Lactic and glycolic acid both belong to a class of organic compounds called alpha-hydroxy acids, meaning they have a type of chemical group called a hydroxyl group attached to a carbon atom immediately adjacent to a carboxyl group. While they share certain similarities, their chemistry and biochemistry is also different in some very important ways.
Properties
Glycolic acid is the smallest of the alpha-hydroxy acids; its chemical formula is C2O3H4. Lactic acid has one more carbon atom than glycolic acid, so its chemical formula is C3O3H6. Lactic acid melts at 17 degrees Celsius and boils at 122 degrees Celsius, whereas glycolic acid melts at 75 degrees Celsius and decomposes before you reach the boiling point. Both compounds are weakly acidic and highly soluble in water.
Chirality
Unlike glycolic acid, lactic acid is chiral, which means it contains an asymmetrical carbon. If you draw the lactic acid molecule so that all three carbons and the two oxygens in the carboxyl group are in the plane of the page, the hydroxyl group could be either pointing into the page or out of it. One of these two configurations is called the L-configuration and the other is called the D-configuration. The naturally occurring form in your body is L.
Biochemistry
Lactic acid is a normal product of metabolism in your muscles during high-intensity workouts and sprints. Many different organisms use lactic acid fermentation to break down sugars when oxygen is in short supply. Glycolic acid is found in very low concentrations in unripe grapes and sugar beets, among other places. In victims of ethylene glycol or antifreeze poisoning, the liver converts the ethylene glycol first to glycolic acid, then to glyoxylic acid and finally to oxalic acid. These metabolites are responsible for ethylene glycol's toxicity.
Uses
Lactic acid is found in many food products such as yogurt, cheese and wine; it's also produced by bacteria in your mouth when they ferment sugar from your food. It enjoys a variety of uses in industry, especially in leather tanning and chemical manufacturing. Glycolic acid is likewise employed in leather tanning and chemical synthesis. It also finds use in water treatment, as an ink and paint additive, and in cosmetics, where it acts as an exfoliant to promote shedding of dead skin cells.
References
- ChemicalLand21: Glycolic Acid
- ChemicalLand21: Lactic Acid
- "Organic Chemistry, Structure and Function"; Peter Vollhardt et al.; 2011
- "Chemical Principles: The Quest for Insight"; Peter Atkins et al.; 2008
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