Unsaturated

Unsaturated is a term used in chemistry to describe molecules that are not saturated with hydrogen. In practice, it often refers to hydrocarbons that contain one or more carbon–carbon multiple bonds (double or triple) or ring systems that change the hydrogen count relative to the fully saturated form. In hydrocarbon series, alkanes are saturated and have the formula CnH2n+2. Alkenes have CnH2n, and alkynes have CnH2n−2. Aromatic compounds also count as unsaturated, with conjugated pi systems that affect their reactivity and hydrogen content.

The degree of unsaturation, also called the index of hydrogen deficiency, measures how many rings and/or multiple bonds a molecule has. It can be calculated by IHD = (2C + 2 + N − H − X)/2, where C is carbon, N is nitrogen, H is hydrogen, and X represents halogens. Each ring or pi bond increases the IHD by one. For example, ethene (C2H4) has IHD = 1, while benzene (C6H6) has IHD = 4.

Reactivity of unsaturated compounds is largely governed by the presence of pi bonds. They commonly undergo addition reactions across double or triple bonds, such as hydrogenation (adding H2 to convert alkenes or alkynes to alkanes) or halogenation. They can also participate in polymerization and various cycloaddition or oxidation reactions. Conjugation and aromaticity can stabilize certain unsaturated systems, influencing both their reactivity and physical properties.

In nutrition and biochemistry, unsaturated fats contain one or more carbon–carbon double bonds. They are typically liquids at room temperature and exhibit cis/trans isomerism, which affects texture, flavor, and health considerations.