Ethanol

• 1. Introduction

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  Ethanol, the molecule that makes alcoholic drinks intoxicating, turns up in the breath of healthy volunteers at an average of 196 parts per billion. It also forms an icy coating on dust grains in interstellar clouds. This one plain liquid, built from two carbon atoms, sits in a wine glass, a hand sanitizer, a Brazilian fuel tank and the propellant system of an early rocket. People call it ethyl alcohol, grain alcohol, drinking alcohol, or just alcohol. It is the second most widely used drug in the world, behind caffeine. How did a word for a powdered cosmetic become the name of a drink? Who first worked out what the molecule looks like? Why does the body treat it as both a sedative and a poison? And what makes the same liquid worth burning, worth cleaning with and hard to purify?

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• 2. A Borrowed Name

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  In 1834 the chemist Justus Liebig coined the word ethyl for a group of atoms, building it from the German name Aether. Aether is the compound that English speakers call ether. According to the Oxford English Dictionary, ethyl contracts two Greek words: aithēr, meaning upper air, and hylē, meaning wood or raw material, and so matter or substance.

  The modern name ethanol came much later. A resolution on naming alcohols and phenols was adopted at an international chemical nomenclature conference in Geneva in April 1892. Under the systematic scheme, the prefix eth- marks two carbon atoms, the infix -an- marks a single bond between them, and the suffix -ol marks the attached OH group.

  The word alcohol has a stranger path. It is a medieval loan from the Arabic al-kuḥl, a powdered ore of antimony used since antiquity as a cosmetic. Middle Latin kept that meaning. The phrase alcohol of wine was first recorded for ethanol in 1753, and before the late 18th century the word generally meant any sublimated substance. Chemistry now uses alcohol for a wide class of compounds, yet in everyday speech it still means ethanol.

  The ether that gave ethyl its name would later take over one of ethanol's oldest jobs, when surgeons began using diethyl ether as an anesthetic in the 1840s.

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• 3. Water of Life

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  Dried residue on 9,000-year-old pottery found in China suggests that Neolithic people were already drinking alcoholic beverages. Turning sugar into ethanol through fermentation is one of the earliest biotechnologies humans used. Over the centuries the liquid collected names such as spirit of wine, ardent spirits and aqua vitae, which is Latin for water of life.

  Aristotle, Theophrastus and Pliny the Elder all knew that the fumes rising from wine could catch fire. That knowledge did not lead straight to isolating ethanol, even after distillation methods improved in second- and third-century Roman Egypt. A clue appears in a writing attributed to Jābir ibn Ḥayyān in the ninth century. Adding salt to boiling wine raises its relative volatility, which makes the vapors more flammable.

  Distilling wine is attested in Arabic works attributed to al-Kindī and al-Fārābī. It also appears in the 28th book of al-Zahrāwī's Kitāb al-Taṣrīf, which was later translated into Latin as Liber servatoris. By the twelfth century, Latin works began to carry recipes for aqua ardens, or burning water, made by distilling wine with salt. By the end of the thirteenth century, chemists across Western Europe knew it well.

  Taddeo Alderotti, who lived from 1223 to 1296, described repeated fractional distillation through a water-cooled still. His method reached an ethanol purity of 90%. Arnald of Villanova studied the medicinal properties of the spirit. So did John of Rupescissa, who called it the quintessence of wine and regarded it as a life-preserving substance able to prevent all diseases.

  Distillation also developed far from Europe. In China, archaeological evidence points to true distillation of alcohol under the Jin or Southern Song dynasties, and a 12th-century still has been found at an archaeological site in Qinglong, Hebei. In India the technique arrived from the Middle East, and by the 14th century it was in wide use in the Delhi Sultanate.

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• 4. Pinning Down the Formula

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  In 1796 the German-Russian chemist Johann Tobias Lowitz obtained pure ethanol. He mixed partially purified ethanol with an excess of anhydrous alkali, then distilled the mixture over low heat. The French chemist Antoine Lavoisier described ethanol as a compound of carbon, hydrogen and oxygen. In 1807 Nicolas-Théodore de Saussure determined its chemical formula.

