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In chemistry, an alcohol is any organic compound in which a hydroxyl group (-OH) is bound to a carbon atom of an alkyl or substituted alkyl group. The general formula for a simple acyclic alcohol is CnH2n+1OH. In general usage, alcohol (from Arabic : ) refers almost always to ethanol, also known as grain alcohol, a strongly-smelling, colorless, volatile liquid formed by the fermentation of sugars. It also often refers to any beverage that contains ethanol (see alcoholic beverage). This sense underlies the term alcoholism (addiction to alcohol). Other forms of alcohol are usually described with a clarifying adjective, as in isopropyl alcohol or by the suffix -ol, as in isopropanol. Etymology The word dates to the 16th century when it was used to refer to any chemical substance arrived at by sublimation. This derived from the Medieval Latin alcohol ("powdered ore of antimony"), originating from Arabic (), which is also the source of kohl, based on a root , attested already in Akkadian as referring to eye makeup. Structure
Primary, secondary, and tertiary alcohols
Methanol and ethanol The simplest and most commonly used alcohols are methanol (common name methyl alcohol) and ethanol (ethyl alcohol), with the structures shown above. Methanol was formerly obtained by the distillation of wood and called "wood alcohol." It is now a cheap commodity, the chemical product of carbon monoxide reacting with hydrogen under high pressure. In common usage, "alcohol" often refers to ethanol or "grain alcohol." Methylated spirits ("Meths"), also called "surgical spirits," is a form of ethanol rendered undrinkable by the addition of methanol. Aside from its primary use in alcoholic beverages, ethanol is also used as a highly controlled industrial solvent and raw material. Automotive Alcohol is often used as a automotive fuel. Ethanol and methanol can be made to burn more cleanly than gasoline or diesel. Alcohol was once commonly used as an anti-freeze in automobile radiators. And to add to an internal combustion engines performance Methanol may be injected into turbocharged and supercharged engines to cool the air intake charge. Doing this provides a denser air charge. Scientific, medical, and industrial Alcohols are in wide use in industry and science as reagents solvents. Because of its low toxicity and ability to dissolve non-polar substances, ethanol is often used as a solvent in medical drugs, perfumes, and vegetable essences such as vanilla. In organic synthesis, alcohols frequently serve as versatile intermediates. Ethanol is often used as an antiseptic, to disinfect the skin before injections are given, often along with iodine. Ethanol-based soaps are now becoming commonplace within restaurants and are particularly convenient as they do not require drying due to the volatility of the molecule. Cuisine In the kitchen, alcoholic beverages are added to dishes not only for their inherent flavors, but also because the alcohol dissolves flavor compounds that water cannot. Ethanol is commonly used in beverages to promote flavor, reduce social inhibitions, or induce a euphoric intoxication commonly known as "drunkenness", intoxication, or "being drunk" Effects of alcohol on the body Ethanol is a drug, with potential for overdose or toxic poisoning if taken in excessive quantities. Alcoholism, the physiological or psychological dependency on ethanol, is one of the most common drug addictions (caffeine causes chemical dependency, but not the mental longing known as addiction) in the world. Upon cessation or decrease of use, the physiological dependency can lead to physical withdrawal symptoms, such as restlessness, trouble sleeping, "the shakes," or even death. Not everyone who abuses alcohol becomes physiologically dependent upon it, but can become psychologically addicted to it. Psychological addiction produces no physical withdrawal symptoms upon cessation of drinking alcohol, but the urge, or craving, to drink again can become quite intense and irresistable. Alcohol and politics Ethanol for consumption has been regulated by taxation. Those who manufacture it for other purposes often avoid this expense by "denaturing" it in a manner that renders it unfit for drinking. A common way to do this is by the addition of denatonium benzoate. "SD-40" and "SD Alcohol" sometimes followed by "40-B" are designations that were established by the United States' Bureau of Alcohol, Tobacco, and Firearms for this formulation. Sources Many alcohols can be created by fermentation of fruits or grains with yeast, but only ethanol is commercially produced this way — chiefly for fuel and drink. Other alcohols are generally produced by synthetic routes from natural gas, petroleum, or coal feed stocks; for example, via acid catalyzed hydration of alkenes. For more details see Preparation of alcohols. Systematic names
Etymology The word "alcohol" almost certainly comes from the Arabic language (the "al-" prefix being the Arabic definite article); however, the precise origin is unclear. It was introduced into Europe, together with the art of distillation and the substance itself, around the 12th century by various European authors who translated and popularized the discoveries of Islamic alchemists *. A popular theory, found in many dictionaries, is that it comes from الكحل = ALKHL = al-kuhul, originally the name of very finely powdered antimony sulfide Sb2S3 used as an antiseptic and eyeliner. The powder is prepared by sublimation of the natural mineral stibnite in a closed vessel. According to this theory, the meaning of alkuhul would have been first extended to distilled substances in general, and then narrowed to ethanol. This conjectured etymology has been circulating in England since 1672 at least (OED). However, this derivation is suspicious since the current Arabic name