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_nH_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 (carbon-carbon 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_nH_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_nH_2n-2, 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:

For non hydrocarbon elements:
Oxygen--ignore
Halides (F, Cl, Br, I)--count as a hydrogen
Nitrogen--count 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:

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).