Information AboutAlcohol |
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In Chemistry , an alcohol is any Organic Compound in which a Hydroxyl Group (''- O H '') 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 refers almost always to Ethanol , also known as '''grain alcohol''', a 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). As a Drug , ethanol is known to have a Depressing Effect that decreases the responses of the Central Nervous System (see Effects Of Alcohol On The Body ). 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. Alcohol was discovered in Iran for the first time by "Zakaria ye Razi," an Iranian physician. The word "alcohol" has an Arabic root because, around that time, people in Iran (Persia) were not allowed to talk or to write books in their native language (i.e., "Persian" or " فارسی "). STRUCTURE The Functional Group of an alcohol is a Hydroxyl Group bonded to an sp³ hybridized carbon. It can therefore be regarded as a derivative of Water , with an Alkyl group replacing one of the hydrogens. If an Aryl group is present rather than an alkyl, the compound is generally called a Phenol rather than an alcohol. Also, if the Hydroxyl Group is bonded to one of the sp&2 hybridized carbons of an alkenyl group, the compound is referred to as an Enol . The oxygen in an alcohol has a bond angle of around 109° (c.f. 104.5° in water), and two nonbonded electron pairs. The O-H bond in methanol (CH3OH) is around 96 pico Metre s long. Primary, secondary, and tertiary alcohols There are three major subsets of alcohols- 'primary' (1°), 'secondary' (2°) and 'tertiary' (3°), based upon the number of carbons the C-OH carbon (shown in red) is bonded to. Methanol is the simplest 'primary' alcohol. The simplest secondary alcohol is Isopropanol (propan-2-ol), and a simple tertiary alcohol is ''tert''- Butanol (2-methylpropan-2-ol). : The Phenols with parent compound Phenol have a hydroxyl group (attached to an Benzene ring) just like alcohols but differ sufficiently in properties to warrant a separate treatment. Methanol & ethanol The simplest and most commonly used alcohols are Methanol and Ethanol (common names Methyl alcohol and Ethyl alcohol, respectively), which have the structures shown above. Methanol was formerly obtained by the distillation of wood, and was called "wood alcohol." It is now a cheap commodity, chemical produced by the high pressure reaction of Carbon Monoxide with Hydrogen . In common usage, "alcohol" often refers simply 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 major use in alcoholic beverages, ethanol is also used (although highly controlled) as an industrial solvent and raw material. USES Alcohols are in wide use in industry and science as reagents, Solvent s, and Fuels . Ethanol and methanol can be made to burn more cleanly than Gasoline or Diesel . Because of its low toxicity and ability to dissolve non-polar substances, ethanol is often used as a solvent in medical drugs, Perfume s, and vegetable essences such as Vanilla . In Organic Synthesis , alcohols frequently serve as versatile intermediates. In the kitchen, alcoholic beverages are added to dishes not only for their inherent flavors, but because the alcohol dissolves flavor compounds that water can not. Ethanol is commonly used in beverages to promote flavor, reduce social inhibitions, or induce a euphoric intoxication commonly known as "drunkenness" or "being drunk." The consumption of ethanol is illegal in some countries, mainly Islamic. Alcohol is a Drug , with potential for overdose or toxic poisoning if taken in excessive quantities. Alcoholism , the physiological or psychological dependency on alcohol, 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. For the full article on this topic see Effects Of Alcohol On The Body . 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 Fruit s 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 Alkene s. For more details see Preparation Of Alcohols . NOMENCLATURE Systematic names In the , Ketone or Carboxylic Acid ), then it is necessary to use the prefix "hydroxy", for example: 1-hydroxy-2-propanone (CH3COCH2OH). Some examples of simple alcohols and how to name them: Common names for alcohols usually take the name of the corresponding Alkyl group and add the word "alcohol", e.g. Methyl alcohol, Ethyl alcohol or ''tert''-butyl alcohol. Propyl alcohol may be ''n''-propyl alcohol or isopropyl alcohol depending on whether the hydroxyl group is bonded to the 1st or 2nd carbon on the propane chain. Isopropyl alcohol is also occasionally called ''sec''-propyl alcohol. As mentioned above alcohols are classified as primary (1°), secondary (2°) or tertiary (3°), and common names often indicate this in the alkyl group prefix. For example (CH3)3COH is a tertiary alcohol is commonly known as ''tert''-butyl alcohol. This would be named 2-methylpropan-2-ol under IUPAC rules, indicating a propane chain with methyl and hydroxyl groups both attached to the middle (#2) carbon. An alcohol with two hydroxyl groups is commonly called a "glycol", e.g. HO-CH2-CH2-OH is Ethylene Glycol . The IUPAC name is ethane-1,2-diol, "diol" indicating two hydroxyl groups, and 1,2 indicating their bonding positions. Geminal glycols (with the two hydroxyls on the same carbon atom), such as ethane-1,1-diol, are generally unstable. For three or four groups, "triol" and "tetraol" are used. 