Encyclopedia
This article is about the chemical compound. For drinks containing ethanol, see
alcoholic beverage. For the use of ethanol as a fuel, see
ethanol fuel. For its physiological effects, see effects of alcohol on the body.
| Ethanol |
|---|
| |
| General |
|---|
| Systematic name | Ethanol |
| Other names | Ethyl alcohol,
grain alcohol,
hydroxyethane,
EtOH |
| Molecular formula | C2H6O |
| SMILES | CCO |
| Molar mass | 46.06844 g/mol |
| Appearance | clear 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 |
| Boiling point | 78.4 °C |
| Acidity | 15.9 |
| Viscosity | 1.200 cP at 20 °C |
| Dipole moment | 1.69 D |
| Hazards |
|---|
| MSDS | External MSDS |
| EU classification | Flammable |
| NFPA 704 | |
| R-phrases | |
| S-phrases | , , |
| Flash point | 13 °C |
| RTECS number | KQ6300000 |
| Supplementary data page |
|---|
| Structure & properties | n, er, 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
|
|
Ethanol, also known as
ethyl alcohol or grain alcohol, is a flammable, colorless, mildly toxic chemical compound with a distinctive
odor, and is the
alcohol found in
alcoholic beverages. In common usage, it is often referred to simply as
alcohol. Its
molecular formula is
C2H6O, variously represented as EtOH, C
2H
5OH or as its empirical formula C
2H
6O.
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 most notable of whom was
Al-Razi. The writings attributed to
Jabir Ibn Hayyan mention the flammable vapors of boiled wine.
Al-Kindi 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, and fifty years later, 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 Great 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.
Ethanol served as lamp fuel in pre-Civil War United States and helped power early Model T automobiles. But the fuel couldn't compete with the low cost and availability of petroleum, and ethanol faded from the public eye. The recent rise in oil prices has spurred renewed interest.
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 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.
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 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
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 , by reaction with an alkali metal such as
sodium. This reaction evolves
hydrogen gas:
- 2CH3CH2OH + 2Na ? 2CH3CH2ONa + H2
| Triple point [i] || 13.8033 K, 7.042 kPa
...
Nucleophilic substitution
In aprotic solvents, ethanol reacts with
hydrogen halides to produce 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 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
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.
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,
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 product
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 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 25% ethanol .
During the fermentation process, it is important to prevent oxygen getting to the ethanol, since otherwise the ethanol would be oxidised to
acetic acid . 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.
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.
After distillation ethanol can be further purified by "drying" it using lime or salt. Lime, , when mixed with the water in ethanol will form
calcium hydroxide, which then can be separated. 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 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 , 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 and bacteria 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 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; in the BRI process, the heat released by gasification can be used to co-produce electricity with ethanol.
Another prospective technology is the closed-loop ethanol plant. 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.
Ethanol
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, which consists of ethanol blended with various additives to render it unfit for human consumption. These additives, called denaturants, are generally either toxic or have unpleasant tastes or odors .
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 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 89.66% ethanol, 9.46% methanol, 0.50%
pyridine, 0.38% naphtha, and is dyed purple with
methyl violet.
Hydrous and anhydrous ethanol
Hydrous and anhydrous ethanol are terms used to describe ethanol by the type of process used to covert biomass into fuel. There are different prices for each anhydrous and hydrous ethanol depending on market demands.
The term hydrous pyrolysis is sometimes used to encompass thermolysis in the presence of water, such as steam cracking of oil, or more generally hydrous pyrolysis. An example of the latter is thermal depolymerization of organic waste into light
crude oil.
Anhydrous pyrolysis can be used to produce liquid fuel similar to
diesel from solid biomass. The most common technique uses very low residence times and high heating rates using a temperature between 350-500 °C. It is called either fast or flash pyrolysis.'
Anhydrous Alcohol can also be produced from hydrous alcohol using drying agents like molecular sieves, or by azeotropic distillation, extractive distillation techniques.
Absolute ethanol
Absolute or anhydrous alcohol generally refers to purified ethanol, containing no more than one percent
water.
It is not possible to obtain absolute alcohol by simple
fractional distillation, because a mixture containing around 95.6%
alcohol and 4.4% water becomes a constant
boiling mixture . In one common industrial method to obtain 100% pure alcohol, a small quantity of
benzene is added to
rectified spirit and the mixture is then distilled. Absolute alcohol is obtained in third fraction that distills over at 78.2
°C .
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.
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.
Currently, the most popular method of purification past 95.6% purity is desiccation using adsorbents such as starch or
zeolites. These adsorb water preferentially.
Feedstocks
Currently the main feedstock in the United States for the production of ethanol is corn, but trials of a new crop,
switchgrass, are showing much greater yields.
The dominant ethanol feedstock in warmer regions is
sugarcane.
In some parts of Europe, particularly France and Italy, wine is used as a feedstock due to massive oversupply.
Use
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 . One method of production is through fermentation of
sugar. Ethanol has a lower energy content than
gasoline. In the United States, the color yellow has become associated with the fuel and is commonly used on fuel pumps and labels.
Alcoholic beverages
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 grain 
