Ethanol

Ethanol, also called ethyl alcohol, grain alcohol, or simply alcohol, is an organic compound with the formula CH3CH2OH (also written as C2H5OH, C2H6O, or EtOH). It is a volatile, flammable, colorless liquid with a pungent taste. As a central nervous system depressant, it is the active ingredient in alcoholic beverages and ranks as the second most consumed drug worldwide, after caffeine.

Ethanol occurs naturally when yeasts ferment sugars, and it can also be made through petrochemical processes like ethylene hydration. Historically, it served as a general anesthetic, and today it is used medically as an antiseptic, disinfectant, solvent for certain medications, and an antidote for methanol and ethylene glycol poisoning. It also functions as a chemical solvent, a feedstock for synthesizing other organic compounds, and a fuel for lamps, stoves, and internal combustion engines. Ethanol can be dehydrated to produce ethylene, an important industrial chemical. In 2023, global ethanol fuel production reached 112.0 gigalitres, with the United States contributing 51% and Brazil 26%.

The name "ethanol" comes from the ethyl group, coined in 1834, and was officially adopted in 1892. The word "alcohol" originally referred to a powdered cosmetic and only later came to mean ethanol specifically. Ethanol appears naturally as a byproduct of yeast metabolism in overripe fruit and palm blossoms, during plant germination without oxygen, in interstellar space, in human breath, and, in rare cases, is produced internally due to auto-brewery syndrome.

Humans have used ethanol as an intoxicant since ancient times. Its production through fermentation and distillation evolved over centuries across many cultures, while chemical identification and synthetic production began in the 19th century.

The systematic name "ethanol" was defined by the International Union of Pure and Applied Chemistry for a compound with a two-carbon alkyl group (prefix "eth-"), a single bond between them (infix "-an-"), and an attached -OH group (suffix "-ol"). The "eth-" prefix and the qualifier "ethyl" in "ethyl alcohol" originated from the name "ethyl," assigned in 1834 by Justus Liebig to the group C2H5−. He coined it from the German name "Aether" for the compound C2H5−O−C2H5 (diethyl ether). According to the Oxford English Dictionary, "ethyl" is a contraction of the Ancient Greek αἰθήρ (aithḗr, "upper air") and ὕλη (hýlē, "wood, raw material," hence "matter, substance"). "Ethanol" was coined following a resolution on naming alcohols and phenols adopted at the International Conference on Chemical Nomenclature in Geneva, Switzerland, in April 1892. The term "alcohol" now refers to a broader class of substances in chemistry, but in common speech it remains the name for ethanol. It is a medieval loan from Arabic al-kuḥl, a powdered antimony ore used as a cosmetic since antiquity, a meaning retained in Middle Latin. The use of "alcohol" for ethanol (specifically "alcohol of wine") was first recorded in 1753. Before the late 18th century, "alcohol" generally meant any sublimated substance.

As a recreational drug, ethanol is a central nervous system depressant and one of the most commonly consumed psychoactive drugs. Despite its psychoactive, addictive, and carcinogenic properties, it is widely available and legal in many countries, subject to laws regulating sale, export/import, taxation, manufacturing, consumption, and possession—most commonly excise taxes and prohibitions for minors. In mammals, ethanol is primarily metabolized in the liver and stomach by ADH enzymes, which catalyze its oxidation into acetaldehyde: CH3CH2OH + NAD+ → CH3CHO + NADH + H+. At significant concentrations, this metabolism is also aided by the cytochrome P450 enzyme CYP2E1 in humans, with trace amounts processed by catalase. The intermediate acetaldehyde is a known carcinogen and is significantly more toxic to humans than ethanol itself. Many symptoms of alcohol intoxication and many long-term health hazards from ethanol consumption are attributed to acetaldehyde toxicity. Acetaldehyde is then oxidized into acetate by aldehyde dehydrogenase (ALDH) enzymes. A mutation in the ALDH2 gene that produces an inactive or dysfunctional form of this enzyme affects about 50% of East Asian populations, causing the characteristic alcohol flush reaction—temporary reddening of the skin and other unpleasant symptoms of acetaldehyde toxicity. This mutation is often accompanied by another mutation in the ADH1B enzyme in roughly 80% of East Asians, which improves the efficiency of converting ethanol into acetaldehyde.

Medically, ethanol is the oldest known sedative, used as an oral general anesthetic in ancient Mesopotamia and medieval times. Mild intoxication begins at a blood alcohol concentration of 0.03–0.05%, and anesthetic coma occurs at 0.4%. This use carries high risks of deadly alcohol intoxication, pulmonary aspiration, and vomiting, leading to alternatives in antiquity such as opium and cannabis, and later diethyl ether starting in the 1840s. Ethanol is used as an antiseptic in medical wipes and hand sanitizer gels due to its bactericidal and anti-fungal effects; it kills microorganisms by dissolving their membrane lipid bilayer.