Ethanol

Ethanol (C₂H₅OH), also called ethyl alcohol, is a volatile, flammable, colorless liquid belonging to the family of organic alcohols. Its hydroxyl group allows it to form hydrogen bonds and to mix with water in all proportions. It is the intoxicating component of alcoholic beverages, and it is also a common solvent, disinfectant, chemical intermediate, and fuel. Because it is produced both biologically, through the fermentation of sugars by yeast, and industrially, it is one of the largest-volume chemicals in the world.

The substance matters for several reasons. Biofuel ethanol has become a major component of the global transport fuel supply, and its production links agriculture, renewable energy, and climate policy. Ethanol is also a useful model for understanding enzyme-catalyzed metabolism in living organisms, because the human body oxidizes it in a two-step pathway. Its physical properties, including its boiling point, flammability, and azeotrope with water, shape how it is handled, purified, and measured in laboratories and factories.

This entry first describes the molecule's structure and properties, then the biochemical and industrial routes by which it is made. It goes on to cover its main uses in fuel, beverages, and industry, its metabolism and toxicity in humans, its natural occurrence, and the history of its discovery and production. It closes with the environmental and policy debates over biofuels and with the methods used to analyze and safely handle it.

Molecular structure and physical properties

Ethanol, with the formula $\mathrm{C_2H_5OH}$ (also written $\mathrm{CH_3CH_2OH}$), is the simplest alcohol with two carbon atoms. It consists of an ethyl group, a two-carbon chain, joined to a hydroxyl group (–OH) through a covalent bond between carbon and oxygen. The carbon that carries the hydroxyl group is bonded to only one other carbon atom, which makes ethanol a primary alcohol. By contrast, isopropanol is secondary because its carbinol carbon is bonded to two other carbons, and tert-butanol is tertiary. The C–O and O–H bonds are polar, and this polarity governs much of the molecule's behaviour. The hydroxyl group is also the site of most of its chemical reactions, including oxidation to acetaldehyde and acetic acid and esterification with acids. The molar mass is about 46.07 g/mol.

The hydroxyl group allows ethanol molecules to form hydrogen bonds with one another, since the O–H hydrogen can bond to the oxygen atom of a neighbouring molecule. These bonds, in addition to ordinary van der Waals forces, account for ethanol's relatively high boiling point of about 78 °C. Its isomer dimethyl ether, which has no O–H group, boils near –24 °C. The hydrogen-bonded network also raises viscosity; ethanol's dynamic viscosity is about 1.07 mPa·s at 25 °C. Because the hydroxyl group can hydrogen-bond with water in any proportion, ethanol is fully miscible with water, and mixing the two contracts the total volume. The ethyl chain is nonpolar enough that ethanol also dissolves in many nonpolar liquids such as benzene and diethyl ether, although it is a poor solvent for fats and oils.

Key physical constants are summarised below (values at 1 atm unless noted):

The flash point near room temperature means that ethanol releases flammable vapour under ordinary conditions. The heat of combustion is the energy released when the liquid is completely oxidised to carbon dioxide and water, and it underlies ethanol's use as a fuel.

Ethanol and water form a minimum-boiling azeotrope at about 95.6% ethanol by weight, which boils at about 78.2 °C, slightly below the boiling point of pure ethanol. At this composition the vapour and the liquid have the same composition, so ordinary distillation cannot raise the concentration further. Fermentation broths of roughly 10–15% ethanol can be concentrated to near the azeotrope, but anhydrous (absolute) ethanol requires an additional step. Historically, a third component such as benzene was added to carry off the water, but this was costly and involved a toxic substance. Today, the usual method is adsorptive dehydration over a molecular sieve, typically a zeolite that binds water far more strongly than ethanol and can be regenerated by pressure or vacuum cycling.

Ethanol is routinely identified by its spectroscopic signatures. In infrared spectroscopy, the most diagnostic feature is a broad O–H stretching band between about 3200 and 3550 cm⁻¹, whose breadth reflects hydrogen bonding; this band narrows and shifts when the sample is dilute in a nonpolar solvent. A strong C–O stretch appears near 1050 cm⁻¹. In proton NMR, ethanol gives three characteristic signals: a triplet near 1.2 ppm for the methyl protons, a quartet near 3.7 ppm for the methylene protons adjacent to oxygen, and a broad, concentration- and solvent-dependent singlet for the hydroxyl proton. The hydroxyl signal disappears when the sample is shaken with deuterium oxide, which exchanges the labile proton. Carbon-13 NMR shows two peaks, one near 18 ppm for the methyl carbon and one near 58 ppm for the carbon bonded to oxygen.

Biochemical formation by fermentation

In living cells, ethanol is formed mainly through fermentation, a family of anaerobic pathways in which an organic molecule acts as both electron donor and electron acceptor, so no external oxidant such as oxygen is needed. The ethanol route begins with glycolysis, the ten-step sequence that splits one glucose molecule into two molecules of pyruvate. Glycolysis yields a net gain of two ATP and two molecules of reduced NADH per glucose.

From pyruvate to ethanol

Under anaerobic conditions, NADH cannot be reoxidized through a respiratory chain, so the cell needs another route to recycle it. Pyruvate is first decarboxylated by pyruvate decarboxylase, an enzyme that requires thiamine pyrophosphate; this releases carbon dioxide and leaves acetaldehyde. Alcohol dehydrogenase (ADH) then transfers hydrogen from NADH to acetaldehyde, producing ethanol and regenerating NAD⁺. The regenerated NAD⁺ is required by the glyceraldehyde-3-phosphate dehydrogenase step of glycolysis, so sugar breakdown can continue. The reduction of acetaldehyde is therefore a redox reaction whose main purpose is to keep the glycolytic cycle running rather than to store energy.

