#"Olefins........."C_nH_(2n)#
Alkanes are saturated molecules. Their formulae correspond to #C_nH_(2n+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, #H_2C=CH_2,# and #H_3C-CH=CH_2#, with that of their saturated precursors, #H_3C-CH_3#, and #H_3C-CH_2CH_3#. What are the #C_nH_m# 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. For a practical example of how we can use this idea to interrogate a formula see here.
