If we take acetone, #H_3C-C(=O)CH_3#, or butanone, #C_4H_8O#, we can see that these simple formula do conform to the general formula given above. The carbonyl group, #C=O#, represents a degree of unsaturation; that is 2 hydrogens less than that possessed by the comparable alkane. For propanone, with 3 carbon atoms, the comparative saturated species is propane, #C_3H_8#.
A degree of unsaturation represents a double bond (to #C=C#, or #C=O#, or #C=N#, OR a ring structure (i.e. the carbon termini bind to each other, cf. cyclopropane, #C_3H_6#, compared to propane, #C_3H_8#. If there is a nitrogen in the molecule (as often occurs in organic chemistry), we subtract #NH# from the formula before we calculate the degree of unsaturation.
