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Alcohol

2007 Schools Wikipedia Selection. Related subjects: General Chemistry

   Functional group of an alcohol molecule. The carbon atom is attached to
   other carbon or hydrogen atoms.
   Enlarge
   Functional group of an alcohol molecule. The carbon atom is attached to
   other carbon or hydrogen atoms.

   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
   C[n]H[2n+1]OH.

   In general usage, alcohol 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.

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 (CH[3]OH) is around 96 pico 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).

          Some common alcohols

          Enlarge
          Some common alcohols

   The phenols with parent compound phenol have a hydroxyl group (attached
   to a benzene ring) just like alcohols but differ sufficiently in
   properties to warrant a separate treatment.

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 an automotive fuel. Ethanol and methanol can
   be made to burn more cleanly than gasoline or diesel. Alcohol was once
   commonly used as an antifreeze in automobile radiators. And to add to
   an internal combustion engine's 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 flavours, 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.

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,
   similar to marijuana. 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 irresistible.
   Alcohol is proven to relax your body causing your reactions to be
   slower and as your body is relaxed you are not as affected in accidents
   as others may be.

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 or
   methanol. "SD-40" and "SD Alcohol" sometimes followed by "40-B" are
   designations that were established by the United States' Bureau of
   Alcohol, Tobacco, Firearms and Explosives for this formulation.

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 CH[3]CH[2]CH[2]OH,
   propan-2-ol for CH[3]CH(OH)CH[3]. 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 (CH[3]COCH[2]OH).

   Some examples of simple alcohols and how to name them:
   Examples of alcohols & their names
   Enlarge
   Examples of alcohols & their names

   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 (CH[3])[3]COH 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-CH[2]-CH[2]-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. A Persian physician named Rhazes discovered this
   substance, but as he spoke Arabic under Arab rule, the word remains of
   Arabic origin. 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 al-kuḥl, 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 al-kuḥūl, does not derive from al-kuḥl. The
   Qur'an, in verse 37:47, uses the word الغول‎ al-ghawl — properly
   meaning " spirit" or " demon" — with the sense "the thing that gives
   the wine its headiness". The word al-ghawl is also the origin of 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 al-kuḥl,
   but rather to Western alchemists and authors confusing the two words
   al-kuḥl and al-ghawl, which have indeed been transliterated in many
   different and overlapping ways.

Physical and chemical properties

   The hydroxyl group generally makes the alcohol molecule polar. Those
   groups can form hydrogen bonds to one another and to other compounds.
   Two opposing solubility trends in alcohols are: the tendency of the
   polar OH to promote solubility in water, and of the carbon chain to
   resist it. Thus, methanol, ethanol, and propanol are miscible in water
   because the hydroxyl group wins out over the short carbon chain.
   Butanol, 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 hydrocarbons and ethers. The boiling point of
   the alcohol Ethanol is 78.29 °C, compared to 69 °C for the hydrocarbon
   Hexane (a common constituent of gasoline), and 34.6 °C for Diethyl
   ether. All simple alcohols are miscible in organic solvents. This
   hydrogen bonding means that alcohols can be used as protic solvents.

   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 pK[a] of around 16-19 they are generally slightly
   weaker acids 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 alkoxides, with the general formula RO^-
   M^+.

   Meanwhile the oxygen atom has lone pairs of nonbonded electrons that
   render it weakly basic in the presence of strong acids such as sulfuric
   acid. For example, with methanol:

   Acidity & basicity of methanol

   Alcohols can also undergo oxidation to give aldehydes, ketones or
   carboxylic acids, or they can be dehydrated to alkenes. They can react
   to form ester compounds, 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 odour 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 LD[50] 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 LD[50] 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.

Preparation of alcohols

Laboratory

   Several methods exist for the preparation of alcohols in the
   laboratory.
     * Primary alkyl halides react with aqueous NaOH or KOH mainly to
       primary alcohols in nucleophilic aliphatic substitution. (Secondary
       and especially tertiary alkyl halides will give the elimination
       (alkene) product instead).
     * Aldehydes or ketones are reduced with sodium borohydride or lithium
       aluminium hydride (after an acidic workup). Another reduction by
       aluminumisopropylates is the Meerwein-Ponndorf-Verley reduction.
     * Alkenes engage in an acid catalysed hydration reaction using
       concentrated sulfuric acid as a catalyst which gives usually
       secondary or tertiary alcohols.
     * The hydroboration-oxidation and oxymercuration-reduction of alkenes
       are more reliable in organic synthesis.
     * Grignard reagents react with carbonyl groups to secondary and
       tertiary alcohols
     * Noyori asymmetric hydrogenation is the asymmetric reduction of
       β-keto-esters

   The formation of a secondary alcohol via reduction and hydration is
   shown:
   Preparation of a secondary alcohol

Industrial

   Industrially alcohols are produced in several ways:
     * By fermentation using glucose produced from sugar from the
       hydrolysis of starch, in the presence of yeast and temperature of
       less than 37°C to produce ethanol. For instance the conversion of
       invertase to glucose and fructose or the conversion of glucose to
       zymase and ethanol.
     * By direct hydration using ethene or other alkenes from cracking of
       fractions of distilled crude oil. Uses a catalyst of phosphoric
       acid under high temperature and pressure.
     * Methanol is producted from water gas: It is manufactured from
       synthesis gas, where carbon monoxide and 2 equivalents of hydrogen
       gas are combined to produce methanol using a copper, zinc oxide and
       aluminium oxide catalyst at 250°C and a pressure of 50-100 atm.

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^+ + H[2]↑

          2 R-OH + 2Na → 2R-O^−Na + H[2]

          E.g. 2 CH[3]CH[2]-OH + 2 Na → 2 CH[3]-CH[2]-O^−Na + H[2]

   Water is similar in pK[a] 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^+ + H[2]O (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

   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−OH[2]^+, 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 halides, 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.^

   Some simple conversions of alcohols to alkyl chlorides

   Alcohols may likewise be converted to alkyl bromides using hydrobromic
   acid or phosphorus tribromide, for example:

          3 R-OH + PBr[3] → 3 RBr + H[3]PO[3]

   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−OH[2]^+ can react with ROH
   to produce ethers 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
   alkenes. 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 require a higher temperature.

   This is a diagram of acid catalysed dehydration of ethanol to produce
   ethene:

   A more controlled elimination reaction is the Chugaev elimination with
   carbon disulfide and iodomethane.

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 + H[2]O

   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 H[2]SO[4] 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 CH[3]-CH(-OH)-CH[3] + K[2]Cr[2]O[7] + 4 H[2]SO[4] → 3
          CH[3]-C(=O)-CH[3] + Cr[2](SO[4])[3] + K[2]SO[4] + 7 H[2]O

   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.

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