  Fifty years after that, Archibald Scott Couper published the structural formula of ethanol. It was one of the first structural formulas ever determined.

  Michael Faraday made ethanol synthetically in 1825, though he did not know it. He found that sulfuric acid could absorb large volumes of coal gas, and he handed the resulting solution to Henry Hennell, a British chemist. In 1826 Hennell found that it contained what he called sulphovinic acid, now known as ethyl hydrogen sulfate. In 1828 Hennell and the French chemist Georges-Simon Serullas independently discovered that sulphovinic acid could be decomposed into ethanol. Faraday had unwittingly shown that ethylene, a component of coal gas, could become ethanol through acid-catalyzed hydration.

  Industry would not use a related route until 1930, when Union Carbide first practiced, at industrial scale, an indirect hydration of ethylene through sulfuric acid.

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• 5. Sedative and Antiseptic

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  Roughly 50% of east Asian populations carry a mutation in the ALDH2 gene that produces an inactive or dysfunctional enzyme. To see why that matters, follow what happens after a drink. In mammals, ADH enzymes in the liver and stomach oxidize ethanol into acetaldehyde, also called ethanal. Acetaldehyde is a known carcinogen and is far more toxic to humans than ethanol itself. Many symptoms of intoxication, and many hazards of long-term drinking, can be traced to it.

  ALDH enzymes normally finish the job by oxidizing acetaldehyde into acetate. With the ALDH2 mutation, acetaldehyde lingers, producing the alcohol flush reaction, which can redden the skin and bring other unpleasant symptoms. The mutation is typically paired with a second one, in the ADH1B enzyme, found in roughly 80% of east Asians. It makes the conversion of ethanol into acetaldehyde more efficient.

  Ethanol also behaves like a nutrient. Like proteins, fats and carbohydrates, it provides calories, contributing 7 kilocalories per gram. The toll is heavy. Globally, ethanol accounted for 1.8 million deaths in 2021, and long-term ingestion can cause serious liver damage.

  Medicine has used the same molecule on purpose. Ethanol is the oldest known sedative, and it served as an oral general anesthetic during surgery in ancient Mesopotamia and in medieval times. Mild intoxication begins at a blood alcohol concentration of 0.03-0.05%, while an anesthetic coma sets in at 0.4%. The risks of deadly intoxication and of vomiting with pulmonary aspiration pushed physicians toward alternatives such as opium and cannabis.

  Today its best-known medical role is as an antiseptic, in medical wipes and hand sanitizer gels. Ethanol kills microorganisms by dissolving the lipid bilayer of their membranes and denaturing their proteins. It works against most bacteria, fungi and viruses but not against bacterial spores, which call for hydrogen peroxide. A 70% solution beats pure ethanol, because the alcohol relies on water molecules for its best antimicrobial effect. Absolute ethanol may inactivate microbes without destroying them, since it cannot fully pass through the membrane.

  Some medicinal solutions of ethanol go by an older name: tinctures.

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• 6. Fuel for Engines

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  Gasoline sold in Brazil contains at least 25% anhydrous ethanol. Engine fuel and fuel additive is the largest single use of ethanol, and Brazil leans on it heavily. Hydrous ethanol, about 95% ethanol and 5% water, can power more than 90% of the new gasoline-fueled cars sold there. More than 20% of Brazilian cars can run on 100% ethanol, counting both ethanol-only engines and flex-fuel engines, which accept any mixture of ethanol and gasoline. A national infrastructure built on domestically grown sugarcane supports this fleet.

  The United States and many other countries mainly use E10, a 10% ethanol blend sometimes called gasohol, and E85, which is 85% ethanol. American flex-fuel vehicles can run on 0% to 85% ethanol, since higher blends are not yet allowed or efficient. Australian law limits the use of pure ethanol from sugarcane waste to 10% in automobiles. As of 2023, world production of ethanol fuel was 112.0 GL, with 51% coming from the United States and 26% from Brazil.

