Ethanol

Ethanol, also known as ethyl alcohol, drinking alcohol or grain alcohol, is a flammable, colorless, mildly toxic chemical compound, and is best known as the alcohol found in alcoholic beverages. In common usage, it is often referred to simply as alcohol. Its molecular formula is variously represented as EtOH, CH₃CH₂OH, C₂H₅OH or as its empirical formula C₂H₆O (which it shares with dimethyl ether).

After the use of fire, fermentation of sugar into ethanol is perhaps the earliest organic reaction known to humanity, and the intoxicating effects of ethanol consumption have been known since ancient times. In modern times ethanol intended for industrial use has also been produced from byproducts of petroleum refining.

Because of ethanol's ease of production and because exposure to low amounts does negligible harm, it has widespread use as a solvent for substances intended for human contact or consumption, including scents, flavorings, colorings, and medicines. In chemistry it is both an essential solvent and a feedstock for the synthesis of other products. Because it burns cleanly, ethanol has a long history as a fuel, including as a fuel for internal combustion engines.

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History

Ethanol has been used by humans since prehistory as the intoxicating ingredient in alcoholic beverages. Dried residues on 9000-year-old pottery found in China imply the use of alcoholic beverages even among Neolithic people.^[1] Its isolation as a relatively pure compound was first achieved by Muslim chemists who developed the art of distillation during the Abbasid caliphate, the most notable of whom were Jabir ibn Hayyan (Geber), Al-Kindi (Alkindus) and al-Razi (Rhazes). The writings attributed to Jabir ibn Hayyan (721-815) mention the flammable vapors of boiled wine. Al-Kindi (801-873) unambiguously described the distillation of wine.^[2] Absolute ethanol was obtained in 1796 by Johann Tobias Lowitz, by filtering distilled ethanol through charcoal.

Antoine Lavoisier described ethanol as a compound of carbon, hydrogen, and oxygen, and in 1808, Nicolas-Théodore de Saussure determined ethanol's chemical formula. Fifty years later Archibald Scott Couper published a structural formula for ethanol, which places ethanol among the first of chemical compounds to have its chemical structures determined.^[3]

Ethanol was first prepared synthetically in 1826, through the independent efforts of Henry Hennel in Great Britain and S.G. Sérullas in France. Michael Faraday prepared ethanol by the acid-catalyzed hydration of ethylene in 1828, in a process similar to that used for industrial ethanol synthesis today.^[4]

Ethanol served as lamp fuel in the United States as early as 1840, although taxes levied during the Civil War on industrial alcohol rendered the practice uneconomical.^[5] The tax was not repealed until 1906,^[5] and from 1908 Ford Model T automobiles could be adapted to run on ethanol.^[6] With the advent of Prohibition in 1920 though, sellers of ethanol fuel were accused of being allies of moonshiners,^[5] and ethanol fuel once again faded from the public eye. The recent rise in oil prices has spurred renewed interest.

Political support has also increased recently for more ethanol based products. For example, Hilary Clinton originally opposed ethanol research efforts, stating in 2002 that “there is no sound public policy reason for mandating the use of ethanol.” ^[7] Now, Senator Clinton, and many other leaders from both the Republican and Democratic party are supporters of ethanol-based fuel solutions.

Physical properties

The properties of ethanol stem primarily from the presence of its hydroxyl group and the shortness of its carbon chain. Ethanol's hydroxyl group is able to participate in hydrogen bonding, rendering it more viscous and less volatile than less polar organic compounds of similar molecular weight. Ethanol, like most short-chain alcohols, is flammable, colorless, has a strong odor, and is volatile.

