Alkenes, olefins, have the $"ONE DEGREE OF UNSATURATION...."$
...............A very useful idea that is used to rationalize the formulae of organic compounds is the $"degree of unsaturation"$. An alkane is fully $"saturated"$, and it contains the MAXIMUM ALLOWABLE number of $C-H$ bonds.
From a prior answer on this topic....
Alkanes are $"FULLY saturated"$ and have a general formula of $C_nH_(2n+2)$. Try this out for $"methane,"$ $"ethane,"$ .........$"pentane, etc."$ The number of oxygen atoms present in the formula does not affect this calculation.
Each double bond, each olefinic bond or carbonyl group, OR a ring junction, corresponds to $1""^@$ of unsaturation; i.e. 2 hydrogens LESS than the saturated formula. So according to the scheme, $"ethane"$ has the saturated formula of $H_3C-CH_3$, but $"ethylene"$, $H_2C=CH_2$, and $"acetaldehyde"$, $H_3C-C(=O)H$ has $1^@$ of unsaturation. Halogen atoms count for one hydrogen; for nitrogen atoms, substract $NH$ from the formula before assessing unsaturation; i.e. for $"ethylamine,"$ $H_2NCH_2CH_3$ $rarr C_2H_6$, i.e. $"no degrees of unsaturation"$.
