Degree of Unsaturation
Count rings + double bonds from formula
Formula
DU = (2C + 2 + N - H - X) / 2
- DU = degree of unsaturation
- C = number of carbons
- N = number of nitrogens
- H = number of hydrogens
- X = number of halogens (F, Cl, Br, I)
- O and S ignored (valence = 2)
Example: C₆H₆
Given: benzene, C₆H₆.
DU = (2×6 + 2 + 0 - 6 - 0) / 2 = (12 + 2 - 6) / 2 = 8 / 2 = 4
Benzene has 3 double bonds + 1 ring = 4 degrees of unsaturation.
Answer: DU = 4
Notes
- Each ring or double bond counts as 1 DU; each triple bond counts as 2 DU.
- Useful in structure elucidation from molecular formula + NMR/IR data.
- Always round to nearest integer; non-integer suggests formula error.