The net process can be written as:

Because the reaction balances mass exactly, the theoretical maximum yield is about 0.51 g of ethanol per gram of glucose, with roughly 0.49 g of CO₂ released. Practical yields are lower, because part of the sugar is diverted into cell biomass, glycerol, and other byproducts.

Yeast and other microorganisms

The principal industrial organism is the yeast microorganism Saccharomyces cerevisiae. It tolerates acidic conditions and moderate ethanol concentrations, grows reliably in large tanks, and is well understood genetically. A notable feature is the Crabtree effect: this yeast ferments sugar to ethanol even when oxygen is available, provided sugar concentrations are high. Other yeasts, such as some non-Saccharomyces species, are used in specialized processes, and their flavor profiles can differ.

Bacteria also produce ethanol. Zymomonas mobilis uses the Entner–Doudoroff pathway instead of classic glycolysis, which yields only one ATP per glucose but supports rapid sugar uptake and high ethanol productivity. Several other bacteria, including some clostridia and enteric bacteria such as Escherichia coli, produce ethanol as one of several end products in mixed-acid fermentation, and engineered strains have been developed for fuel production.

Limits set by sugar, oxygen, and ethanol toxicity

Fermentation does not continue indefinitely. Ethanol is toxic to the cell that makes it: at higher concentrations it disrupts cell membranes, denatures proteins, and inhibits enzymes. As a result, fermentation typically stops at roughly 10–18% ethanol by volume, and many industrial strains stall near the lower end of that range. Reaching higher concentrations requires distillation, which is discussed elsewhere in this article. Other constraints include sugar depletion, osmotic stress from very high sugar levels, shortages of nitrogen or vitamins, unfavorable temperature or pH, and the need for small amounts of oxygen to synthesize sterols and unsaturated fatty acids for the cell membrane.

Comparison with aerobic respiration

Aerobic cellular respiration oxidizes the same sugar completely:

In respiration, pyruvate is converted to acetyl-CoA and fed into the citric acid cycle, and electrons from NADH are passed along an electron transport chain to oxygen. The complete oxidation releases roughly 2,800–2,900 kJ per mole of glucose and yields on the order of 30 ATP, compared with a net 2 ATP from fermentation. Fermentation therefore extracts only a small fraction of the available chemical energy, which is why it is a low-yield process for the cell. The contrast was studied in the nineteenth century by Louis Pasteur, whose observation that oxygen tends to suppress fermentation in yeast is known as the Pasteur effect, though the Crabtree effect shows that this suppression is not universal.

Industrial production and purification

Nearly all industrial ethanol is made by microbial fermentation of sugars, and the rest is made by hydrating ethylene. Fermentation plants use whatever sugar source is cheapest locally. Sugarcane juice and molasses contain fermentable sugars directly, whereas grains and tubers such as corn, wheat, and cassava store their energy as starch, which must first be broken down into sugars. Sugar beets are a further sugar source. Corn dominates in the United States because of its large grain harvest, while sugarcane is the main feedstock in Brazil. Feedstock choice therefore follows regional harvests, land availability, and the price of sugar and grain more than any single technical advantage.

Dry-milling and wet-milling

In the United States, corn is processed in two main ways. Dry-grind (dry-milling) plants grind the whole kernel into meal, mix it with water to form a mash, and add enzymes: α-amylase first liquefies the starch, and glucoamylase then converts it to glucose. Ammonia is commonly added for pH control and as a yeast nutrient. Yeast ferments the glucose, and the resulting beer is distilled. The solid residue is dried into distillers' grains, a feed co-product. Dry milling needs less capital, which is why it accounts for most US ethanol capacity, and a bushel of corn yields about 2.7 US gallons of ethanol. Wet-milling instead steeps the kernel for roughly two days in dilute sulfurous acid, separates germ, fiber, gluten, and starch, and then hydrolyzes the purified starch (hydrolysis) before fermentation. Its ethanol yield is similar, about 2.65 gallons per bushel, but its equipment is expensive, so it is justified only when the starch, syrup, and oil co-products are valuable.

Second-generation (cellulosic) ethanol

Crop residues such as corn stover and wheat straw, along with wood, grasses, and bagasse, contain far more sugar than grain starch, but that sugar is locked in cellulose and hemicellulose, which are wrapped in lignin. Lignocellulose must therefore be pretreated before enzymes can reach the polysaccharides. Common pretreatments include dilute acid or alkali treatment, steam explosion, and hot-water treatment, which disrupt the lignin seal and loosen the cellulose. A cocktail of cellulase enzymes then hydrolyzes the cellulose to glucose, while hemicellulose yields pentose sugars such as xylose that only some microbes ferment well. Pretreatment also produces inhibitory compounds, and enzyme cost, low sugar yields, and high capital costs have made commercial cellulosic production difficult to sustain.