Ethanol is slightly more refractive than water with a refractive index of 1.36242 (at λ=589.3 nm and 18.35 °C).^[8]

Ethanol is a versatile solvent, miscible in all proportions with water and many organic solvents, including acetic acid, acetone, benzene, carbon tetrachloride, chloroform, diethyl ether, ethylene glycol, glycerol, nitromethane, pyridine, and toluene.^[8][9] It is also miscible with light aliphatic hydrocarbons such as pentane and hexane, as well as aliphatic chlorides such as trichloroethane and tetrachloroethylene.^[9] Ethanol's miscibility with water is in contrast to longer chain alcohols (five or more carbons), whose water solubility decreases rapidly as the number of carbons increases.^[10]

Hydrogen bonding causes pure ethanol to be hygroscopic to the extent that it readily absorbs water from the air. The polar nature of the hydroxyl group causes ethanol to dissolve many ionic compounds, notably sodium and potassium hydroxides, magnesium chloride, calcium chloride, ammonium chloride, ammonium bromide, and sodium bromide.^[9] Sodium and potassium chlorides are slightly soluble in ethanol.^[9] Because the ethanol molecule also has a nonpolar end, it also dissolves nonpolar substances, including most essential oils,^[11] as well as numerous flavoring, coloring, and medicinal agents.

Several unusual phenomena are associated with mixtures of ethanol and water. Ethanol-water mixtures have less volume than their individual components. A mixture of equal volumes ethanol and water has only 95.6% of the volume of equal parts ethanol and water, unmixed (at 15.56 °C).^[8] The addition of even a few percent of ethanol to water sharply reduces the surface tension of water. This property partially explains the tears of wine phenomenon. When wine is swirled in a glass, ethanol evaporates quickly from the thin film of wine on the wall of the glass. As its ethanol content decreases, its surface tension increases, and the thin film beads up and runs down the glass in channels rather than as a smooth sheet.

Ethanol and mixtures with water greater than about 50% ethanol are flammable and easily ignited. This principle was used for the alcoholic proof, which initially consisted on adding gunpowder to a given liquor: if the mixture ignited, it was considered to be "100 proof". Ethanol-water solutions below 50% ethanol by volume may also be flammable if the solution is vaporized by heating (as in some cooking methods that call for wine to be added to a hot pan, causing it to flash boil into a vapor, which is then ignited to "burn off" excessive alcohol).

Chemistry

For more details on this topic, see Alcohol.

Ethanol is classified as a primary alcohol, meaning that the carbon to which its hydroxyl group is attached has at least two hydrogen atoms attached to it as well.

The chemistry of ethanol is largely that of its hydroxyl group.

Acid-base chemistry

Ethanol's hydroxyl proton is very weakly acidic; it is an even weaker acid than water. Ethanol can be quantitatively converted to its conjugate base, the ethoxide ion (CH₃CH₂O⁻), by reaction with an alkali metal such as sodium:^[10]

2CH₃CH₂OH + 2Na → 2CH₃CH₂ONa + H₂

Halogenation

Under special conditions, ethanol reacts with hydrogen halides to produce ethyl halides such as ethyl chloride and ethyl bromide:

CH₃CH₂OH + HCl → CH₃CH₂Cl + H₂O

HCl reaction requires a catalyst such as zinc chloride.^[12]

CH₃CH₂OH + HBr → CH₃CH₂Br + H₂O

HBr requires refluxing with a sulfuric acid catalyst.^[12]

Ethyl halides can also be produced by reacting ethanol with more specialized halogenating agents, such as thionyl chloride for preparing ethyl chloride, or phosphorus tribromide for preparing ethyl bromide.^[10][12]

Ester formation

Under acid-catalyzed conditions, ethanol reacts with carboxylic acids to produce ethyl esters and water:

RCOOH + HOCH₂CH₃ → RCOOCH₂CH₃ + H₂O

For this reaction to produce useful yields it is necessary to remove water from the reaction mixture as it is formed.

Ethanol can also form esters with inorganic acids. Diethyl sulfate and triethyl phosphate, prepared by reacting ethanol with sulfuric and phosphoric acid respectively, are both useful ethylating agents in organic synthesis. Ethyl nitrite, prepared from the reaction of ethanol with sodium nitrite and sulfuric acid, was formerly a widely-used diuretic.

Dehydration

Strong acid desiccants, such as sulfuric acid, cause ethanol's dehydration to form either diethyl ether or ethylene:

2 CH₃CH₂OH → CH₃CH₂OCH₂CH₃ + H₂O

CH₃CH₂OH → H₂C=CH₂ + H₂O

Which product, diethyl ether or ethylene, predominates depends on the precise reaction conditions.

