unsaturated

Meaning: An unsaturated compound has at least one pi bond (C=C or C≡C), so it carries fewer hydrogens than the alkane of the same carbon count (CₙH₂ₙ₊₂, which is saturated). Each pi bond or ring removes two hydrogens. Pi bonds make alkenes and alkynes reactive toward addition, such as H₂ with a metal catalyst or Br₂. Unsaturated fatty acids have C=C bonds, usually cis in nature, whose kinks lower melting point. In general chemistry, an unsaturated solution holds less solute than its solubility limit.

Equation: Degrees of unsaturation = (2C + 2 + N − H − X)/2, where C, N, H and X are the numbers of carbon, nitrogen, hydrogen and halogen atoms in the formula. It is a unitless count. Oxygen is ignored.

On the MCAT: You are given a formula such as C₆H₁₀ (2 degrees) and asked which rings or pi bonds fit it. Other questions ask you to compare fatty-acid melting points or to predict hydrogenation products. A reagent such as Br₂ losing its color suggests a C=C.

Common confusion: A ring of single bonds has no pi bond, yet it counts as one degree in the formula, so degrees of unsaturation is not the same as the number of double bonds. Benzene, C₆H₆, has 4 degrees but is aromatic and resists addition.