Olefins.........CnH2n
Alkanes are saturated molecules. Their formulae correspond to CnH2n+2. Try it out with methane, ethane,.....hexane. Olefins have ONE so-called degrees of unsaturation with respect to equivalent alkanes. For instance, compare the formula of ethylene and propylene, H2C=CH2, and H3C−CH=CH2, with that of their saturated precursors, H3C−CH3, and H3C−CH2CH3. What are the CnHm formula? How many degrees of unsaturation does acetylene have?
A degree of unsaturation corresponds to the presence of an olefinic bond, HC=CH, OR a ring junction (the ring junction reduces the overall hydrogen count by 2). Halogens count for 1 hydrogen, oxygens we ignore (though a carbonyl bond could be the source of unsaturation), and if there is nitrogen present we subtract NH from the formula before we assess saturation.
For more details, see [here.](https://socratic.org/questions/how-do-you-classify-alkanes-alkenes-and-alkynes) For a practical example of how we can use this idea to interrogate a formula see [here.](https://socratic.org/questions/how-many-and-what-are-the-isomers-of-c-4h-8?source=search)
