Ethanol

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Ethanol
Ethanol
General
Systematic name Ethanol
Other names Ethyl alcohol,
grain alcohol,
hydroxyethane
Molecular formula C2H6O
SMILES CCO
Molar mass 46.07 g/mol
Appearance Colorless liquid
CAS number [64-17-5]
Properties
Density and phase 0.789 g/cm3, liquid
Solubility in water Fully miscible
Melting point −114.3 °C (158.8 K)
Boiling point 78.4 °C (351.6 K)
Acidity (pKa) 15.9 (H+ from OH group)
Viscosity 1.200 c P at 20 °C
Dipole moment 1.69 D (gas)
Hazards
MSDS External MSDS
EU classification Flammable (F)
Irritant (Xi)
NFPA 704 Image:nfpa_h1.pngImage:nfpa_f3.pngImage:nfpa_r0.png
R-phrases R11
S-phrases S2, S7, S16
Flash point 13 °C (55.4 °F)
RTECS number KQ6300000
Supplementary data page
Structure & properties n, εr, etc.
Thermodynamic data Phase behaviour
Solid, liquid, gas
Spectral data UV, IR, NMR, MS
Related compounds
Related alcohols Methanol, 1-Propanol
Other heteroatoms Ethylamine, Ethyl chloride,
Ethyl bromide, Ethanethiol
Substituted ethanols Ethylene glycol, Ethanolamine,
2-Chloroethanol
Other compounds Acetaldehyde, Acetic acid
Except where noted otherwise, data are given for
materials in their standard state (at 25°C, 100 kPa)
Infobox disclaimer and references

Ethanol, also known as ethyl alcohol or grain alcohol, is a flammable, colorless chemical compound, one of the alcohols that is most often found in alcoholic beverages. In common parlance, it is often referred to simply as alcohol. Its chemical formula is C2H5 OH, also written as C2H6O.

This article is mostly about ethanol as a chemical compound. For beverages containing ethanol, see alcoholic beverage. For the use of ethanol as a fuel, see alcohol fuel. For its physiological effects, see effects of alcohol on the body.

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 northern China imply the use of alcoholic beverages even among Neolithic peoples. Its isolation as a relatively pure compound was first achieved by Islamic alchemists who developed the art of distillation during the Abbasid caliphate. The writings attributed to Jabir Ibn Hayyan (Geber) (721-815) mention the flammable vapors of boiled wine. Al-Kindī (801-873) unambiguously described the distillation of wine. Distillation of ethanol from water yields a product that is at most 96% ethanol, because ethanol forms an azeotrope with water. Absolute ethanol was first 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. In 1858, Archibald Scott Couper published a structural formula for ethanol: this places ethanol among the first chemical compounds to have their chemical structures determined.

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

Physical properties

Ethanol's hydroxyl group is able to participate in hydrogen bonding. At the molecular level, liquid ethanol consists of hydrogen-bonded pairs of ethanol molecules; this phenomenon renders ethanol more viscous and less volatile than less polar organic compounds of similar molecular weight. In the vapor phase, there is little hydrogen bonding; ethanol vapor consists of individual ethanol molecules.

Ethanol is a versatile solvent. It is miscible with water and with most organic liquids, including nonpolar liquids such as aliphatic hydrocarbons. Organic solids of low molecular weight are usually soluble in ethanol. Among ionic compounds, many monovalent salts are at least somewhat soluble in ethanol, with salts of large, polarizable ions being more soluble than salts of smaller ions. Most salts of polyvalent ions are practically insoluble in ethanol.

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 96% of the volume of equal parts ethanol and water, unmixed. The addition of even a few percent ethanol to water sharply reduces the surface tension of water. This property partially explains the tears of wine phenomenon: when wine is swirled inside 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.

Chemistry

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 (C2H5O), by reaction with an alkali metal such as sodium. This reaction evolves hydrogen gas:

CH3CH2OH + Na → CH3CH2ONa + ½ H2
Nucleophilic substitution

In aprotic solvents, ethanol reacts with the hydrogen halides to give ethyl halides such as ethyl chloride and ethyl bromide via nucleophilic substitution:

CH3CH2OH + HCl → CH3CH2Cl + H2O
CH3CH2OH + HBr → CH3CH2Br + H2O

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

Esterification

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

RCOOH + HOCH2CH3 RCOOCH2CH3 + H2O

The reverse reaction, hydrolysis of the resulting ester back to ethanol and the carboxylic acid, limits the extent of reaction, and high yields are unusual unless water can be removed from the reaction mixture as it is formed. Esterification can also be carried out using more a reactive derivative of the carboxylic acid, such as an acyl chloride or acid anhydride.

