Compare ethane to ethylene to acetylene,
i.e. H_3C-CH_3, H_2C=CH_2, HC-=CHH3C−CH3,H2C=CH2,HC≡CH
Ethane is fully saturated, and has a formula of C_nH_(2n+2)CnH2n+2; here n=2n=2, and there are 6 hydrogen atoms.
Ethylene has a so-called 1^@1∘ degree of unsaturation, with 22 hydrogens LESS than the equivalent saturated formula. That is ethylene has 4 hydrogens, 2 hydrogens LESS than the saturated formula.
And acetylene has 2^@2∘ so-called degrees of unsaturation, with 44 hydrogens LESS than the equivalent saturated formula.
And so for a given formula each "degree of unsaturation"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 NHNH from the formula before assessment. Halogens count for 1 hydrogen.
And thus cyclohexanone, C_6H_10OC6H10O has 2^@2∘ of unsaturation. Acetylene, H-C-=CHH−C≡CH, also has 2^@2∘ of unsaturation. Napthalene, C_10H_8C10H8 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,"degree of unsaturation, and accurately forecast the number of olefinic bonds, and ring junctions the molecule is likely to have.
