Ethanol

The chemical compound ethanol, also known as ethyl alcohol or grain alcohol, is the bio-alcohol found in alcoholic beverages. When non-chemists refer to "alcohol", they almost always mean ethanol. Its chemical formula is C2H5OH and its molecular structure is

      H H
      | |
    H-C-C-O-H
      | |
      H H

Table of contents
1 Properties
2 Production
3 Legalities
4 Social and religious customs
5 Effects on the human body
6 Ethanol as fuel
7 See also
8 External links

Properties

Pure ethanol is a flammable, colorless liquid with a boiling point of 78.5° C. Its low melting point of -114.5° C allows it to be used in antifreeze products. It has a pleasant odor reminiscent of whiskey.

Its density is 789 g/L, about 20% less than that of water. It is easily soluble in water and is itself a good solvent, used in perfumes, paints and tinctures. Alcoholic drinks have a large variety of tastes, since various flavor compounds are dissolved during brewing.

A solution of 70-85% of ethanol is commonly used as a disinfectant; it kills organisms by denaturing their proteins and dissolving their lipids: it is effective against most bacteria and fungi, and many viruses, but is ineffective against bacterial spores. This disinfectant property of ethanol is the reason that alcoholic beverages can be stored for a long time.

Ethanol can lose a proton from the hydroxyl group and is a very weak acid, weaker than water.

The CAS number of ethanol is 64-17-5 and its UN number is UN 1170.

Production

Ethanol for use in alcoholic beverages is produced by fermentation: it is a product of sugar metabolism in certain species of yeast in the absence of oxygen. The process of culturing yeast under conditions to produce alcohol is referred to as brewing. Yeasts can grow in the presence of up to only about 14% alcohol, but the concentration of alcohol in the final product can be increased by distillation.

Ethanol is also used as a fuel and in a wide variety of industrial processes. Ethanol for industrial use is often made from petroleum feedstocks, typically from ethylene; this is cheaper than the production by fermentation.

Ethanol for industrial use is normally denatured, meaning small amounts of unpleasant or toxic substances (such as methanol) are added so that it cannot be consumed by humans, thus avoiding the relevant taxes. Denatured ethanol has the UN number UN 1987 and toxic denatured ethanol has UN 1986.

Legalities

Beverages containing ethanol are among the most widely used recreational drugs. Their use is legal in the western world, but illegal in Muslim countries. During the period known as Prohibition, from 1919 to 1933, it was also illegal in the United States.

Most countries have laws against drunk driving, driving with a certain concentration of ethanol in the blood. The legal threshold of blood alcohol content ranges from 0.1% to 0.08%, 0.05% and even 0% in different countries.

Most countries also specify a legal drinking age, below which the consumption of alcohol is prohibited.

In many countries, production of alcoholic beverages requires a license, and alcohol production is taxed. In the U.S., the Bureau of Alcohol, Tobacco, Firearms, and Explosives and the Alcohol and Tobacco Tax and Trade Bureau (formerly one organization known as the Bureau of Alcohol, Tobacco and Firearms) enforce regulations related to alcohol. In the UK the Customs and Excise department issues distilling licences.

Social and religious customs

Some religious groups such as Muslims and Mormons prohibit the consumption of ethanol. Some religious groups such as Roman Catholicism and Judaism use alcoholic beverages in their sacred rituals.

Effects on the human body

Several studies have confirmed that regular consumption of small amounts of alcohol has a beneficial effect, as it lowers the incidence of coronary heart disease.

In small amounts, ethanol causes a mild euphoria and removes inhibitions. In larger doses, ethanol acts as a depressant and causes drunkenness (at a blood ethanol content of about 0.1%), coma and death. A blood ethanol content above 0.4% is generally fatal, although regular heavy drinkers can tolerate higher levels.

Alcoholism, the dependency on alcohol, is a major public health problem. Alcoholics develop a number of health problems, with cirrhosis of the liver being the most important one.

If a mother drinks too much alcohol during pregnancy, the unborn child can develop mental and physical defects. This is known as fetal alcohol syndrome.

Action on the brain

Ethanol is quickly absorbed into the bloodstream and reaches the brain. As a small molecule, it is able to cross the blood-brain barrier. The euphorizing effects of ethanol are probably due to its causing the release of endorphins, natural "feel-good" molecules.

The depressing effect is mostly due to ethanol's acting on the GABA receptors. GABA is an inhibitory neurotransmitter, meaning it acts to slow down or inhibit nerve impulses. Ethanol increases the effectiveness of the GABA receptors. When used over a long time, ethanol changes the number and type of GABA receptors, and this is thought to be the cause of the violent withdrawal effects of alcoholics.

Ethanol also interferes with synaptic firing and causes the death of brain cells. This cell death is caused by an increased concentration of intracellular calcium which weakens the electrochemical gradient across the cell membranes. It is this gradient which is the motive force of membrane pumps and channels (cells, especially neurons, quickly die without proper membrane pump and channel function). There is also direct damage to cell membranes from free-radicals that are produced from alcohol metabolism.