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 acids, such as sulfuric acid, can catalyse 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 catalysed by enzymes. In the laboratory, aqueous soluations 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 overoxidation to acetic acid, by pyridinium chromic chloride.

Production

94% denatured ethanol sold in a secure bottle for household use
94% denatured ethanol sold in a secure bottle for household use

Ethanol is produced both as a petrochemical, through the hydration of ethylene, and biologically, by fermenting sugars with yeast.

Ethylene hydration

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

C2H4 + H2O → CH3CH2OH

The catalyst is most commonly phosphoric acid, absorbed 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. Solid catalysts, mostly various metal oxides, have also been mentioned in the chemical literature.

In an older process, first practised on the industrial scale in 1930 by Union Carbide, but now almost entirely obsolete, ethene was hydrated indirectly by reacting it with concentrated sulfuric acid to product ethyl sulfate, which was then hydrolysed to yield ethanol and regenerate the sulphuric acid:

C2H4 + H2SO4 CH3CH2SO4
CH3CH2SO4 + H2O → CH3CH2OH + H2SO4

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 overall chemical reaction conducted by the yeast may be represented by the chemical equation

C6H12O6 → 2 CH3CH2OH + 2 CO2

The process of culturing yeast under conditions to produce alcohol is referred to as brewing. Brewing can only produce relatively dilute concentrations of ethanol in water; concentrated ethanol solutions are toxic to yeast. The most ethanol-tolerant strains of yeast can survive in up to about 20% ethanol (by volume).

In order to produce ethanol from starchy materials such as cereal grains, the starch must first be broken down into sugars. In brewing beer, this has traditionally been accomplished allowing the grain to germinate, or malt. In the process of germination, the seed produces enzymes that can break its starches into sugars. For fuel ethanol, this hydrolysis of starch into glucose is accomplished more rapidly by treatment with dilute sulfuric acid, fungal amylase enzymes, or some combination of the two.

At petroleum prices like those that prevailed through much of the 1990s, ethylene hydration was a decidedly more economical process than fermentation for producing purified ethanol. Recent increases in petroleum prices, coupled with perennial uncertainty in agricultural prices, make forecasting the relative production costs of fermented versus petrochemical ethanol difficult at the present time.

Purification

The product of either ethylene hydration or brewing is an ethanol-water mixture. For most industrial and fuel uses, the ethanol must be purified. Fractional distillation can concentrate ethanol to 96% volume; the mixture of 96% ethanol and 4% water is an azeotrope with a boiling point of 78.2 °C, and cannot be further purified by distillation. Therefore, 95% ethanol in water is a fairly common solvent.

Several approaches are used to produce absolute ethanol. The ethanol-water azeotrope can be broken by the addition of a small quantity of benzene. Benzene, ethanol, and water form a ternary azeotrope with a boiling point of 64.9 °C. Since this azeotrope is more volatile than the ethanol-water azeotrope, it can be fractionally distilled out of the ethanol-water mixture, extracting essentially all of the water in the process. The bottoms from such a distillation is anhydrous ethanol, with several parts per million residual benzene. Benzene is toxic to humans, and cyclohexane has largely supplanted benzene in its role as the entrainer in this process.

Alternatively, a molecular sieve can be used to selectively absorb the water from the 96% ethanol solution. Synthetic zeolite in pellet form can be used, as well as a variety of plant-derived absorbents, including cornmeal, straw, and sawdust. The zeolite bed can be regenerated essentially an unlimited number of times by drying it with a blast of hot carbon dioxide. Cornmeal and other plant-derived absorbents cannot readily be regenerated, but where ethanol is made from grain, they are often available at low cost. Absolute ethanol produced this way has no residual benzene, and can be used as fuel, or, when diluted, can even be used to fortify port and sherry in traditional winery operations.

At pressures less than atmospheric pressure, the composition of the ethanol-water azeotrope shifts to more ethanol-rich mixtures, and at pressures less than 70 torr, there is no azeotrope, and it is possible to distill absolute ethanol from an ethanol-water mixture. While vacuum distillation of ethanol is not presently economical, pressure-swing distillation is a topic of current research. In this technique, a reduced-pressure distillation first yields an ethanol-water mixture of more than 96% ethanol. Then, fractional distillation of this mixture at atmospheric pressure distills off the 96% azeotrope, leaving anhydrous ethanol at the bottoms.