Oxidation

  Ethanol can be oxidized to acetaldehyde, and further oxidized to acetic acid. In the human body, these oxidation reactions are catalyzed by enzymes. In the laboratory, aqueous solutions of strong oxidizing agents, such as chromic acid or potassium permanganate, oxidize ethanol to acetic acid, and it is difficult to stop the reaction at acetaldehyde at high yield. Ethanol can be oxidized to acetaldehyde, without over oxidation to acetic acid, by reacting it with pyridinium chromic chloride.^[12]

Chlorination

When exposed to chlorine, ethanol is both oxidized and its alpha carbon chlorinated to form the compound, chloral.

4Cl₂ + C₂H₅OH → CCl₃CHO + 5HCl

Combustion

Combustion of ethanol forms carbon dioxide and water:

C₂H₅OH + 3 O₂ → 2 CO₂ + 3 H₂O

Production

Ethanol is produced both as a petrochemical, through the hydration of ethylene, and biologically, by fermenting sugars with yeast.^[13] Which process is more economical is dependent upon the prevailing prices of petroleum and of grain feed stocks.

Ethylene hydration

Ethanol for use as industrial feedstock is most often made from petrochemical feed stocks, typically by the acid-catalyzed hydration of ethylene, represented by the chemical equation

C₂H_4(g) + H₂O_(g) → CH₃CH₂OH_(l)

The catalyst is most commonly phosphoric acid,^[14] adsorbed onto a porous support such as diatomaceous earth or charcoal. This catalyst was first used for large-scale ethanol production by the Shell Oil Company in 1947.^[15] The reaction is carried out at with an excess of high pressure steam at 300 °C.

In an older process, first practiced on the industrial scale in 1930 by Union Carbide,^[16] but now almost entirely obsolete, ethylene was hydrated indirectly by reacting it with concentrated sulfuric acid to produce ethyl sulfate, which was then hydrolyzed to yield ethanol and regenerate the sulfuric acid:^[12]

C₂H₄ + H₂SO₄ → CH₃CH₂SO₄H

CH₃CH₂SO₄H + H₂O → CH₃CH₂OH + H₂SO₄

Fermentation

For more details on this topic, see Ethanol fermentation.

Ethanol for use in alcoholic beverages, and the vast majority of ethanol for use as fuel, is produced by fermentation. When certain species of yeast, most importantly, Saccharomyces cerevisiae, metabolize sugar in the absence of oxygen, they produce ethanol and carbon dioxide. The chemical equation below summarizes the conversion:

C₆H₁₂O₆ → 2 CH₃CH₂OH + 2 CO₂

The process of culturing yeast under conditions to produce alcohol is called brewing. Ethanol's toxicity to yeast limits the ethanol concentration obtainable by brewing. The most ethanol-tolerant strains of yeast can survive up to approximately 15% ethanol by volume.^[17]

The fermentation process must exclude oxygen. If oxygen is present, yeast undergo aerobic respiration which produces carbon dioxide and water rather than ethanol.

In order to produce ethanol from starchy materials such as cereal grains, the starch must first be converted into sugars. In brewing beer, this has traditionally been accomplished by allowing the grain to germinate, or malt, which produces the enzyme, amylase. When the malted grain is mashed, the amylase converts the remaining starches into sugars. For fuel ethanol, the hydrolysis of starch into glucose can be accomplished more rapidly by treatment with dilute sulfuric acid, fungally produced amylase, or some combination of the two.^[18]

Cellulosic ethanol

Main article: Cellulosic ethanol

Sugars for ethanol fermentation can be obtained from cellulose. Until recently, however, the cost of the cellulase enzymes capable of hydrolyzing cellulose has been prohibitive. The Canadian firm, Iogen, brought the first cellulose-based ethanol plant on-stream in 2004.^[19] Its primary consumer so far has been the Canadian government, which, along with the United States Department of Energy, has invested heavily in the commercialization of cellulosic ethanol. Deployment of this technology could turn a number of cellulose-containing agricultural byproducts, such as corncobs, straw, and sawdust, into renewable energy resources. Other enzyme companies are developing genetically engineered fungi that produce large volumes of cellulase, xylanase and hemicellulase enzymes. These would convert agricultural residues such as corn stover, wheat straw and sugar cane bagasse and energy crops such as switchgrass into fermentable sugars.^[20]

