Calculating a Molecules Degree of Unsaturation

Because of its double bond, an alkene has fewer hydrogens than an alkane with the same number of carbons—C H2 for an alkene versus C H2 . 2 for an alkane—and is therefore referred to as unsaturated. Ethylene, for example, has the formula C2H4, whereas ethane has the formula C2H6. 

In general, each ring or double bond in a molecule corresponds to a loss of two hydrogens from the alkane formula C H2 +2. Knowing this relation- 

Let s assume that we want to find the structure of an unknown hydrocarbon. A molecular weight determination on the unknown yields a value of 82, which corresponds to a molecular formula of CgHio. Since the saturated Cg alkane (hexane) has the formula CgH, the unknown compound has two fewer pairs of hydrogens (H14 - Hjo = H4 = 2 H2), and its degree of unsaturation is two. The unknown therefore contains two double bonds, one ring and one double bond, two rings, or one triple bond. There s still a long way to go to establish structure, but the simple calculation has told us a lot about the molecule. 

Similar calculations can be carried out for compounds containing elements other than just carbon and hydrogen. 

C5H9N = CgHg Two unsaturations one ring and one double bond 

4-Methyl-13-pentadiene Cyclohexene Bicyelo[3.1.0jhexane 4-Methyl-2-pentyne 

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