Prospective technologies

Glucose for fermentation into ethanol can also be obtained from cellulose. Until recently, however, the cost of the cellulase enzymes that could hydrolyse cellulose has been prohibitive. The Canadian firm Iogen brought the first cellulose-based ethanol plant on-stream in 2004. The primary consumer thus far has been the Canadian government, which, along with the United States government (particularly the Department of Energy's National Renewable Energy Laboratory), has invested millions of dollars into assisting the commercialization of cellulosic ethanol. Realization of this technology would turn a number of cellulose-containing agricultural byproducts, such as corncobs, straw, and sawdust, into renewable energy resources.

Cellulosic materials typically contain, in addition to cellulose, other polysaccharides including hemicellulose. When hydrolysed, hemicellulose breaks down into mostly five-carbon sugars such as xylose. S. cerevisiae, the yeast most commonly used for ethanol production, cannot metabolize xylose. Other yeasts ( for example) and bacteria ( for example) are under investigation to metabolize xylose and so improve the ethanol yield from cellulosic material.

The anaerobic bacterium Clostridium ljungdahlii, recently discovered in commercial chicken wastes, can produce ethanol from single-carbon sources including carbon monoxide and a mixture of hydrogen and carbon dioxide. Use of these bacteria to produce ethanol from synthesis gas has progresed to the pilot plant stage at the BRI Energy, LLC facility in Fayetteville, Arkansas. Synthesis gas is a mixture of carbon monoxide and hydrogen that can be generated from the partial combustion of either fossil fuels or biomass; the heat released by gasification can be used to co-produce electricity with ethanol in the BRI process.

Denatured alcohol

In most jurisdictions, the sale of ethanol, as a pure substance or in the form of alcoholic beverages, is heavily taxed. In order to relieve non-beverage industries of this tax burden, governments specify formulations for denatured alcohol, ethanol blended with various additives to render it unfit for human consumption. These additives, called denaturants, are generally either toxic (such as methanol) or have unpleasant tastes or odors (such as denatonium benzoate).

Specialty denatured alcohols are denatured alcohol formulations intended for a particular industrial use, containing denaturants chosen so as not to interfere with that use. While they are not taxed, purchasers of specialty denatured alcohols must have a government-issued permit for the particular formulation they use and must comply with other regulations.

Completely denatured alcohols are denatured alcohol formulations that can be purchased for any legal purpose, without permit, bond, or other regulatory compliance. It is intended that it be difficult to isolate a product fit for human consumption from completely denatured alcohol. For example, the completely denatured alcohol formulation used in the United Kingdom contains (by volume) 89.66% ethanol, 9.46% methanol, 0.50% pyridine, 0.38% naphtha, and is dyed purple with methyl violet.

Use

A car "fueled by clean burning ethanol" (New York City, New York, USA).
A car "fueled by clean burning ethanol" ( New York City, New York, USA).

As a fuel

The largest single use of ethanol is as a motor fuel and fuel additive. The largest national fuel ethanol industries exist in Brazil and the United States. The Brazilian ethanol industry is based on sugarcane; as of 2004, Brazil produces 14 billion liters annually, enough to replace about 40% of its gasoline demand. Most new cars sold in Brazil are flexible-fuel vehicles that can run on ethanol, gasoline, or any blend of the two.

The United States fuel ethanol industry is based largely on corn. As of 2005, its capacity is 15 billion liters annually, although the Energy Policy Act of 2005 requires U.S. fuel ethanol production to increase to 7.5 billion gallons (28 billion liters) by 2012. In the United States, ethanol is most commonly blended with gasoline as a blend of up to 10% ethanol, nicknamed "gasohol". This blend is widely sold throughout the U.S. Midwest, which contains the nation's chief corn-growing centers.

Chemicals derived from ethanol

Ethyl esters

In the presence of an acid catalyst (typically sulfuric acid) ethanol reacts with carboxylic acids to produce ethyl esters:

CH3CH2OH + RCOOH → RCOOCH2CH3 + H2O

The two largest-volume ethyl esters are ethyl acrylate (from ethanol and acrylic acid) and ethyl acetate (from ethanol and acetic acid). Ethyl acrylate is a monomer used to prepare acrylate polymers for use in coatings and adhesives. Ethyl acetate is a common solvent used in paints, coatings, and in the pharmaceutical industry; its most familiar application in the household is as a solvent for nail polish. A variety of other ethyl esters are used in much smaller volumes as artificial fruit flavorings.