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 Sb 2 S 3 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 ") 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 The , with a four-carbon chain, is moderately soluble because of a balance between the two trends. Alcohols of five or more carbons ( Pentanol and higher) are effectively insoluble because of the hydrocarbon chain's dominance. Because of Hydrogen Bonding , alcohols tend to have higher boiling points than comparable Hydrocarbon s and Ether s. All simple alcohols are miscible in organic solvents. This hydrogen bonding means that alcohols can be used as Protic Solvent s. The lone pairs of electrons on the oxygen of the hydroxyl group also makes alcohols nucleophiles. Alcohols, like water, can show either acidic or basic properties at the O-H group. With a PKa of around 16-19 they are generally slightly weaker Acid s than Water , but they are still able to react with strong bases such as Sodium Hydride or reactive metals such as Sodium . The salts that result are called Alkoxide s, with the general formula R O- M +. Meanwhile the oxygen atom has Lone Pair s of nonbonded electrons that render it weakly Basic in the presence of strong acids such as Sulfuric Acid . For example, with methanol: Alcohols can also undergo Oxidation to give Aldehyde s, Ketone s or Carboxylic Acid s, or they can be dehydrated to Alkene s. They can react to form Ester Compound s, and they can (if activated first) undergo Nucleophilic Substitution reactions. For more details see the Reactions Of Alcohols section below. TOXICITY Alcohols often have an odor described as 'biting' that 'hangs' in the nasal passages. Ethanol in the form of Alcoholic Beverage s 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 and, depending on the dose and regularity of use, can cause acute respiratory failure or death and with chronic use has medical repercussions. 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 Enzyme s 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. This will bind to alcohol dehydrogenase, preventing methanol from binding and thus acting as a Substrate . Any formaldehyde will be converted to Formic Acid and excreted before it causes damage. PREPARATION OF ALCOHOLS Laboratory Several methods exist for the preparation of alcohols in the laboratory.
The formation of a secondary alcohol via reduction and hydratation is shown: Industrial Industrially alcohols are produced in several ways.
REACTIONS OF ALCOHOLS 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 + 2 Na → 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 Group s 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 Halide s alkoxides give rise to Ether s in the Williamson Ether Synthesis . Nucleophilic substitution The OH group is not a good Leaving Group in Nucleophilic Substitution reactions, so neutral alcohols do not react in such reactions. However if the oxygen is first protonated to give R−OH2+, the leaving group ( Water ) is much more stable, and nucleophilic substitution can take place. For instance, tertiary alcohols react with Hydrochloric Acid to produce tertiary Alkyl Halide s, where the Hydroxyl Group is replaced by a Chlorine atom. If primary or secondary alcohols are to be reacted with Hydrochloric Acid , an activator such as Zinc Chloride is needed. Alternatively the conversion may be performed directly using Thionyl Chloride . {Link without Title} Alcohols may likewise be converted to alkyl bromides using Hydrobromic Acid or Phosphorus Tribromide , for example: : 3 R-OH + PBr3 → 3 RBr + H3PO3 In the Barton-McCombie Deoxygenation an alcohol is deoxygenated to an Alkane with Tributyltin Hydride or a Trimethylborane -water complex in a Radical Substitution reaction. Dehydration Alcohols are themselves nucleophilic, so R−OH2+ can react with ROH to produce Ether s and water in a Dehydration Reaction , although this reaction is rarely used except in the manufacture of Diethyl Ether . More useful is the E1 Elimination Reaction of alcohols to produce Alkene s. The reaction generally obeys Zaitsev's Rule , which states that the most stable (usually the most substituted) alkene is formed. Tertiary alcohols eliminate easily at just above room temperature, but primary alcohols requre a higher temperature. This is a diagram of acid catalysed dehydration of ethanol to produce Ethene : 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 Aldehyde s or Carboxylic Acid s upon Oxidation , while secondary alcohols give Ketone s. 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. 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
REFERENCES EXTERNAL LINKS
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