The formula, and what each term does
C is carbon, H is hydrogen, N is nitrogen and X is halogens (F, Cl, Br, I). Each ring or pi bond removes two hydrogens from the saturated formula, which is why the whole thing is halved. Oxygen and sulfur are divalent and drop out, so you leave them out entirely.
Worked examples
| Molecule | Formula | DoU | Structure |
|---|---|---|---|
| Ethanol | C₂H₆O | 0 | fully saturated |
| Cyclohexane | C₆H₁₂ | 1 | one ring |
| Benzene | C₆H₆ | 4 | ring + 3 double bonds |
| Aniline | C₆H₇N | 4 | ring + 3 double bonds |
| Naphthalene | C₁₀H₈ | 7 | two fused rings |
Aniline has a nitrogen, so the +N term keeps its DoU at 4 even though it has one more hydrogen than benzene.
How to read the result
- 0 — fully saturated: only single bonds, no rings.
- 1 — one ring or one double bond.
- 2 — one triple bond, or two rings, or a ring plus a double bond.
- 4 or more — a strong hint of an aromatic ring. Benzene rings each contribute 4.
- Ions. Add +1 for a cation or −1 for an anion to the result.
Common questions
What does the degree of unsaturation tell me?
It counts the total number of rings plus pi bonds in a molecule. Zero means fully saturated. One means a single ring or one double bond. Four is the classic value for a benzene ring, which is one ring plus three double bonds.
Do oxygen and sulfur affect the count?
No. Oxygen, sulfur and the other divalent chalcogens cancel out of the formula and are simply ignored. Halogens count like hydrogen (subtract them), and nitrogen and phosphorus are added in the numerator.
What if I get a fraction?
A correct degree of unsaturation for a neutral molecule is always a whole number. A fraction almost always means an error in the atom counts, so recheck the formula.