Cellulose-bearing materials typically also contain other polysaccharides, including hemicellulose. When hydrolyzed, hemicellulose decomposes into mostly five-carbon sugars such as xylose. S. cerevisiae, the yeast most commonly used for ethanol production, cannot metabolize xylose. Other yeasts and bacteria are under investigation to ferment xylose and other pentoses into ethanol.^[21]

Prospective technologies

The anaerobic bacterium Clostridium ljungdahlii, recently discovered in commercial chicken wastes, can produce ethanol from single-carbon sources including synthesis gas, a mixture of carbon monoxide and hydrogen that can be generated from the partial combustion of either fossil fuels or biomass. Use of these bacteria to produce ethanol from synthesis gas has progressed to the pilot plant stage at the BRI Energy facility in Fayetteville, Arkansas.^[22]

Another prospective technology is the closed-loop ethanol plant.^[23] Ethanol produced from corn has a number of critics who suggest that it is primarily just recycled fossil fuels because of the energy required to grow the grain and convert it into ethanol. However, the closed-loop ethanol plant attempts to address this criticism. In a closed-loop plant, the energy for the distillation comes from fermented manure, produced from cattle that have been fed the by-products from the distillation. The leftover manure is then used to fertilize the soil used to grow the grain. Such a process is expected to have a much lower fossil fuel requirement.^[24]

Though in an early stage of research, there is some development of alternative production methods that use feed stocks such as municipal waste or recycled products, rice hulls, sugarcane bagasse, small diameter trees, wood chips, and switchgrass.^[25]

Testing

  Breweries and biofuel plants employ two methods for measuring ethanol concentration. Infrared ethanol sensors measure the vibrational frequency of dissolved ethanol using the CH band at 2900 cm⁻¹. This method uses a relatively inexpensive solid state sensor that compares the CH band with a reference band to calculate the ethanol content. The calculation makes use of the Beer-Lambert law. Alternatively, by measuring the density of the starting material and the density of the product, using a hydrometer, the change in specific gravity during fermentation indicates the alcohol content. This inexpensive and indirect method has a long history in the beer brewing industry.

Purification

Main article: Ethanol purification

Ethylene hydration or brewing produces an ethanol-water mixture. For most industrial and fuel uses, the ethanol must be purified. Fractional distillation can concentrate ethanol to 95.6% by weight (89.5 mole%). This mixture is an azeotrope with a boiling point of 78.1 °C, and cannot be further purified by distillation.

In one common industrial method to obtain absolute alcohol, a small quantity of benzene is added to rectified spirit and the mixture is then distilled. Absolute alcohol is obtained in the third fraction, which distills over at 78.3 °C (351.4 K).^[10] Because a small amount of the benzene used remains in the solution, absolute alcohol produced by this method is not suitable for consumption, as benzene is carcinogenic.^[26]

There is also an absolute alcohol production process by desiccation using glycerol. Alcohol produced by this method is known as spectroscopic alcohol — so called because the absence of benzene makes it suitable as a solvent in spectroscopy.

Other methods for obtaining absolute ethanol include desiccation using adsorbents such as starch or zeolites, which adsorb water preferentially, as well as azeotropic distillation and extractive distillation.

Types of ethanol

Denatured alcohol

Main article: Denatured alcohol

Pure ethanol and alcoholic beverages are heavily taxed. Ethanol has many applications that do not involve human consumption. To relieve the tax burden on these applications, most jurisdictions waive the tax when agents have been added to the ethanol to render it unfit for human consumption. These include bittering agents such as denatonium benzoate, as well as toxins such as methanol, naphtha, and pyridine.^[27][28]

Absolute ethanol

Absolute or anhydrous alcohol generally refers to purified ethanol, containing no more than one percent water. Absolute alcohol not intended for human consumption often contains trace amounts of toxic benzene (used to remove water by azeotropic distillation).