Synthetic ethanol

Synthetic ethanol is made from ethylene, which is obtained by cracking petroleum fractions or natural gas liquids. In the dominant direct hydration process, ethylene and steam pass over a solid phosphoric acid catalyst, usually supported on silica, in a fixed-bed reactor. The reaction is exothermic and gives a crude ethanol stream that must then be purified. An older route absorbed ethylene in sulfuric acid to form ethyl sulfate, which was then hydrolyzed and fractionated to about 95% ethanol. Synthetic ethanol held a large share of the market in the nineteenth century, but its share fell after the 1980s as fermentation expanded. For 2010, one estimate put the synthetic share at about 7% of a global output of roughly 102 billion litres. The balance between the two routes depends on the relative prices of petroleum-derived ethylene and sugar or grain, on the supply of molasses, and on policy. Synthetic production has declined in the United States, where cheap feedstock is scarce, but has grown in regions with inexpensive ethylene, such as Saudi Arabia.

Distillation and dehydration

Fermentation beer typically contains only around 10% ethanol. A beer column strips the ethanol into vapour, and a rectifying column concentrates it. The ethanol–water distillation stops at an azeotrope near 95.6% ethanol by weight (about 96% by volume) at atmospheric pressure, because the liquid and vapour then share the same composition. Ordinary distillation therefore cannot yield pure ethanol, and the azeotrope must be broken by a separate step. Historically, benzene was added as an entrainer, but this was costly and toxic. Calcium oxide and salt rectification were also tried, with limited success. The modern method is adsorptive dehydration over molecular sieves, typically 3A-type potassium aluminosilicate zeolites with pores of about 3 Å. Water is retained in the pores while ethanol passes through. Plants use paired beds, so that one dries product while the other is regenerated using a portion of dried ethanol vapour, which is then recycled to the column. Such systems produce anhydrous ethanol containing less than 0.5% water.

Fuel use and blending

Most ethanol reaches motorists as a component of gasoline rather than as a standalone fuel. Roughly nine-tenths of a typical American regular-grade pump fuel comes from crude oil, with ethanol making up the remainder. Higher blends are far less common. E15 and E85 have limited availability in the United States, and E85 is also misleadingly named. Federal rules allow anywhere from 51 to 85 percent ethanol in E85, and in winter the ethanol share is lowered toward 51%, because ethanol vaporizes less readily than gasoline in cold conditions.

Vehicles that accept these variable blends are called flexible-fuel vehicles. Their fuel systems measure the blend directly: an ethanol-content sensor informs the engine control unit, which then sets fueling, timing and related parameters for the blend actually present. Cold starting remains a weak point, since one engineering study cited by SAE International found that an E85 engine needs roughly three times the start-up fuel of a gasoline engine.

Brazil is the reference case for a national program. Its ethanol push began with the PROALCOOL initiative of the 1970s, which lost competitiveness in the 1990s before regaining momentum with the spread of flex-fuel vehicles from the early 2000s. Brazil's flex-fuel technology, established in 2003, lets consumers choose between E30 and E100. The mandated share has risen in steps: in 1993 a law fixed the blend at 22% anhydrous ethanol, and since August 1, 2025, the mandatory level has been 30% in regular gasoline and 25% in premium gasoline. The permitted range is between 22% and 35%, and under the Fuels of the Future Law, new ethanol and biodiesel mandates require approval by the National Energy Policy Council (CNPE). The government presented the increase as a way to reduce reliance on gasoline imports, lower emissions, and cut fuel costs.

The United States instead relies on the Renewable Fuel Standard, which sets volume obligations rather than a fixed blend level. The Environmental Protection Agency reported that the RFS required 15.25 billion gallons of biofuel use in 2023. Because obligated parties can meet the target with several renewable fuels and tradable credits, the ethanol share of American gasoline is not set directly by the standard. The contrast between a blend-level mandate in Brazil and a volume mandate in the United States is one reason the two programs produce different market outcomes.

Ethanol carries less energy per unit volume than gasoline. Its lower heating value is about 27% below that of E0 gasoline, which is why E85 tends to give lower fuel economy. Its advantage lies in octane. Gasoline has an octane rating between 87 and 93, whereas E85 generally reaches about 100 to 105. This anti-knock property is the main reason ethanol has been used as a gasoline additive. After MTBE was found to contaminate groundwater, manufacturers moved to ethanol in the early 2000s.

Aviation and marine uses are at an earlier stage. In aviation, ethanol is being studied as a feedstock for sustainable aviation fuel through alcohol-to-jet routes, in which ethanol is converted into hydrocarbons that can be blended with conventional jet fuel under industry specifications. Marine use remains limited to pilot projects and niche applications, partly because of ethanol's low flash point and its affinity for water.

Production figures vary considerably by source and unit. The U.S. Alternative Fuels Data Center lists American output at over 16 billion gallons in 2024, and says that together, the United States and Brazil produce 80% of the world's ethanol. For Brazil, the same dataset gives about 8.98 billion gallons for 2024, roughly 34 billion litres, whereas REN21 reports a total production of 32.5 billion litres for the same year, a gap that reflects unit conversion and differing definitions. Global totals also diverge: the Renewable Fuels Association tabulates a total of 32,000 (million gallons) for 2025, while one market-research summary cites 104 billion liters in 2023. Volume totals also overstate ethanol's energy contribution, since a litre of ethanol holds noticeably less energy than a litre of gasoline.

Beverages and alcohol content

The alcoholic beverages people drink owe their effects to ethanol. The World Health Organization describes the ethanol in these drinks as psychoactive, toxic, and capable of producing dependence (Alcohol and alcoholic beverages contain ethanol, which is a psychoactive and toxic substance with dependence-producing properties.). In beer, wine, and cider, ethanol is made by fermentation: yeast converts the sugars in grapes, grain, or fruit into ethanol and carbon dioxide, and the process halts once the rising ethanol concentration becomes toxic to the yeast, typically at around 10–18% by volume. Typical strengths therefore vary by style. The average ABV for beer is as low as 5% but can be higher than 10%. The average ABV for wine is 11-13%.