for alcohol, الكحول = ALKHWL = al???, does not derive from al-kuhul. The Qur'an in verse 37:47 uses the word الغول = ALGhWL = al-ghawl — properly meaning "spirit" ("spiritual being") or "demon" — with the sense "the thing that gives the wine its headiness". The word al-ghawl also originated the English word "ghoul", and the name of the star Algol. This derivation would, of course, be consistent with the use of "spirit" or "spirit of wine" as synonymous of "alcohol" in most Western languages. (Incidentally, the etymology "alcohol" = "the devil" was used in the 1930s by the U.S. Temperance movement for propaganda purposes.) According to the second theory, the popular etymology and the spelling "alcohol" would not be due to generalization of the meaning of ALKHL, but rather to Western alchemists and authors confusing the two words ALKHL and ALGhWL, which have indeed been transliterated in many different and overlapping ways. Physical and chemical properties
Toxicity Alcohols often have an odor described as 'biting' that 'hangs' in the nasal passages. Ethanol in the form of alcoholic beverages has been consumed by humans since pre-historic times, for a variety of hygienic, dietary, medicinal, religious, and recreational reasons. While infrequent consumption of ethanol in small quantities may be harmless or even beneficial, larger doses result in a state known as drunkenness or intoxication (which may lead to a hangover the next day) and, depending on the dose and regularity of use, can cause acute respiratory failure or death and with chronic use has medical repercussions. Alcohol has also been known to be a catalyst for reckless behaviors that may have undesirable results, such as accidents, fighting, and unprotected sex. The LD50 of ethanol in rats 11,300 mg/kg.* This ratio would correspond to an 80kg (176.4lb) man drinking 65 shots of 80 proof alcohol, although the LD50 does not necessarily translate directly to humans. Other alcohols are substantially more poisonous than ethanol, partly because they take much longer to be metabolized, and often their metabolism produces even more toxic substances. Methanol, or wood alcohol, for instance, is oxidized by alcohol dehydrogenase enzymes in the liver to the poisonous formaldehyde, which can cause blindness or death. An effective treatment to prevent formaldehyde toxicity after methanol ingestion is to administer ethanol. Alcohol dehydrogenase has a higher affinity for ethanol, thus preventing methanol from binding and acting as a substrate. Any remaining methanol will then have time to be excreted through the kidneys. Remaining formaldehyde will be converted to formic acid and excreted. Laboratory
Industrial Industrially alcohols are produced in several ways: Deprotonation Alcohols can behave as weak acids, undergoing deprotonation. The deprotonation reaction to produce an alkoxide salt is either performed with a strong base such as sodium hydride or ''n''-butyllithium, or with sodium or potassium metal. 2 R-OH + 2 NaH → 2 R-O-Na+ + H2↑ 2 R-OH + 2Na → 2R-O−Na + H2 E.g. 2 CH3CH2-OH + 2 Na → 2 CH3-CH2-O−Na + H2 Water is similar in pKa to many alcohols, so with sodium hydroxide there is an equilibrium set up which usually lies to the left: R-OH + NaOH <=> R-O-Na+ + H2O (equilibrium to the left) It should be noted, though, that the bases used to deprotonate alcohols are strong themselves. The bases used and the alkoxides created are both highly moisture sensitive chemical reagents. The acidity of alcohols is also affected by the overall stability of the alkoxide ion. Electron-withdrawing groups attached to the carbon containing the hydroxyl group will serve to stabilize the alkoxide when formed, thus resulting in greater acidity. On the other hand, the presence of electron-donating group will result in a less stable alkoxide ion formed. This will result in a scenario whereby the unstable alkoxide ion formed will tend to accept a proton to reform the original alcohol. With alkyl halides alkoxides give rise to ethers in the Williamson ether synthesis. Nucleophilic substitution
Dehydration
Esterification To form an ester from an alcohol and a carboxylic acid the reaction, known as Fischer esterification, is usually performed at reflux with a catalyst of concentrated sulfuric acid: R-OH + R'-COOH → R'-COOR + H2O In order to drive the equilibrium to the right and produce a good yield of ester, water is usually removed, either by an excess of H2SO4 or by using a Dean-Stark apparatus. Esters may also be prepared by reaction of the alcohol with an acid chloride in the presence of a base such as pyridine. Other types of ester are prepared similarly- for example tosyl (tosylate) esters are made by reaction of the alcohol with p-toluenesulfonyl chloride in pyridine. Oxidation Primary alcohols generally give aldehydes or carboxylic acids upon oxidation, while secondary alcohols give ketones. Traditionally strong oxidants such as the dichromate ion or potassium permanganate are used, under acidic conditions, for example: 3 CH3-CH(-OH)-CH3 + K2Cr2O7 + 4 H2SO4 → 3 CH3-C(=O)-CH3 + Cr2(SO4)3 + K2SO4 + 7 H2O Frequently in aldehyde preparations these reagents cause a problem of over-oxidation to the carboxylic acid. To avoid this, other reagents such as PCC, Dess-Martin periodinane, 2-Iodoxybenzoic acid, TPAP or methods such as Swern oxidation and Corey-Kim oxidation are now preferred. In the Guerbet reaction aliphatic alcohols dimerize with an initial oxidation step. Alcohols with a methyl group attached to the alcohol carbon can also undergo a haloform reaction (such as the iodoform reaction) in the presence of the halogen and a base such as sodium hydroxide. Tertiary alcohols resist oxidation, but can be oxidised by reagents such as 2,3-dichloro-5,6-dicyano-1,4-benzoquinone. See also | |||||||||||||||||||||
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