Vinegar

Vinegar is a dilute solution of acetic acid prepared by the action of Acetobacter bacteria on ethanol solutions. Although traditionally prepared from alcoholic beverages including wine, apple cider, and unhopped beer, vinegar can also be made from solutions of industrial ethanol. Vinegar made from distilled ethanol is called "distilled vinegar", and is commonly used in food pickling and as a condiment.

Ethylamines

When heated to 150–220 °C over a silica- or alumina-supported nickel catalyst, ethanol and ammonia react to produce ethylamine. Further reaction leads to diethylamine and triethylamine:

CH3CH2OH + NH3 CH3CH2NH2 + H2O
CH3CH2OH + CH3CH2NH2 (CH3CH2)2NH + H2O
CH3CH2OH + (CH3CH2)2NH → (CH3CH2)3N + H2O

The ethylamines find use in the synthesis of pharmaceuticals, agricultural chemicals, and surfactants.

Other chemicals

Ethanol is a versatile chemical feedstock, and in the past has been used commercially to synthesize dozens of other high-volume chemical commodities. At the present, it has been supplanted in many applications by less costly petrochemical feedstocks. However, in markets with abundant agricultural products, but a less developed petrochemical infrastructure, such as China, India, and Brazil, ethanol can be used to produce chemicals that would be produced from petroleum in the West, including ethylene and butadiene.

Other uses

It is easily soluble in water in all proportions with a slight overall decrease in volume when the two are mixed. Absolute ethanol and 95% ethanol are themselves good solvents, somewhat less polar than water and used in perfumes, paints and tinctures. Other proportions of ethanol with water or other solvents can also be used as a solvent. Alcoholic drinks have a large variety of tastes because various flavor compounds are dissolved during brewing. When ethanol is produced as a mixing beverage it is a neutral grain spirit.

Ethanol is used in medical wipes and in most common antibacterial hand sanitizer gels at a concentration of about 62%. Oddly enough, the peak of the disinfecting power occurs around 70% ethanol; stronger and weaker solutions of ethanol have a lessened ability to disinfect. Solutions of this strength are often used in laboratories for disinfecting work surfaces. 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.

Wine with less than 16% ethanol cannot protect itself against bacteria. Because of this, port is often fortified with ethanol to at least 18% ethanol by volume to halt fermentation for retaining sweetness and in preparation for aging, at which point it becomes possible to prevent the invasion of bacteria into the port, and to store the port for long periods of time in wooden containers that can 'breathe', thereby permitting the port to age safely without spoiling. Because of ethanol's disinfectant property, alcoholic beverages of 18% ethanol or more by volume can be safely stored for a very long time.

Metabolism and toxicology

In the human body, ethanol is first oxidized to acetaldehyde, and then to acetic acid. The first step is catalysed by the enzyme alcohol dehydrogenase, and the second by acetaldehyde dehydrogenase. Some individuals have less effective forms of one or both of these 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.

BAC (mg/dL) Symptoms
50 Euphoria, talkativeness, relaxation
100 Central nervous system depression, impaired motor and sensory function, impaired cognition
>140 Decreased blood flow to brain
300 Stupefaction, possible unconsciousness
400 Possible death
>550 Death highly likely

The amount of ethanol in the body is typically quanitified 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 judgment. At higher dosages (BAC > 0.10), 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.

The initial product of ethanol metabolism, acetaldehyde, is more toxic than ethanol itself. The body can quickly detoxify some acetaldehyde by reaction with glutathione and similar thiol-containing biomolecules. When acetaldehyde is produced beyond the capacity of the body's glutathione supply to detoxify it, it accumulates in the bloodstream until further oxidized to acetic acid. The headache, nausea, and malaise associated with an alcohol hangover stem from a combination of dehydration and acetaldehyde poisoning; many health conditions associated with chronic ethanol abuse, including liver cirrhosis, alcoholism, and some forms of cancer, have been linked to acetaldehyde.[ citation needed] Some medications, including paracetamol ( acetaminophen), as well as exposure to organochlorides, can deplete the body's glutathione supply, enhancing both the acute and long-term risks of even moderate ethanol consumption.

Hazards

  • Ethanol and mixtures with water greater than about 50% ethanol are flammable and easily ignited, although there are some solvents and organic compounds which are even more flammable. It is possible to burn even 40% ethanol solution (such as hard liquor) with a gas stove[ citation needed].
  • Ethanol has been shown to increase the growth of Acinetobacter baumannii, the bacteria responsible for pneumonia, meningitis and urinary tract infections. This finding may contradict the common misconception that drinking alcohol can kill off a budding infection. (Smith and Snyder, 2005)

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