Stronger spirits such as whisky, rum, gin, and vodka are produced by distillation. Because ethanol boils at a lower temperature than water, heating a fermented liquid and condensing the vapour yields a liquid far richer in ethanol than the original brew. Repeated or carefully controlled distillation can concentrate the spirit further, which is why spirits carry much higher alcohol content than fermented drinks.

Alcohol strength is expressed in two main ways. Alcohol by volume (ABV) is the percentage of the beverage's volume that is ethanol. It is usually determined from the liquid's density and temperature, because both change with alcohol content. Proof is a separate scale that varies by country. In the United States, policymakers define proof as twice the percentage of alcohol by volume (ABV). In the United Kingdom, the proof figure is roughly 1.75 times the ABV, a relic of an 1816 standard (The term was originally used in England, and from 1816 was equal to about 1.75 times the percentage of alcohol by volume (ABV).). The word itself has tax origins: the term "proof" dates back to 16th-century England, when spirits were taxed based on strength. A traditional but poorly documented account holds that excise officers tested spirits by trying to ignite a sample mixed with gunpowder. Because of these differing conventions, a figure such as "80 proof" means 40% ABV in the United States but a different strength in the UK, and the labelling of alcohol content is regulated by law in many countries.

Before the twentieth century, the commercial alcohol trade separated several categories. Drinking spirits were grouped by base material, such as grain spirits (whisky, gin, vodka) and fruit spirits (brandy and kirsch), and both were distinct from industrial alcohol. Industrial alcohol was sold for use in factories and was not intended for drinking. Before 1920, American factories used undrinkable spirit as a solvent and cleaning agent and as a raw material for detergents, flavourings, and perfumes (For years before Prohibition, industrial-grade (and undrinkable) alcohol was used in factories as a solvent and cleaning fluid, and to manufacture detergent, flavoring extracts and perfumes.).

Denaturing was the mechanism that kept industrial alcohol out of beverages. Because beverage alcohol carried heavy excise duties, governments exempted spirit from those duties only when it was made unfit to drink. In the United States, a process borrowed from Europe added "denaturants," or substances that made grain-based (ethyl) alcohol taste or smell bad, to deter its use in drinks, and tax-free use began on 1 January 1907. Early American formulas used wood (methyl) alcohol, a harmful liquid that could cause blindness or death if ingested. The system had a deadly side effect during Prohibition. Bootleggers obtained denatured spirit and sold it as whiskey, and in January 1920 The magazine Literary Digest, in its January 10, 1920 issue, reported that scores of people had recently died, including 57 in Hartford, Connecticut. Today, US rules still allow tax-free use of denatured spirits, though spirits that are denatured (i.e., treated with substances to make them unsuitable for human beverage consumption) may be used free of tax by a person who holds an industrial use permit. However, no permit is needed to use completely denatured alcohol or an approved article made from denatured spirits.

Governments regulate beverage alcohol heavily. In many countries, sales of alcoholic beverages are heavily taxed for revenue and public health policy purposes (see Pigovian tax). Alcohol's public-health effects are reported regularly by international bodies. According to the WHO's 2024 fact sheet, 2.6 million deaths were attributable to alcohol consumption in 2019, of which 2 million were among men and 0.6 million among women. In relative terms, 2.6 million deaths per year were attributable to alcohol consumption, accounting for 4.7% of all deaths. The burden is concentrated among younger adults, with 13% of alcohol-attributable deaths globally among young people ages 20 to 39. The WHO also estimates that an estimated 400 million people, or 7% of the world's population aged 15 years and older, lived with alcohol use disorders. Figures vary with year and method: an earlier WHO report cited some 3 million deaths (5.3% of all deaths) worldwide for 2016. The WHO also notes that most alcohol related harms come from heavy episodic or heavy continuous alcohol consumption.

Solvent, chemical feedstock, and disinfection uses

Solvent applications

Ethanol is a polar protic solvent. Its hydroxyl group forms hydrogen bonds, so it mixes with water and with many organic liquids, while its ethyl group lets it dissolve substances that water handles poorly. This combination makes it one of the most widely used solvents in manufacturing. Paint, coating, adhesive, and ink producers use it to dissolve resins, oils, fats, and waxes, and it evaporates at moderate temperatures. Ethanol is also a more acceptable carrier for some ink-jet inks than older solvents, since it has low odor and comparatively low toxicity. Its weaker power to dissolve certain resins and dyes, however, makes formulation harder, and ethanol-based inks absorb moisture in humid conditions.

In perfumery, ethanol is the usual carrier for fragrance oils. Its low odor and low boiling point let it evaporate quickly from skin and release the scent, and it is used in similar roles in cosmetic and hair products. In flavorings and botanical extracts, it dissolves aroma and colour compounds. In pharmaceuticals, it dissolves active ingredients in tinctures and liquid preparations and serves as a preservative in some injectable and oral products. Aqueous ethanol is also a practical extraction solvent for plant-derived drugs; ethanol-assisted extraction has been used to recover the antimalarial artemisinin from Artemisia annua.

