaturated at some point in the molecule. What this little rule of thumb doesn’t tell us is where that unsaturation is or what type of unsaturation it is, a double bond or a ring.
Dr. Michael Pa
Dr. Michael Pa got a bachelors degree in chemistry from Binghamton University, a masters degree in organic chemistry from the University of Arizona and a Ph.D. in organic chemistry from the University of Arizona. His research focus was on novel pain killers which were more potent than morphine but designed to have fewer side effects. There may even be a patent or two that came out of it. Prior to all of this, he was a chemist at Procter and Gamble. After all of that, he (briefly) worked as a post-doctoral assistant at Syracuse University, working on novel organic light-emitting diodes (OLEDs). In between, he did NOT compete at the 1996 Olympics, make the Atlanta Braves opening day roster, or become the head coach of the Indiana Pacers, as he had intended. #fail During this entire time, he always loved helping students, especially if they were struggling with organic chemistry. In 2006, Dr. Pa founded AceOrganicChem.com in order to make learning organic chemistry fast and easy. 14 years and about 60,000 students later we are still helping students to learn organic chemistry one reaction at a time at https://www.aceorganicchem.com
