Unsaturation

Unsaturation refers to the presence of fewer hydrogen atoms in a compound than would be required for a fully saturated structure. In organic chemistry, unsaturation arises from double or triple bonds and from ring closures that reduce the number of hydrogen atoms relative to an acyclic alkane. Saturated hydrocarbons (alkanes) have the maximum number of hydrogens for a given carbon skeleton; any deviation signals unsaturation.

The concept is often quantified by the degree of unsaturation, also called double bond equivalents (DBE) or hydrogen deficiency index (HDI). For a molecule consisting of carbon, hydrogen, and optionally nitrogen, DBE = C − H/2 + N/2 + 1. Each ring or pi bond adds one to the DBE; a double bond contributes 1, a triple bond contributes 2, and rings contribute 1 each. Examples: ethene (C2H4) has DBE 1; cyclohexane (C6H12) has DBE 1; benzene (C6H6) has DBE 4; ethyne (C2H2) has DBE 2. Halogens are typically counted as hydrogens for the purpose of this calculation.

Detection and measurement methods include infrared spectroscopy, where C=C and C≡C bonds show characteristic absorptions, and hydrogen-deficiency calculations used in mass spectrometry and formula analysis. The concept is widely used in organic synthesis, structural determination, and natural products chemistry.

In biological systems, unsaturation commonly refers to the presence of carbon–carbon double bonds in fatty acids. The degree of unsaturation affects physical properties like melting point and reactivity, and it governs processes such as hydrogenation and oxidation.