Ethylene glycol, the main ingredient in antifreeze, is often confused with the ethanol found in alcoholic beverages due to its sweet taste and initial intoxicating effects. Chemically, ethylene glycol is classified as an alcohol, but it is a highly toxic type. Its danger lies not in the parent compound itself, but in the poisonous byproducts it creates once consumed.
Understanding the Chemical Definition of an Alcohol
The term “alcohol” in chemistry refers to a class of organic compounds that contain at least one hydroxyl group (–OH) attached to a saturated carbon atom. The presence of this functional group is the molecular requirement for a compound to be chemically defined as an alcohol.
Alcohols are classified by the number of hydroxyl groups they possess. Ethanol, the alcohol consumed in beverages, is a monohydric alcohol because it contains only one –OH group. Ethylene glycol is a dihydric alcohol, also known as a diol or glycol, because it has two separate hydroxyl groups bonded to adjacent carbon atoms. Its systematic chemical name, ethane-1,2-diol, reflects this structure.
Ethylene Glycol’s Key Properties and Applications
Ethylene glycol is a syrupy, viscous substance with a high boiling point (nearly 198°C) and a relatively low freezing point (about -12°C). These characteristics are due to the two hydroxyl groups, which enable strong hydrogen bonds.
The most common application is as the primary ingredient in commercial antifreeze and engine coolants. When mixed with water, it significantly lowers the freezing point, preventing ice formation and engine damage. It also raises the boiling point of the coolant, which helps prevent the engine from overheating. Ethylene glycol is also utilized in de-icing fluids for aircraft and is a starting material in the manufacturing of polyethylene terephthalate (PET).
The Metabolic Pathway: Why Ethylene Glycol is Poisonous
Ethylene glycol itself is relatively non-toxic until the body begins to process it. The danger lies in its metabolic pathway, which is initiated by the liver enzyme alcohol dehydrogenase (ADH)—the same enzyme that metabolizes ethanol.
The first step converts ethylene glycol into glycoaldehyde, which is then rapidly oxidized into glycolic acid. Glycolic acid is the primary culprit in the severe anion gap metabolic acidosis that characterizes the poisoning. This acid accumulation lowers the body’s pH, leading to serious systemic dysfunction.
The process continues as glycolic acid is further metabolized into glyoxylic acid and, finally, into oxalic acid. Oxalic acid combines with calcium to form insoluble calcium oxalate crystals. These crystals are deposited throughout the body, particularly in the renal tubules of the kidneys, leading to acute kidney injury and kidney failure.
The competition between ethylene glycol and ethanol for the ADH enzyme is why ethanol is sometimes administered as a medical antidote. By saturating the enzyme with ethanol, the metabolism of ethylene glycol is slowed. This allows the less-toxic parent compound to be excreted unchanged by the kidneys.