  The environmental record is mixed. An industry advocacy group says ethanol cuts tailpipe emissions of carbon monoxide, particulate matter and oxides of nitrogen. Argonne National Laboratory compared greenhouse gas emissions across many fuel and engine combinations. It found reductions of 8% for a B20 biodiesel blend, 17% for conventional E85 and 64% for cellulosic ethanol, all against pure gasoline. Ethanol also has a much higher research octane number than gasoline, so it resists pre-ignition and allows more torque and efficiency.

  The exhaust has a darker side. Burning ethanol yields significantly larger amounts of formaldehyde and acetaldehyde than gasoline does, which means more ground-level ozone. The Clean Fuels Report puts ozone from ethanol exhaust at 2.14 times that of gasoline exhaust. Its Localized Pollution Index rates ethanol at 1.7 against 1.0 for gasoline. In 2008 the California Air Resources Board recognized control standards for formaldehydes as an emissions control group.

  Ethanol carries less energy than gasoline, at 21.2 megajoules per liter against 34.8. Its fondness for water causes trouble too. It cannot travel through modern pipelines the way liquid hydrocarbons do, and mechanics link increased damage to small-engine carburetors with the water that ethanol fuel retains.

  The fuel has a long record. Ethanol lit lamps in the United States as early as 1840, until a tax on industrial alcohol during the Civil War made that use uneconomical. The tax was repealed in 1906. In 1908 the Ford Model T could run on petrol or on ethanol.

  Rockets used it as well. The German A-4 ballistic rocket of World War II, better known by its propaganda name V-2, burned ethanol as the main part of its B-Stoff. The ethanol was mixed with 25% water to lower the combustion chamber temperature. The V-2 design team later helped develop American rockets, including the ethanol-fueled Redstone, which launched the first American astronaut on a suborbital spaceflight. Alcohols fell out of favor as more energy-dense propellants arrived, though ethanol powered some recent experimental lightweight rocket-powered racing aircraft.

  Fuel cells are a newer frontier, with ethanol attractive for its wide availability, low cost, high purity and low toxicity. Most of the work is still at a research level. At the household scale, ethanol fireplaces can heat a home or simply decorate it, and ethanol also works as a cooking stove fuel.

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• 7. Making Ethanol

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  Bertam palm blossoms host symbiotic yeast that makes ethanol, so the liquid has natural sources far from any distillery. Yeast produces it as a byproduct of its metabolism, which means ethanol is present in any yeast habitat, including overripe fruit. Most animals show no interest in ethanol-containing food or avoid it, but the pentailed treeshrew seeks it out. Many plants also make ethanol while germinating, because of natural anaerobiosis. In the rare condition called auto-brewery syndrome, or gut fermentation syndrome, intoxicating quantities form through fermentation inside the digestive system.

  Industrially there are two routes. One is the petrochemical hydration of ethylene, and the other is fermentation of sugars by yeast. Prevailing prices of petroleum and grain feed stocks decide which is more economical.

  World production in 2006 was 51 GL, and Brazil and the United States together supplied 69% of it. Brazilian ethanol comes from sugarcane. Sugarcane holds about 30% more sucrose than corn and is much easier to extract. The leftover bagasse is burned in power plants, and that burning accounts for around 9% of the electricity produced in Brazil.

  The American story went the other way. In the 1970s most industrial ethanol in the United States was petrochemical, but in the 1980s the country introduced subsidies for corn-based ethanol. According to the Renewable Fuels Association, as of the 30th of October 2007, 131 grain ethanol bio-refineries could produce 7 billion US gallons a year. Another 72 construction projects promised 6.4 billion gallons of new capacity within 18 months.

