Alcohol
From Principles of Applied Arts
This article is about organic compounds containing -OH groups.
In chemistry, alcohol (via Medieval Latin alcohol "powdered ore of antimony" from Arabic Template:ArabDIN Template:Ar "kohl", from a root k-ḥ-l attested in Hebrew and Akkadian words for makeup for the eyes; the English word is attested from the 16th century, referring to any chemical substance arrived at by sublimation) is any organic compound in which a hydroxyl group]] (-OH) is bound to a carbon atom, which in turn is bound to other hydrogen and/or carbon atoms. 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, and often to any beverage that contains ethanol . 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. Other forms of alcohol are usually described with a clarifying adjective, as in isopropyl alcohol or by the suffix -ol, as in isopropanol.
Contents |
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 sp² 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 picot metres 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).
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 (though highly controlled) as an industrial solvent and raw material.
Uses
Alcohols are in wide use in industry and science as reagents, solvents, and as 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, perfumes, and vegetable essences such as vanilla. In organic synthesis, alcohols frequently serve as versatile intermediates.
Ethanol is also commonly used in beverages after fermentation to promote flavor or induce a euphoric intoxication commonly known as "drunkenness" or "being drunk". The use of ethanol for this purpose is illegal in some jurisdictions. In such instances of consumption, alcohol is a Psychoactive drug, with immediate potential for overdose, toxic poisoning, and physiological dependency (known as alcoholism). Alcoholism has become one of the most common drug addictions (if not second to caffeine) in the world. The physiological dependency caused by alcoholism means that the user experiences physical withdrawal (in the form of a headache known as a "hangover," extremely high anxiety known as "the shakes," and restlessness or trouble sleeping) upon cessation or decrease of use. For the full article on this topic see effects of alcohol on the body.
Because of such particular uses, historically, ethanol 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 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.
Nomenclature
Systematic names
In the IUPAC system, the name of the alkane chain loses the terminal "e" and adds "ol", e.g. "methanol" and "ethanol". When necessary, the position of the hydroxyl group is indicated by a number between the alkane name and the "ol": propan-1-ol for CH3CH2CH2OH, propan-2-ol for CH3CH(OH)CH3. Sometimes, the position number is written before the IUPAC name: 1-propanol and 2-propanol. If a higher priority group is present (such as an aldehyde, ketone or carboxylic acid), then it is necessary to use the prefix "hydroxy", for example: 1-hydroxy-2-propanone (CH3COCH2OH).
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.
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 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 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. 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.
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 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.
References
External links
- What Is Alcohol, Anyway? Interesting information about alcohols.