Chemical feedstock and organic synthesis

As a chemical intermediate, ethanol is oxidized to acetaldehyde, which is a precursor for acetic acid and other chemicals. Esterification with acetic acid yields ethyl acetate, a common coatings solvent. Dehydration over suitable catalysts can produce ethylene, and ethanol is also converted into ethyl acrylate, glycol ethers, ethylamines, and higher alcohols such as butanol. In the laboratory, ethanol serves both as a reagent, supplying the ethoxy group in ethyl ethers and esters, and as a protic reaction medium, for example for the reduction of carbonyl compounds by sodium borohydride. Its use as a feedstock is one reason the bulk of industrial production is carried out at large scale and then purified by distillation.

Disinfection and antisepsis

Ethanol kills many vegetative bacteria, fungi, and enveloped viruses, including influenza viruses and coronaviruses. It is thought to act mainly by denaturing proteins and disrupting membrane lipids, and water is needed to carry it into the cell. This is why the most effective formulations are aqueous. Solutions of roughly 70–85% are generally cited as the most effective range, and reports note that absolute ethanol evaporates too quickly to maintain contact and is less reliable on bacteria. Ethanol has no useful action against bacterial spores.

Its efficacy is narrower than the term "disinfectant" implies. A suspension study on enterovirus 71, a non-enveloped virus, found that 70% and 75% ethanol barely reduced viral titres within ten minutes, while 95% ethanol gave a much larger reduction but still did not fully inactivate the virus. Studies of norovirus have also reported that 70% ethanol affected only some genotypes. Some regulatory documents, by contrast, describe ethanol as superior to propanols against certain non-enveloped viruses, so the results depend on the virus, concentration, and test method.

Laboratory and specimen use

Ethanol is a routine laboratory solvent for extraction and sample preparation. Biological specimens are commonly stored in it, and histological tissue is dehydrated through graded ethanol series before embedding. For analytical work, reagent-grade material is used as a reference and calibration standard, including for instruments such as gas chromatographs and breath analysers. Reagent-grade ethanol is typically the most expensive grade, reflecting the tighter analytical purity required.

Pharmaceutical-grade and food-grade specifications

Food-grade and pharmaceutical-grade ethanol differ in their governing standards and documentation. Food-grade ethanol is generally held to the Food Chemicals Codex (FCC), is non-denatured, and is used for beverages, flavorings, and botanical extraction. Pharmaceutical-grade ethanol must meet a monograph in a pharmacopoeia such as the United States Pharmacopeia (USP) or the European Pharmacopoeia, and it is used in injectables, oral solutions, and antiseptics. Pharmaceutical production also requires batch-level impurity testing and quality-system documentation that food-grade material does not necessarily need.

Published specifications for the two grades vary. Some trade sources give minimum purities of about 95% v/v for both, while others list different values and impurity limits; some state that the FCC and USP monographs set identical methanol limits in certain specifications. Because these figures come from suppliers rather than the official monographs, the current pharmacopoeial and FCC texts should be consulted for exact limits. Industrial-grade ethanol, which may contain denaturants, is unsuitable for both food and pharmaceutical use.

Metabolism and toxicity in humans

Absorption and distribution

Ethanol is absorbed from the stomach and small intestine, with the small intestine doing most of the work, and it spreads through the body's water rather than being stored in fat. Its blood concentration therefore tracks total body water, which is why the same amount of alcohol produces higher levels in smaller or leaner people. Because the molecule is small and dissolves readily in water, it crosses the blood–brain barrier by simple diffusion and reaches the central-nervous-system within minutes. Elimination is slow and fairly steady: Rate of ETOH elimination is 15-30mg/dL/hr (depending on degree of chronic alcohol intake).

Two-step oxidation

Most ethanol is oxidised in the liver. The first step is catalysed by alcohol dehydrogenase (ADH), which passes hydrogen from the alcohol to the cofactor NAD⁺, reducing it to NADH. ADH also is capable of oxidizing a variety of primary, secondary and tertiary aliphatic alcohols, but ethanol is its main substrate in the body. The product, acetaldehyde, is a recognised carcinogen, so the body moves it on quickly: it is quickly broken down to a less toxic compound called acetate by aldehyde dehydrogenase (ALDH). This second reaction is effectively one-way, since the enzyme reaction is irreversible.

Acetate then leaves the body in urine or re-enters central metabolism through acetyl-CoA. Both oxidation steps add to the cell's reducing load: two NADH molecules are produced per molecule of acetate generated. A surplus of NADH and acetyl-CoA can feed fatty acid synthesis, which is one reason heavy long-term drinking promotes fatty liver. When intake is high, the enzyme CYP2E1 takes on a larger share of the work, while in the brain, catalase is estimated to generate between 60 and 70 % of acetaldehyde.

Genetic variation

Variation in the genes for ADH and ALDH changes how quickly people process ethanol. Among the most consequential are variants of ADH1B and ALDH2 that are common in East Asian populations. Certain ADH1B and ADH1C variants commonly found in East Asian populations lead to more rapid ethanol breakdown and acetaldehyde accumulation in the body. The ALDH2 variant is more widespread: in East Asian countries, ~40% of the population carry a genetic variant in the aldehyde dehydrogenase 2 (ALDH2) enzyme that metabolizes acetaldehyde to acetic acid. Accumulated acetaldehyde produces facial reddening, nausea and a racing pulse. Carriers of these alleles are less likely to drink and have a lower risk of alcohol dependence, although many heterozygous ALDH2 variant carriers have developed tolerance to the acetaldehyde-induced flushing response and become habitual heavy drinkers. The health consequences are serious: even moderate alcohol consumption for ALDH2*2 carriers increases the odds ratio for developing aerodigestive track cancer and esophageal cancer. Pronounced flushing is far less common among Europeans.

