Defining
saturation and unsaturation
All alkanes
have the exact same empirical formula. Specifically, for every
carbon in an alkane there will be twice as many hydrogens
plus two, and so every alkane has the formula C_{n}H_{2n+2},
where n represents the number of carbons. Alkanes are said
to be hydrogen saturated since they have the maximum number
of hydrogens possible. Hydrocarbons that contain a single
alkene (carboncarbon double bond) or contain a single ring
are said to contain a single degree of unsaturation because
adding either a ring or a double bond requires loss of two
hydrogens, and so these molecules have the formula C_{n}H_{2n}.
Hydrocarbons that contain two double bonds, 2 rings, or an
alkyne (a triple bond) are said to contain two degrees of
unsaturation and have the formula C_{n}H_{2n2},
representing a loss of an additional two hydrogens.
Determining degrees of unsaturation from a
formula
A general
formula for calculating the degrees of unsaturation from a
molecular formula is the following:
Degrees
of Unsaturation = [(Number of Carbons x 2) + 2 
Number of Hydrogens] / 2

For non
hydrocarbon elements:
Oxygenignore
Halides (F, Cl, Br, I)count as a hydrogen
Nitrogencount as one half of a carbon
Determining
degrees of unsaturation from a structure
Determining
the degrees of unsaturation from a structure rather than a
formula is easy:

Double bonds add one degree of unsaturation

Rings
add one degree of unsaturation

Triple
bonds add two degrees of unsaturation

The total
unsaturation in a molecule is the sum of each of the molecules
elements of unsaturation. For example, a hydrocarbon containing
a ring and an alkyne is said to have three degrees of unsaturation
(one for the ring and two for the triple bond).
Knowing
the degrees of unsaturation in a molecule is quite useful
when trying to determine a structure, because it tells whether
double or triple bonds or rings are present, and how many
(although it doesn't tell you which ones are present).
PRACTICE
PROBLEM:
Determine the degrees of unsaturation from the compounds or
formulas below. Click on the picture for the answer. (Requires
Javascript).
