Frequently Asked Questions
What does degree of unsaturation (DBE) tell you about a molecule?
Degree of unsaturation (also called index of hydrogen deficiency or IHD) counts the total number of pi bonds and rings. DBE = 0 means the molecule is fully saturated (all single bonds, no rings). DBE = 1 means one ring or one double bond. DBE = 4 suggests a benzene ring (3 double bonds + 1 ring). DBE helps narrow down the possible structures when interpreting spectral data.
What is the formula for degree of unsaturation?
DBE = (2C + 2 + N - H - X) / 2, where C = carbons, N = nitrogens, H = hydrogens, X = halogens (F, Cl, Br, I - each counts as one hydrogen). Oxygen and sulfur do not appear in the formula because they can substitute into chains without changing the hydrogen deficiency. The result must be a non-negative integer or half-integer (half-integers indicate radicals).
How do you calculate DBE for benzene (C6H6)?
DBE = (2(6) + 2 + 0 - 6 - 0) / 2 = (12 + 2 - 6) / 2 = 8/2 = 4. Benzene has 4 degrees of unsaturation: 1 for the ring and 3 for the three formal double bonds in the Kekule structure (or equivalently, 3 pi bonds in the delocalized structure). This is why DBE >= 4 and the formula CnH(2n-6) strongly suggest an aromatic ring.
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