Saturation (chemistry)

In chemistry, saturation (from the Latin word saturare, meaning 'to fill')^[1] has diverse meanings, all based on the idea of reaching a maximum capacity.

Organic chemistry

Hydrocarbons

In organic chemistry, a saturated compound is a hydrocarbon which has no double or triple bonds. For example, consider three similar organic compounds that are progressively less saturated: ethane, ethylene, and ethyne. Ethane, a completely saturated compound, has only single bonds between its hydrogen and carbon atoms (C₂H₆). Ethylene, an unsaturated compound of ethane, has a carbon double bond, having lost two hydrogen single bonds (C₂H₄). Finally ethyne, the completely unsaturated compound of ethane, has a carbon triple bond, having lost an additional pair of hydrogen single bonds (C₂H₂).

The concept of saturation can be described using various naming systems, formulas, and analytical tests. For instance, IUPAC nomenclature is a system of naming conventions used to describe the type and location of unsaturation within organic compounds. The "degree of unsaturation" is a formula used to summarize and diagram the amount of hydrogen that a compound can bind. Unsaturation can be determined by NMR, mass spectrometry and IR spectroscopy, or by determining a compound's bromine number.^[2]

Fatty acids

The term saturation is applied similarly to the fatty acid constituents of fats, which can be either saturated or unsaturated, depending on whether the constituent fatty acids contain carbon-carbon double bonds. Tallow consists mainly of triglycerides (fats), whose major constituents are derived from the saturated stearic and monounsaturated oleic acids.^[3] Many vegetable oils contain fatty acids with one (monounsaturated) or more (polyunsaturated) double bonds in them.

Organometallic chemistry

In organometallic chemistry, an unsaturated complex has fewer than 18 valence electrons and thus is susceptible to oxidative addition or coordination of an additional ligand. Unsaturation is characteristic of many catalysts because it is usually a requirement for substrate activation.^[4] In contrast, a coordinatively saturated complex resists undergoing substitution and oxidative addition reactions.

Physical chemistry

Solutions

In physical chemistry, saturation is the point at which the solute of a substance can dissolve no more of that substance and additional amounts of it will appear as a separate phase (as a precipitate,^[5] if solid, or as effervescence or inclusion, if gaseous). This point of maximum concentration, the saturation point, depends on the temperature and pressure of the solution as well as the chemical nature of the substances involved. This can be used in the process of recrystallisation to purify a chemical: it is dissolved to the point of saturation in hot solvent, then as the solvent cools and the solubility decreases, excess solute precipitates. Impurities, being present in much lower concentration, do not saturate the solvent and so remain dissolved in the liquid. If a change in conditions (e.g. cooling) means that the concentration is actually higher than the saturation point, the solution has become supersaturated.

Surfaces

In physical chemistry, when referring to surface processes, saturation denotes the degree at which a binding site is fully occupied. For example, base saturation refers to the fraction of exchangeable cations that are base cations.

Biochemistry

Proteins

In biochemistry, the term saturation refers to the fraction of total protein binding sites that are occupied at any given time.

Ecological stoichiometry

In environmental soil science, nitrogen saturation means that an ecosystem, such as a soil, cannot store anymore nitrogen.

See also

• Saturated fat
• Unsaturated fat
• Hydrogenation
• Organic redox reaction

References