Pure ethanol is classed as 200 proof in the USA, equivalent to 175 degrees proof in the (now rarely used) UK system.

Use

As a fuel

Main article: Ethanol fuel

The largest single use of ethanol is as a motor fuel and fuel additive. The largest national fuel ethanol industries exist in Brazil (gasoline sold in Brazil contains at least 20% ethanol and anhydrous ethanol is also used as fuel).^[32]

Today, almost half of Brazilian cars are able to use 100% ethanol as fuel, which includes ethanol-only engines and flex-fuel engines. Flex-fuel engines are able to work with all ethanol, all gasoline, or any mixture of both. Brazil supports this population of ethanol-burning automobiles with large national infrastructure that produces ethanol from domestically grown sugar cane. Sugar cane not only has a greater concentration of sucrose than corn (by about 30%), but is also much easier to extract. The bagasse generated by the process is not wasted, but is utilized in power plants as a surprisingly efficient fuel to produce electricity.

World production of ethanol in 2006 was 51 billion liters, (13.5 billion gallons), with 69% of the world supply coming from Brazil and the United States.^[33]

The United States fuel ethanol industry is based largely on maize. According to the Renewable Fuels Association, as of October 30, 2007, 131 grain ethanol biorefineries in the United States have the capacity to produce 7.0 billion gallons of ethanol per year. An additional 72 construction projects underway (in the U.S.) can add 6.4 billion gallons of new capacity in the next 18 months. Over time, it is believed that a material portion of the ~150 billion gallon per year market for gasoline will begin to be replaced with fuel ethanol.^[34]

The Energy Policy Act of 2005 requires that 4 billion gallons of "renewable fuel" be used in 2006 and this requirement will grow to a yearly production of 7.5 billion gallons by 2012.^[35]

  In the United States, ethanol is most commonly blended with gasoline as a 10% ethanol blend nicknamed "gasohol". This blend is widely sold throughout the U.S. Midwest, and in cities required by the 1990 Clean Air Act to oxygenate their gasoline during the winter.

Controversy

As reported in "The Energy Balance of Corn Ethanol: an Update,"^[36] the energy returned on energy invested EROEI for ethanol made from corn in the U.S. is 1.34 (it yields 34% more energy than it takes to produce it). Input energy includes natural gas based fertilizers, farm equipment, transformation from corn or other materials, and transportation. However, other researchers report that the production of ethanol consumes more energy than it yields.^[37][38]

Environmentalists, livestock farmers, and opponents of subsidies say that increased ethanol production won't meet energy goals and may damage the environment as food prices soar. Some of the controversial subsidies in the past have included more than $10 billion to Archer-Daniels-Midland since 1980.^[39] Critics also speculate that as ethanol is more widely used, changing irrigation practices could greatly increase pressure on water resources. In October 2007, 28 environmental groups decried the Renewable Fuels Standard (RFS), a legislative effort intended to increase ethanol production, and said that the measure will "lead to substantial environmental damage and a system of biofuels production that will not benefit family farmers...will not promote sustainable agriculture and will not mitigate global climate change."^[40][41] Recent articles have also blamed subsidized ethanol production for the nearly 200% increase in milk prices since 2004,^[42] although that is disputed by some.

Oil has historically had a much higher EROEI than agriculturally produced ethanol. Apart from this, the amount of ethanol needed to run the United States, for example, is greater than its own farmland could produce, even if fields now used for food were converted for production of non-food-grade corn. It has been estimated that "if every bushel of U.S. corn, wheat, rice and soybean were used to produce ethanol, it would only cover about 4% of U.S. energy needs on a net basis."^[43]