Acute toxicity and intoxication

Ethanol acts principally as a depressant of the nervous system. Alcohol (ethanol) is a CNS depressant that can cause respiratory depression, coma, or death when consumed rapidly or in large quantities. At the level of neurons, it enhances the inhibitory effect of GABA at GABA-A receptors and dampens excitatory signalling through NMDA-type glutamate receptors, so the net effect is reduced neurotransmitter activity in the brain. These actions underlie slurred speech, impaired coordination and memory loss.

At high doses, the body's metabolic balance is also disrupted. Young children are especially prone to low blood glucose, because hypoglycemia, particularly in young children due to gluconeogenesis inhibition by ethanol metabolism occurs when the NADH surplus blocks the liver from making new glucose. Acidosis can arise as well. Laboratory features are high anion gap metabolic acidosis and increased serum ketoacid levels, particularly β-hydroxybutyrate, a pattern often called alcoholic ketoacidosis. In one documented case, an 18-year-old male was referred to the emergency department in a comatose state following binge drinking, with an extremely elevated osmolal gap of 91 mOsm/kg.

Comparison with methanol and ethylene glycol

Methanol and ethylene glycol are far more dangerous than ethanol, but for a chemically indirect reason. Methanol is not directly toxic, but formic acid is both directly toxic (e.g. direct optic nerve toxicity) and inhibits mitochondrial cytochrome oxidase. Methanol poisoning is therefore characterised by acidosis and visual loss, and a mortality rate of 20% has been reported despite treatment.

Ethanol serves as an antidote because it competes for the same enzyme. This involves competitive inhibition of alcohol dehydrogenase (ADH). The aim is to delay the production of the toxic metabolites. Its use has declined: due to unpredictable ethanol kinetics, CNS depression, and hypoglycemia, ethanol treatment is not preferred. Clinicians now generally favour fomepizole, a more specific ADH inhibitor. Ethanol also complicates diagnosis, since ethanol can mislead the clinician because it further delays the onset of the acidosis, 'explains' the presence of intoxication and also explains the presence of an osmolar gap.

Occurrence in nature and space

Ethanol forms wherever microbes ferment sugars without oxygen. Yeasts and some bacteria produce it during the ripening and spoilage of fruit, and it also appears in nectar and in other fermenting plant material. Its natural occurrence is therefore the same fermentation chemistry that industry later adopted, carried out by microorganisms in ordinary environments.

Production by plants under low oxygen

Higher plants can make ethanol too. When waterlogged soil excludes oxygen from roots, mitochondrial respiration cannot fully oxidize the pyruvate produced by glycolysis. Instead, pyruvate decarboxylase converts pyruvate to acetaldehyde, which alcohol dehydrogenase reduces to ethanol, and this fermentation helps roots keep producing energy. Under hypoxia, pyruvate generated by glycolysis in roots cannot be effectively oxidized by aerobic respiration in mitochondria and instead is decarboxylated to acetaldehyde by PDC. The ethanol does not always stay in the roots. Ethanol made underground can travel up the transpiration stream, and aerial tissues that contain alcohol dehydrogenase can convert it back to acetaldehyde, which is one reason leaves of flooded plants emit these compounds. Genetic work supports the importance of the pathway: In Arabidopsis, the Adh1 null mutant also has reduced tolerance to hypoxia while by contrast, overexpressing PDC1 or PDC2 increases survival.

Detection in interstellar space

Ethanol was among the first organic molecules found in space by radio astronomy. Three transitions of trans-ethyl alcohol (CH3CH2OH) were detected in emission toward the Sagittarius B2 molecular cloud in 1975, and its abundance there was comparable to that of many molecules previously detected there. The higher-energy gauche conformer was seen later, in Orion KL several years later (Pearson et al. 1997). Subsequent surveys have found ethanol in several hot cores and protostellar regions, which shows that it forms readily in the dense, cold gas that precedes star formation. The detection also says something about interstellar chemistry. Alcohols and aldehydes are often found together in space, and one study describes a gas-phase "top-down" link from alcohols to aldehydes that compliments the better studied "bottom-up" routes involving grain-surface H-addition reactions, meaning that ethanol and acetaldehyde can be interconverted in addition to being formed from simpler precursors.

Atmospheric occurrence

Ethanol is present throughout the lower atmosphere, although its global budget has been only loosely constrained by observation. A global model by Goldstein and colleagues gave 5.0 Tg yr−1 from industrial sources and biofuels, 9.2 Tg yr−1 from terrestrial plants, ~0.5 Tg yr−1 from biomass burning, so plants and human-related sources together dominate the input. Because of its reactivity, ethanol does not accumulate. The resulting atmospheric lifetime of ethanol in the model is 2.8 days. Within that model, gas-phase oxidation by the hydroxyl radical (OH) is the primary global sink of ethanol in the model (65%), with deposition removing the remainder.

Environmental fate

In soil and water, ethanol is generally short-lived. It is a simple, water-soluble molecule that a wide range of bacteria and fungi can use as a carbon and energy source, so it is usually degraded rapidly under aerobic conditions and more slowly under anaerobic ones. Because of this, spills of ethanol usually disperse and disappear within days to weeks rather than persisting, though a large release can consume dissolved oxygen in surface water as microbes break it down, and this can be a short-term ecological concern. Its rapid return to the carbon cycle is one reason it is considered environmentally less persistent than many synthetic solvents.

