Ethanol, also known as ethyl alcohol, drinking alcohol or grain alcohol, is a flammable, colorless, slightly toxic chemical compound with a distinctive perfume-like odor, 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.
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, like most short-chain alcohols, is flammable, colorless, has a strong odor, and is volatile.
Ethanol has a refractive index of 1.3614. 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.
Furthermore, ethanol is used as a solvent in dissolving medicines, food flavorings and colorings that do not dissolve easily in water. Once the non-polar material is dissolved in the ethanol, water can be added to prepare a solution that is mostly water. The ethanol molecule has a hydrophilic -OH group that helps it dissolve polar molecules and ionic substances. The short, hydrophobic hydrocarbon chain CH3CH2- can attract non-polar molecules. Thus ethanol can dissolve both polar and non-polar substances.
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. The addition of even a small amount of 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
Chemical formula of ethanol, (C is carbon, the dash is a single bond, H is hydrogen, O is oxygen)
The chemistry of ethanol is largely that of its hydroxyl group.
- Acid-base chemistry
Ethanol’s hydroxyl proton is weakly acidic, having a pKa of only 15.9, compared to water’s 15.7 (Ka of ethanol is a measure of ![scriptstyle frac {[C_2H_5O^-][H^+]} {[C_2H_5OH]}](http://upload.wikimedia.org/math/d/9/5/d95ea137f082edf55769b4bbcd06c7f5.png)
. Note that Ka of water is derived by dividing water’s dissociation constant, ![scriptstyle[H^+][OH^-] = 1.0 times 10^{-14}](http://upload.wikimedia.org/math/c/3/d/c3d893531b5e26c0eb74397bf55094f3.png)
moles2/liter, by its molar density of 55.5 moles/liter). Ethanol can be quantitatively converted to its conjugate base, the ethoxide ion (CH3CH2O−), by reaction with an alkali metal such as sodium. This reaction evolves hydrogen gas:
- Nucleophilic substitution
In aprotic solvents, ethanol reacts with hydrogen halides to produce ethyl halides such as ethyl chloride and ethyl bromide via nucleophilic substitution:
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. A very common ester of ethanol is ethyl acetate, found in for example nailpolish remover.
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
Although sulfuric acid catalyses this reaction, the acid is diluted by the water that is formed, which makes the reaction inefficient. 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 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 overoxidation to acetic acid, by reacting it with pyridinium chromic chloride.
- Combustion
Ethanol combusting in the confines of an evaporating dish
Combustion of ethanol forms carbon dioxide and water:
- C2H5OH + 3 O2 → 2 CO2 + 3 H2O
Production
94% denatured ethanol sold in a 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 ethene, represented by the chemical equation
The catalyst is most commonly phosphoric acid, 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. Solid catalysts, mostly various metal oxides, have also been mentioned in the chemical literature.
In an older process, first practiced 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 produce ethyl sulfate, which was then hydrolysed to yield ethanol and regenerate the sulfuric acid:
- C2H4 + H2SO4 → CH3CH2SO4H
- CH3CH2SO4H + 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
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 15% ethanol (by volume).
During the fermentation process, it is important to prevent oxygen from getting to the ethanol, since otherwise the ethanol would be oxidised to acetic acid (vinegar). Also, in the presence of oxygen, the yeast would undergo aerobic respiration to produce just carbon dioxide and water, without producing ethanol.
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.
Feedstocks
Currently the main feedstock in the United States for the production of ethanol is corn. Approximately 2.8 gallons of ethanol (10 liters) are produced from one bushel of corn (35 liters). While much of the corn turns into ethanol, some of the corn also yields by-products such as DDGS (distillers dried grains with solubles) that can be used to fulfill a portion of the diet of livestock. A bushel of corn produces about 18 pounds of DDGS. Critics of ethanol as fuel decry the use of corn to produce ethanol because corn is an energy-intensive crop that requires petroleum-derived fertilizers; however, using corn to produce alcohol could save farmers additional petroleum if the farmers are feeding the byproduct to livestock and if the excrement from the animals is then used as fertilizer for the corn [Lynn Ellen Doxon; The Alcohol Fuel Handbook]. Although most of the fermentation plants have been built in corn-producing regions, sorghum is also an important feedstock for ethanol production in the Plains states. Pearl millet is showing promise as an ethanol feedstock for the southeastern U.S.