In the United States, preferential regulatory and tax treatment of ethanol automotive fuels introduces complexities beyond its energy economics alone. North American automakers have in 2006 and 2007 promoted a blend of 85% ethanol and 15% gasoline, marketed as E85, and their flex-fuel vehicles, e.g. GM's "Live Green, Go Yellow" campaign.^[44] The apparent motivation is the nature of U.S. Corporate Average Fuel Economy (CAFE) standards, which give an effective 54% fuel efficiency bonus to vehicles capable of running on 85% alcohol blends over vehicles not adapted to run on 85% alcohol blends.^[45] In addition to this auto manufacturer-driven impetus for 85% alcohol blends, the United States Environmental Protection Agency had authority to mandate that minimum proportions of oxygenates be added to automotive gasoline on regional and seasonal bases from 1992 until 2006 in an attempt to reduce air pollution, in particular ground-level ozone and smog.^[46] In the United States, incidents of methyl tert(iary)-butyl ether (MTBE) groundwater contamination have been recorded in the majority of the 50 states,^[47] and the State of California's ban on the use of MTBE as a gasoline additive has further driven the more widespread use of ethanol as the most common fuel oxygenate.^[48]

Rocket fuel

Ethanol was commonly used as fuel in early bipropellant rocket vehicles, in conjunction with an oxidizer such as liquid oxygen. The German V-2 rocket of World War II, credited with beginning the space age, used ethanol, mixed with water to reduce the combustion chamber temperature.^[49][50] The V-2's design team helped develop U.S. rockets following World War II, including the ethanol-fueled Redstone rocket, which launched the first U.S. satellite.^[51] Alcohols fell into general disuse as more efficient rocket fuels were developed.^[50]

Alcoholic beverages

Main article: Alcoholic beverage

Ethanol is the principal psychoactive constituent in alcoholic beverages, with depressant effects to the central nervous system. It has a complex mode of action and affects multiple systems in the brain, most notably ethanol acts as an agonist to the GABA receptors.^[52] Similar psychoactives include those which also interact with GABA receptors, such as gamma-hydroxybutyric acid.^[53]

Alcoholic beverages vary considerably in their ethanol content and in the foodstuffs from which they are produced. Most alcoholic beverages can be broadly classified as fermented beverages, beverages made by the action of yeast on sugary foodstuffs, or as distilled beverages, beverages whose preparation involves concentrating the ethanol in fermented beverages by distillation. The ethanol content of a beverage is usually measured in terms of the volume fraction of ethanol in the beverage, expressed either as a percentage or in alcoholic proof units.

Fermented beverages can be broadly classified by the foodstuff from which they are fermented. Beers are made from cereal grains or other starchy materials, wines and ciders from fruit juices, and meads from honey. Cultures around the world have made fermented beverages from numerous other foodstuffs, and local and national names for various fermented beverages abound.

Distilled beverages are made by distilling fermented beverages. Broad categories of distilled beverages include whiskeys, distilled from fermented cereal grains; brandies, distilled from fermented fruit juices, and rum, distilled from fermented molasses or sugarcane juice. Vodka and similar neutral grain spirits can be distilled from any fermented material (grain or potatoes are most common); these spirits are so thoroughly distilled that no tastes from the particular starting material remain. Numerous other spirits and liqueurs are prepared by infusing flavors from fruits, herbs, and spices into distilled spirits. A traditional example is gin, which is created by infusing juniper berries into a neutral grain alcohol.

In a few beverages, ethanol is concentrated by means other than distillation. Applejack is traditionally made by freeze distillation, by which water is frozen out of fermented apple cider, leaving a more ethanol-rich liquid behind. Eisbier (more commonly, eisbock) is also freeze-distilled, with beer as the base beverage. Fortified wines are prepared by adding brandy or some other distilled spirit to partially-fermented wine. This kills the yeast and conserves some of the sugar in grape juice; such beverages are not only more ethanol-rich, but are often sweeter than other wines.

Alcoholic beverages are sometimes used in cooking, not only for their inherent flavors, but also because the alcohol dissolves hydrophobic flavor compounds which water cannot.

Feedstock

Main article: Chemical derivatives of ethanol

Ethanol is an important industrial ingredient and has widespread use as a base chemical for other organic compounds. These include ethyl halides, ethyl esters, diethyl ether, acetic acid, butadiene, and ethyl amines.