History of discovery and production

People made fermented drinks thousands of years before anyone understood what ethanol was. Residue analysis of pottery from Jiahu in Henan province suggests that by 7000 BC a mixed fermented beverage was already being made there. Later chemical work on the residue pointed to a drink built from grapes, hawthorn berries, honey and rice, dated to roughly 7000–6650 BC. In Mesopotamia and Egypt, brewing became routine: by 3000 BCE, beer and wine were commonly consumed in Mesopotamia and Ancient Egypt, often associated with religious ceremonies and community feasts. The oldest surviving beer recipe is a Sumerian hymn to the brewing goddess Ninkasi, a 3,900-year-old poem that describes making beer from barley bread. These practices relied on empirical knowledge of yeast and sugar, not on chemistry.

Fermentation alone yields only modest alcohol concentrations, so stronger drinks depended on distillation. The sources generally credit Arab scholars with the invention, likely in the 8th or 9th century CE. Distillation enabled the extraction of higher concentrations of ethanol, resulting in spirits or distilled alcoholic beverages. Early spirits were used mainly as medicines, and they later became a trade commodity because, unlike beer and wine, they didn't spoil. Medieval European alchemists adopted and refined the still, which made concentrated ethanol available in Europe.

The chemical identity of ethanol emerged in the early nineteenth century. In 1825, Michael Faraday found that sulfuric acid could absorb large volumes of coal gas, and the resulting solution was passed to Henry Hennell. Hennell reported in 1826 that it contained "sulphovinic acid" (ethyl hydrogen sulfate). In 1828, Hennell and the French chemist Georges-Simon Serullas independently discovered that sulphovinic acid could be decomposed into ethanol. Faraday's route, in which the olefin ethylene from coal gas was absorbed and then converted to alcohol, is regarded as an early version of the acid-catalyzed hydration, a process similar to current industrial ethanol synthesis. Sources differ on the dates: one reference states that Faraday prepared ethanol by this hydration in 1828, so the year attached to this step varies across accounts. Structural questions followed; fifty years later Archibald Scott Couper published a structural formula for ethanol, placing it among the first compounds whose structures were determined.

The biological origin of alcohol was clarified by Louis Pasteur in the 1850s and 1860s. Working on the wine industry, he showed that fermentation was performed by living cells. He published his results in 1857 and a fuller memoir in 1860, and he concluded that yeast have to be alive for alcohol to be produced. Pasteur also distinguished alcoholic fermentation, driven by yeast, from lactic fermentation, driven by bacteria that soured the liquor. His claim that living cells were required met resistance from Liebig and Wöhler, who in 1839 had published a paper on the role of yeast in alcoholic fermentation, and the debate pushed chemists toward explaining fermentation in terms of reactions. The question was resolved in 1897, when Eduard Buchner obtained cell-free yeast juice that could still ferment sugar, showing that the active agents were enzymes rather than intact cells.

Industrial production began to expand in the twentieth century. Synthetic production started with the sulfuric acid route: first practiced on the industrial scale in 1930 by Union Carbide but now almost entirely obsolete, ethylene was hydrated indirectly by reacting it with concentrated sulfuric acid. During both world wars, governments looked to alcohol as a fuel and as a chemical feedstock, including for synthetic rubber. After the wars, large-scale programs, most prominently in Brazil from the 1970s, established ethanol as a fuel on a national scale.

Environmental, economic, and policy debates

Whether ethanol fuel lowers climate emissions depends heavily on where an analysis draws its boundaries. Life-cycle greenhouse-gas accounts cover farming, processing, transport and combustion, and their results shift with the feedstock, the land used, and the energy supplied to the refinery. One retrospective study of US corn-grain plants found the footprint falling from about 58 to 45 g CO₂e per megajoule between 2005 and 2019, excluding land-use change. Corn farming and corn ethanol production account for 28.6 and 25.6 g CO₂e/MJ, respectively, in a separate estimate of the current industry that puts the total at 52.4 g. Method matters as much as the plant itself. In one study, the same corn ethanol came out at 61, 62, and 59 g CO₂e/MJ under three co-product approaches, but ethanol's life-cycle GHG emissions are lower at 46 g CO₂e/MJ under process-level energy allocation. Further cuts are possible: one analysis estimates that adding CCS to a typical dry mill ethanol plant may further reduce corn ethanol CI by 31.4 g CO₂e/MJ.

The most contested element is indirect land-use change (ILUC). That ILUC operates through global markets makes it difficult to attribute land-use emissions to biofuels, and researchers and policymakers have typically relied on models to simulate it. The concern gained momentum after both 2008 studies demonstrated that ILUC could increase carbon emissions following biofuel expansion to such a level that the life-cycle greenhouse gas emissions were higher than with fossil fuels. A widely cited 2008 paper projected about 10.8 million hectares of land conversion and a "carbon debt" that would take 160 years to repay, and critics such as Mathews and Tan questioned its assumptions. Published ILUC values are scattered: they range from about 200% below, up to 1700% above the CF values of fossil fuels. For corn ethanol, the direct land-use contribution is smaller in some modelling scenarios, with at most 20% of life-cycle GHG emissions attributed to it. Regulators have responded unevenly. The EU's 2009 directives initially did not account for emissions due to ILUC, while US and California programs have included an indirect land use change factor when considering the carbon savings with biofuels.

