How To Calculate The Unsaturation Number

In organic chemistry, an "unsaturated" compound is one which contains at least at one "pi" bond–a "double" bond between two of its carbons which uses two electrons from each carbon instead of one. Determining how many pi bonds an unsaturated compound has–its "unsaturation number"–is tedious to do if you choose to draw the compound out by hand. If, on the other hand, you calculate this number using a simple formula chemists have devised, it will take you only a few moments.

Step 1

Replace any halogens–such as bromine, iodine or chlorine–your compound has with hydrogens for the purpose of the calculation. For instance, if your compound is C6H6N3OCl, you would re-write it as C6H7N3O.

Step 2

Disregard any oxygens your compound contains–these are not relevant to the degree of unsaturation calculation. You would now write the example compound as C6H7N3.

Step 3

Subtract each nitrogen from a hydrogen. In this example, you can now represent the compound as C6H4.

Step 4

Calculate the unsaturation number for your compound, which is now in the form CnHm, using this formula Ω = n – (m/2) + 1, where "Ω" is the degree of unsaturation–the number of pi bonds your compound has. For the example compound, C6H4, do this as follows: Ω = 6 – (4/2) + 1 = 6 – 2 + 1 = 5. The compound C6H6N3OCl therefore contains five double bonds.