Antiseptic use

Ethanol is used in medical wipes and in most common antibacterial hand sanitizer gels at a concentration of about 62% (percentage by weight, not volume) as an antiseptic. Ethanol kills organisms by denaturing their proteins and dissolving their lipids and is effective against most bacteria and fungi, and many viruses, but is ineffective against bacterial spores.^[54]

Antidote

Although ethanol has mild toxic effects at high doses, it is sometimes used as an antidote for poisoning by other, more toxic alcohols, in particular methanol^[55] and ethylene glycol. Ethanol competes with other alcohols for the alcohol dehydrogenase enzyme, preventing metabolism into toxic aldehyde and carboxylic acid derivatives.^[56]

Other uses

• Ethanol is easily miscible in water and is a good solvent. Ethanol is less polar than water and used in perfumes, paints and tinctures.
• Ethanol is also used in design and sketch art markers, such as Copic, and Tria.

Effect on humans

Main article: Effects of alcohol on the body

Superficially, ethanol evokes a distinctive heat-like sensation in the mouth and a stinging sensation on the skin. In the body it is metabolized to other substances, affecting the central nervous system in particular. The effect varies between individuals, and can be worse when applied in addition to certain drugs.

Superficial

Pure ethanol evokes no taste sensation, but a strong and distinctive smell sensation. On the other hand, it produces a characteristic heat-like sensation when brought into contact with the tongue or mucous membranes, which explains its effect in alcoholic beverages. When applied to open wounds (as for disinfection) it produces a strong stinging sensation. Pure or highly concentrated ethanol may permanently damage living tissue on contact. Ethanol applied to unbroken skin cools the skin rapidly through evaporation.

Metabolism

Main article: Alcohol metabolism

Ethanol within the human body is converted into acetaldehyde by alcohol dehydrogenase and then into acetic acid by acetaldehyde dehydrogenase. The product of the first step of this breakdown, acetaldehyde,^[58] is more toxic than ethanol. Acetaldehyde is linked to most of the clinical effects of alcohol. It has been shown to increase the risk of developing cirrhosis of the liver,^[53] multiple forms of cancer, and alcoholism.

Cognitive

Ethanol is a central nervous system depressant and has significant psychoactive effects in sublethal doses; for specifics, see effects of alcohol on the body by dose. Based on its abilities to change the human consciousness, ethanol is considered a drug.^[59] Death from ethyl alcohol consumption is possible when blood alcohol level reaches 0.4%. A blood level of 0.5% or more is commonly fatal. Levels of even less than 0.1% can cause intoxication, with unconsciousness often occurring at 0.3-0.4%.^[60]

The amount of ethanol in the body is typically quantified by blood alcohol content (BAC), the milligrams of ethanol per 100 milliliters of blood. The table at right summarizes the symptoms of ethanol consumption. Small doses of ethanol generally produce euphoria and relaxation; people experiencing these symptoms tend to become talkative and less inhibited, and may exhibit poor judgement. At higher dosages (BAC > 100 mg/dl), ethanol acts as a central nervous system depressant, producing at progressively higher dosages, impaired sensory and motor function, slowed cognition, stupefaction, unconsciousness, and possible death.

In America, about half of the deaths in car accidents occur in alcohol-related crashes.^[61] There is no completely safe level of alcohol for driving, since the risk of a fatal car accident rises exponentially with the level of alcohol in the driver's blood.^[62] However, most drunk driving laws governing the acceptable levels in the blood while driving or operating heavy machinery set typical upper limits of between 0.05% or 0.08%.

Drug interaction

Ethanol interacts in harmful ways with a number of other drugs, including barbiturates, benzodiazepines, narcotics, and phenothiazines^[60]

Magnitude of effect

Some individuals have less effective forms of one or both of the metabolizing enzymes, and can experience more severe symptoms from ethanol consumption than others. Conversely, those who have acquired ethanol tolerance have a greater quantity of these enzymes, and metabolize ethanol more rapidly.^[63]

Other effects

Frequent use of alcoholic beverages has also been shown to be a major contributing factor in cases of elevated blood levels of triglycerides.^[64]

Ethanol itself is not a carcinogen,^[65][66] but effects on the liver when ingested can contribute to immune suppression. As such, ethanol consumption can be an aggravating factor in cancers resulting from other causes.

See also

References

Further reading