The competition between food and fuel uses of crops is an older and broader concern, and it helped inspire the ILUC concept in the first place. Diverting maize or sugar to fuel can tighten supplies in food and feed markets and push up grain prices, though the size of that effect is debated, and the distillers' grains returned to livestock feed offset part of the demand. Water and fertilizer issues are largely local but significant: irrigation draws on rivers and aquifers, and nitrogen fertilizer applied to feedstock fields can leach or run off into waterways, contributing to nutrient pollution downstream.

Second-generation (cellulosic) ethanol faces a different economic problem. Capital cost estimates vary considerably. One analysis puts the investment for cellulosic plants at three to four times those for first-generation biofuel plants, while another reports costs roughly 5 times the per-gallon cost of a standard corn ethanol plant. In 2007 the USDA estimated production costs of $2.65 per gallon, compared with $1.65 for corn-based ethanol. Enzymes are a major driver, since enzymes cost can account for up to 30–50% of the total cost of ethanol production. Policy uncertainty has been equally important. The US renewable-fuel mandate originally called for a billion gallons of cellulosic ethanol in 2013, but the Environmental Protection Agency has reduced the target to just six million gallons. A 2026 review attributes the slow progress to technological complexity, limited feedstock availability, high production costs, and modest commercial outcomes, along with inconsistent policy support, competition from lower-cost biofuels, and the collapse of several large-scale ventures.

Subsidies, blending mandates and carbon-pricing schemes shape these economics by changing what producers earn per unit of carbon avoided. Mandates create guaranteed demand, while carbon prices or low-carbon fuel standards reward lower-emission pathways. Because the value of these incentives depends on contested emission estimates, their design is itself part of the debate.

Analysis, safety, and handling

Analytical methods

Quantitative analysis of ethanol relies mainly on chromatography. For blood and other biological specimens, gas chromatography with a flame ionization detector is widely regarded as the reference method, often run in headspace mode. mass-spectrometry can be coupled to the separation as an alternative or confirmatory detector, which helps distinguish ethanol from other volatile compounds that may be present in a sample. Rapid screening instead uses infrared-spectroscopy. Ethanol's C–H and C–O bonds absorb infrared light at characteristic wavelengths, so the degree of absorption indicates concentration. Because hydrocarbons and acetone absorb in overlapping regions, practical instruments use several narrow optical filters and a reference channel to improve specificity.

Breath and blood alcohol testing

Breath analysers have been used to estimate blood alcohol concentration since the 1970s. Most evidential instruments rely on infrared spectrometry near 3.4 µm, and some use electrochemical fuel-cell detectors instead. In a study of a five-filter Swedish instrument, the test was aborted in 27 of 24,072 cases (0.11%) because an interfering substance exceeded a threshold, and police then requested venous blood for gas chromatography. Interference is a real concern: acetone, which can rise in breath after the body metabolises 2-propanol, absorbs at similar wavelengths, and technical ethanol products may also contain other volatile ingredients. Because blood alcohol results carry legal consequences in impaired-driving cases, forensic laboratories treat chromatographic blood analysis as the benchmark against which breath devices are checked.

Flammability and fire hazards

Ethanol is a highly flammable liquid. Reported flash points cluster near 13 °C (55 °F), although values differ by test method, for example 12 °C closed-cup in one safety data sheet and 14 °C in another. Because ambient temperatures normally exceed the flash point, the liquid continuously releases ignitable vapour. Its vapour is about 1.6 times as dense as air, so it spreads along the ground and can flash back to a distant source. The vapour-air explosive range is roughly 3.3% to 19% by volume, which is wider than the range for gasoline. Denatured ethanol has a much lower flash point, around −20 °C, and even a 20% aqueous solution remains flammable with a flash point near 36 °C. Ethanol's boiling point is about 78 °C, so a vessel exposed to nearby fire can, in principle, suffer a boiling liquid expanding vapour explosion. Standard practice therefore calls for grounding and bonding during transfers, explosion-proof equipment, and good ventilation. The U.S. OSHA permissible exposure limit is 1000 ppm as a time-weighted average.

Denaturants and regulation

Industrial ethanol is commonly denatured by adding substances that make it unfit for drinking, which generally allows it to be sold without the excise duties levied on beverage alcohol. Denatured products are not uniform: formulas are specified by national tax and safety authorities, and in the United States the Alcohol and Tobacco Tax and Trade Bureau approves denaturing formulas. Labelling and handling rules then apply to the finished product. Denaturants also matter analytically, because they can appear as interfering compounds in breath and blood tests.

Storage, transport, and firefighting

Ethanol should be kept in approved flammable-liquid storage away from ignition sources and separate from strong oxidisers. Rail and barge spills of denatured ethanol require specific environmental assessment and response planning, since the liquid is soluble in water and its vapour is heavier than air. Firefighting is also distinctive. Water is a poor remedy: one fire-training source notes that about 4,000 to 5,000 gallons of water may be needed to dilute 1,000 gallons of ethanol. Conventional aqueous film-forming foam and protein foam are destroyed by polar solvents. Alcohol-resistant foams instead contain a high-molecular-weight polymer, often a polysaccharide such as xanthan gum, that precipitates at the fuel surface and forms a membrane protecting the foam blanket. Fluorine-free alcohol-resistant foams are now also available, and they are typically applied gently onto the liquid surface rather than plunged into it.

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