  India makes ethanol from sugarcane. Sweet sorghum is a promising alternative that grows in dryland conditions. It needs one-third of the water of sugarcane over the same period, and about 22% less than corn. The International Crops Research Institute for the Semi-Arid Tropics is studying it as a source of fuel, food and animal feed for arid parts of Asia and Africa. The world's first sweet sorghum ethanol distillery began commercial production in 2007 in Andhra Pradesh, India.

  Fermentation relies on yeasts such as Saccharomyces cerevisiae, which turn sugar into ethanol and carbon dioxide at around 35-40 degrees Celsius. Ethanol is toxic to the yeast itself, so brewing alone cannot go far. The most tolerant strains survive up to about 18% ethanol by volume, and higher strengths need fortification or distillation. Starchy grains must first become sugar. Brewers traditionally let the grain germinate, or malt, which produces the enzyme amylase. Mashing then lets the amylase convert the remaining starch to sugar. Cellulose can also supply sugar, which could turn corncobs, straw, sawdust and switchgrass into renewable energy resources.

  The petrochemical route hydrates ethylene with a phosphoric acid catalyst on a porous support such as silica gel or diatomaceous earth. The Shell Oil Company first used this catalyst for large-scale ethanol production in 1947. The reaction runs at 300 degrees Celsius in high-pressure steam, with a 5:3 ratio of ethylene to steam. Union Carbide and others used it industrially. It is no longer practiced in the United States, where corn fermentation costs less.

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• 8. Stubborn Mixtures

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  Fractional distillation at atmospheric pressure can only concentrate ethanol to 95.6% by weight. That mixture is an azeotrope with a boiling point of 78.1 degrees Celsius, which means it boils as a single unit and distillation cannot purify it further. At lower pressure the azeotrope shifts toward more ethanol. Below roughly 70 torr, no azeotrope exists at all, and absolute ethanol can be distilled.

  Chemists found other ways around the wall. An entraining agent such as benzene, cyclohexane or heptane forms a new lower-boiling azeotrope with ethanol and water, and removing it leaves water-free ethanol. Molecular sieves with 3 Å pores, a type of zeolite, trap water molecules and exclude ethanol, and heating the wet sieves drives the water out so they can be reused. Membranes can separate the two without relying on vapor-liquid equilibria. Fractional freezing concentrates traditionally made applejack.

  The resulting grades of ethanol carry different taxes. Pure ethanol and alcoholic beverages are heavily taxed as psychoactive drugs, so most jurisdictions waive the tax on ethanol made unfit to drink. Agents added for this include the bittering compound denatonium benzoate and toxins such as methanol, naphtha and pyridine. The product is called denatured alcohol. Pure ethanol is classed as 200 proof in the United States, equal to 175 degrees proof in the British system. Rectified spirit is a 96% ethanol azeotrope with 4% water, and spirits of wine are about 94% ethanol, or 188 proof.

  Mixing ethanol and water produces small surprises. Equal volumes of the two give only 1.92 volumes of mixture, and the mixing is exothermic, releasing up to 777 joules per mole at 298 kelvin. Pure ethanol readily absorbs water from the air. Even a few percent of ethanol sharply lowers the surface tension of water, which partly explains the tears of wine. Swirl a glass and ethanol evaporates quickly from the thin film on the wall. The film's surface tension rises, and it beads up and runs down in channels.

  The liquid burns with a smokeless blue flame that is not always visible in normal light. Its adiabatic flame temperature in air is 2082 degrees Celsius. Pure ethanol has a flash point of 13 degrees Celsius, so mixtures can ignite below average room temperature, and solutions above 2.35% ethanol by mass count as a Class 3 hazardous flammable liquid.

  It dissolves both polar and nonpolar substances, so labs and industry use it widely, from paints and perfumes to markers and wet specimen preservatives. Chemists use ethanol precipitation in purifying DNA and RNA. Because ethanol freezes at -114 degrees Celsius, laboratories pair it with dry ice as a cooling bath, and alcohol thermometers use it as their working fluid.

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