An unsaturated organic compound has formula #C_nH_(2n+2)#. Each degree of unsaturation REDUCES the hydrogen count by 2, and corresponds to a RING JUNCTION or an olefinic bond. When oxygen appears in the formula, we assess the degree of unsaturation directly (and of course a degree of unsaturation could correspond to #C=O#). Halogens count for #1*H#; we subtract #NH# from the formula if nitrogen is present.
And thus #HC-=CH# has 2 degrees of unsaturation, it is an alkyne.
And #H_3C-CH_3# has 0 degrees of unsaturation.....an alkane.
And #H_2C=CH_2# has 1 degree of unsaturation.....an olefin, or alkene.
And #H_3C-CH_2NH_2-=C_2H_6#...no degrees of unsaturation.
And #C_3H_6# has 1 degrees of unsaturation.....an alkene, i.e. #H_3C-CH-CH_2# OR #"cyclopropane"#.
And #C_3H_8# has 0 degrees of unsaturation.....an alkane.