Metabolism of alcohol and action on the liver

The liver produces a special enzyme (alcohol dehydrogenase) that breaks down alcohols into acetaldehyde, which is turned into acetic acid by the enzyme acetaldehyde dehydrogenase, and then yet another enzyme converts the acid into fatty acids, carbon dioxide and water. The fatty acids are mostly deposited locally which leads to the characteristic "beer belly". Chronic drinkers, however, so tax this metabolic pathway that things go awry: fatty acids build up as plaques in the capillaries around liver cells and those cells begin to die which leads to cirrhosis of the liver. The liver is part of the body's filtration system and if it is damaged then certain toxins build up thus leading to symptoms of jaundice.

The alcohol dehydrogenase of women is less effective than that of men. Combined with the lower amount of water in women's bodies, this means that women typically become drunk earlier than men. Some people, especially in East Asia, have a genetic mutation in their alcohol dehydrogenase gene, resulting in less potent alcohol dehydrogenase. These people are unable to drink much alcohol before becoming drunk, and are therefore less susceptible to alcoholism.

Drugs like Disulfiram block the enzyme acetaldehyde dehydrogenase, thus leading to a buildup of acetaldehyde after alcohol consumption. This causes severe hangover symptoms, and the drugs are therefore used to support treatment of alcoholism.

Dehydration

Consumption of ethanol has a rapid diuretic effect, meaning that more urine than usual is produced. Ethanol inhibits the production of antidiuretic hormone, and this is the cause of the diuretic effect.

Overconsumption can therefore lead to dehydration (the loss of water). It is difficult to replenish the body's fluids using only alcoholic beverages. As large amounts of alcohol are consumed, the diuretic effect causes the body to lose more water than is contained in the beverage.

Hangover

After overconsumption of ethanol, a hangover develops with symptoms of dry mouth, headache, nausea and light sensitivity. These symptoms are partly due to the toxic acetaldehyde produced from alcohol by alcohol dehydrogenase, and partly due to general dehydration.

Ethanol as fuel

Ethanol is flammable and burns more cleanly than many other fuels. When fully combusted its combustion products are only carbon dioxide and water. For this reason, it is favoured for environmentally conscious transport schemes and has been used to fuel public buses. Pure ethanol attacks certain rubber and plastic materials and cannot be used in unmodified car engines.

A mixture of 90% gasoline and 10% ethanol is known as gasohol, and can be used to fuel regular cars. It has a higher octane rating than ordinary gasoline.

The term "E85 ethanol" is used for a mixture of 15% gasoline and 85% ethanol. Beginning with the model year 1999, many cars in the U.S. were manufactured so as to be able to run on E85 fuel without modification.

In Brazil and the United States, the use of ethanol from grain as car fuel has been promoted by government programs. Some individual United States in the corn belt began subsidizing ethanol after the Arab oil embargo of 1973. The Energy Tax Act of 1978 authorized an excise tax exemption for biofuels, chiefly gasohol (a gasoline blend containing at least 10 percent ethanol). The excise tax exemption alone has been estimated as worth $1.4 billion per year. Another U.S. federal program guaranteed loans for the construction of ethanol plants, and in 1986 the U.S. even gave ethanol producers free corn.

Arguments in favor are the search for decreased dependency on oil producing countries and the decreased net ouptut of the greenhouse gas carbon dioxide. Some critics argue that it is mainly a government subsidy for corn-growing agribusiness. The Archer Daniels Midland Corporation of Decatur, Illinois, better known as ADM, the world's largest grain processor, produces 40% of the ethanol used to make gasohol in the U.S.. The company and its officers have been eloquent in their defense of ethanol and generous in contributing to both political parties.

Other critics contend that it is economically absurd to consider ethanol from grain as a replacement for petroleum, when industrial ethanol is made from petroleum feedstocks because it is far cheaper than fermented ethanol.

There is widespread belief that ethanol containing fuel is more environmentally friendly than gasoline without additives. However, there is a controversy over whether requiring ethanol in automotive fuel is wise as it has been argued that the beneficial effects of ethanol can be achieved with other cheaper additives made from petroleum feedstocks. Also, both the Environmental Protection Agency and the National Academy of Sciences have issued "reports showing that adding ethanol to gasoline will at best have no effect on air quality and could even make it worse. Studies show ethanol could even increase emissions of nitrogen oxides and volatile organic compounds, which are major ingredients of smog."[1]

Some studies have found that the total energy needed to produce one gallon of ethanol by fermentation (fertilizing, fuel for farm tractors, harvesting and transporting the grain, building and operating an ethanol plant, the natural gas used to distill corn sugars into alcohol) exceeds the energy content of that gallon of ethanol. Since production energy comes mostly from fossil fuels, gasohol isn't just wasting money but hastening the depletion of nonrenewable resources, critics have averred. In an analysis published in 2001 in the peer-reviewed Encyclopedia of Physical Sciences and Technology, Cornell University ecology professor David Pimentel presented arguments that when all the energy costs are added the ethanol production consumes more energy than the finished product contains.

These findings were contested by ethanol producers and their allies. Michael Graboski, a professor of engineering at the Colorado School of Mines and consultant to the National Corn Growers Association, published a rebuttal of Pimentel's paper, saying he used obsolete data. Pimentel revised his data: in an August 2003 bulletin from Cornell, he now says that making a gallon of ethanol takes only 29% more energy than it provides.

See also

External links




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