Compare ethane to ethylene to acetylene,
i.e. H_3C-CH_3, H_2C=CH_2, HC-=CH
Ethane is fully saturated, and has a formula of C_nH_(2n+2); here n=2, and there are 6 hydrogen atoms.
Ethylene has a so-called 1^@ degree of unsaturation, with 2 hydrogens LESS than the equivalent saturated formula. That is ethylene has 4 hydrogens, 2 hydrogens LESS than the saturated formula.
And acetylene has 2^@ so-called degrees of unsaturation, with 4 hydrogens LESS than the equivalent saturated formula.
And so for a given formula each "degree of unsaturation" corresponds to a double bond OR a ring. When we add oxygen to the formula, we assess the degree of unsaturation directly. Where there is nitrogen we substract NH from the formula before assessment. Halogens count for 1 hydrogen.
And thus cyclohexanone, C_6H_10O has 2^@ of unsaturation. Acetylene, H-C-=CH, also has 2^@ of unsaturation. Napthalene, C_10H_8 has how many degrees of unsaturation?
What is trivial in these simple case, becomes useful in a more complicated formula. If you are given a chemical formula, without knowing ANYTHING else, you can assess the "degree of unsaturation," and accurately forecast the number of olefinic bonds, and ring junctions the molecule is likely to have.
