CHEM 121 glossary

Unsaturation count

The number of rings plus π bonds in a molecule, found from its formula: $\dfrac{2n_\mathrm{C} + 2 + n_\mathrm{N} - n_\mathrm{H} - n_\mathrm{X}}{2}$ , where X counts halogens (O does not appear). Each ring or π bond removes 2 H from the saturated formula $\mathrm{C}_n\mathrm{H}_{2n+2}$ ; it is also called the degree of unsaturation.

Cyclohexene, $\ce{C6H10}$ : one ring + one C=C.

Example

Cyclohexene, $\ce{C6H10}$ : (12 + 2 − 10)/2 = 2, one ring plus one C=C. Benzene, $\ce{C6H6}$ : (12 + 2 − 6)/2 = 4, one ring plus three C=C.