Elements of Unsaturation with Heteroatoms
Heteroatoms are atoms different from carbon and hydrogen.
Halogens: Halogen atoms are counted as hydrogen atoms. E.g. C2H4F2 has no elements of unsaturation.
Oxygen atoms: should be ignored when calculating elements of unsaturation. E.g. C2H4O has one element of unsaturation.
Nitrogen atoms: should be counted as half of a carbon atom. E.g. C4H9N should be treated as C4.5H(9+2).Thus C4H9N is two hydrogens short of the “saturated” formula, and it has one element of unsaturation.