Alkenes, olefins, have the #"ONE DEGREE OF UNSATURATION...."#
...............A very useful idea that is used to rationalize the formulae of organic compounds is the #"degree of unsaturation"#. An alkane is fully #"saturated"#, and it contains the MAXIMUM ALLOWABLE number of #C-H# bonds.
From a prior answer on this topic....
Alkanes are #"FULLY saturated"# and have a general formula of #C_nH_(2n+2)#. Try this out for #"methane,"# #"ethane,"# .........#"pentane, etc."# The number of oxygen atoms present in the formula does not affect this calculation.
Each double bond, each olefinic bond or carbonyl group, OR a ring junction, corresponds to #1""^@# of unsaturation; i.e. 2 hydrogens LESS than the saturated formula. So according to the scheme, #"ethane"# has the saturated formula of #H_3C-CH_3#, but #"ethylene"#, #H_2C=CH_2#, and #"acetaldehyde"#, #H_3C-C(=O)H# has #1^@# of unsaturation. Halogen atoms count for one hydrogen; for nitrogen atoms, substract #NH# from the formula before assessing unsaturation; i.e. for #"ethylamine,"# #H_2NCH_2CH_3# #rarr C_2H_6#, i.e. #"no degrees of unsaturation"#.