Trials of new crops, such as agricultural residues, wood wastes, and various grasses, show much lower yields using conventional, commercialized processes.[citation needed] These crops are cellulosic rather than starchy, and have fewer accessible sugars for fermentation. Newer, more complex processes are necessary to release plant sugars, primarily by disrupting lignin networks. However, the appeal of such crops is their lower requirement for fertilizer and other inputs, and in some cases lower cost or higher availability as “waste” products.
The dominant ethanol feedstock in warmer regions is sugarcane.[citation needed] The directly-accessible sugars simplify the fermentation process.[citation needed] In temperate regions, this accessibility has been somewhat replicated by selective breeding of the sugar beet.[citation needed]
In some parts of Europe, particularly France and Italy, wine is used as a feedstock due to massive oversupply. Japan is hoping to use rice wine (sake) as an ethanol source.
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. Later increases in petroleum prices, coupled with perennial uncertainty in agricultural prices, make forecasting the relative production costs of fermented versus petrochemical ethanol difficult..
Purification
Near infrared spectrum of liquid ethanol.
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 95.6% by weight (89.5 mole%). The mixture of 95.6% ethanol and 4.4% water (percentage by weight) 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.
After distillation ethanol can be further purified by “drying” it using lime or salt. When lime (calcium oxide) is mixed with the water in ethanol, calcium hydroxide forms. The calcium hydroxide can then be separated from the ethanol. Dry salt will dissolve some of the water content of the ethanol as it passes through, leaving a purer alcohol.
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 95.6% 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 to fortify port and sherry in traditional winery operations. Membranes can also be used to separate ethanol and water. The membrane can break the water-ethanol azeotrope because separation is not based on vapor-liquid equilibria. Membranes are often used in the so-called hybrid membrane distillation process. This process uses a pre-concentration distillation column as first separating step. The further separation is then accomplished with a membrane operated either in vapor permeation or pervaporation mode. Vapor permeation uses a vapor membrane feed and pervaporation uses a liquid membrane feed.
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 (9.333 kPa) , 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 95.6% ethanol. Then, fractional distillation of this mixture at atmospheric pressure distills off the 95.6% azeotrope, leaving anhydrous ethanol at the bottoms.
Use
As a fuel
A Ford Taurus “fueled by clean burning ethanol” owned by New York City.
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 hydrous ethanol is also used as fuel). In order for ethanol to be suitable for use as a replacement to petrol in its pure form, it must be distilled to at least 70-80% purity by volume before use. For use as an additive to petrol, almost all water must be removed, otherwise it will separate from the mixture and settle to the bottom of the fuel tank, causing the fuel pump to draw water into the engine, which will cause the engine to stall.
Today almost 50% of Brazilian cars are able to use 100% ethanol as fuel, that 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, giving the buyer a choice for a perfect balance between price/performance issue. That was only possible due to the capability of an efficient sugar cane production. Sugar cane not only has a greater concentration of sucrose (about 30% more than corn) but is also much easier to extract. The bagasse generated by the process is not wasted and it is utilized in power plants becoming a surprisingly efficient source of 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.
One method of production is through fermentation of sugar. Ethanol creates very little pollution when burned. Millions more acres of land are needed if ethanol is to be used to replace gasoline. Pure ethanol has a lower energy content than gasoline (about 30% less energy per unit volume). At gas stations, ethanol is contained in a mix of ethanol and gasoline, otherwise known as gasohol. In the United States, the color yellow (symbolizing the color of corn) has become associated with the fuel and is commonly used on fuel pumps and labels.
According to the Renewable Fuels Association, as of November 2006; 107 grain ethanol biorefineries in the United States have the capacity to produce 5.1 billion gallons of ethanol per year. An additional 56 construction projects underway (in the U.S.) can add 3.8 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. Growth in fuel ethanol in the United States is largely being driven by financial incentives that naturally exist when oil prices are over a certain level, as ethanol typically costs under $1.50 per gallon to manufacture (of course this is sensitive to corn prices) and is exempt from the federal gasoline tax. However, the United States RFS (renewable fuel standard) 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.
