Alcohol

For other uses of "Alcohol", see Alcohol (disambiguation).

In chemistry, alcohol (from Arabical-kukh?l الكحول = "the spirit", "the chemical") is any organic compoundin which a hydroxylgroup(-OH) is bound to a carbonatom, which in turn is bound to other hydrogenand/or carbonatoms. The general formula for a simple acyclicalcohol is C_nH_2n+1OH.

In general usage, alcohol refers almost always to ethanol, also known as grain alcohol, and often to any beverage that contains ethanol (see alcoholic beverage). This sense underlies the term alcoholism(addictionto alcohol). Other forms of alcohol are usually described with a clarifying adjective, as in isopropyl alcohol or by the suffix -ol, as in isopropanol.

As a drug, ethanol is known to have a depressing effectthat decreases the responses of the central nervous system.

Structure

Image:Alcohol general.jpg

The functional groupof an alcohol is a hydroxyl groupbonded to an sp³ hybridized carbon. It can therefore be regarded as a derivative of water, with an alkylgroup replacing one of the hydrogens. If an arylgroup is present rather than an alkyl, the compound is generally called a phenolrather than an alcohol. Also, if the hydroxyl groupis 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 picometreslong.

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. Methanolis 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).

Image:Alcohol common.jpg

Methanol & ethanol

The simplest and most commonly used alcohols are methanoland ethanol(common names methylalcohol and ethylalcohol, 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 monoxidewith 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 fuels. Ethanol and methanol can be made to burn more cleanly than gasolineor 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 not only a 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.

Sources

Many alcohols can be created by fermentationof fruitsor grainswith yeast, but only ethanol is commercially produced this way, chiefly for fueland drink. Other alcohols are generally produced by synthetic routes from natural gas, petroleum, or coalfeed stocks, for example via acid catalyzed hydrationof alkenes. For more details see Chemistry of alcohols

Nomenclature

Systematic names

In the IUPACsystem, 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-olfor CH₃CH₂CH₂OH, propan-2-olfor 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, ketoneor 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:

Image:Alcohol examples.gif

Common names for alcohols usually take the name of the corresponding alkylgroup and add the word "alcohol", e.g. methylalcohol, ethylalcohol or tert-butylalcohol. Propylalcohol 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 distillationand the substance itself, around the 12th centuryby various European authors who translated and popularized the discoveries of Islamicalchemists.

A popular theory, found in many dictionaries, is that it comes from الكحل = ALKHL = al-kuhul, originally the name of very finely powdered antimonysulfideSb₂S₃ used as an antisepticand eyeliner. The powder is prepared by sublimationof the natural mineral stibnitein 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 1672at 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'anin 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 Englishword "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 1930sby the U.S.Temperance Movementfor 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 hydroxyl groupgenerally makes the alcohol molecule polar. Those groups can form hydrogen bondsto 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 (Pentanoland higher) are effectively insoluble because of the hydrocarbon chain's dominance.

Because of hydrogen bonding, alcohols tend to have higher boiling points than comparable hydrocarbonsand ethers. 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_aof around 16-19 they are generally slightly weaker acidsthan water, but they are still able to react with strong bases such as sodium hydrideor reactive metals such as sodium. The salts that result are called alkoxides, with the general formula RO^- M⁺.

Alcohols conjugated to aromatic rings have a lower pKa (around 10). Electron-withdrawing groups also work to make alcohols more acidic. For example, Para-nitro phenol has a pKa of 7.15.

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

Image:Methanol acid base.gif

Alcohols can also undergo oxidationto give aldehydes, ketonesor carboxylic acids, or they can be dehydrated to alkenes. They can react to form ester compounds, and they can (if activated first) undergo nucleophilic substitutionreactions. For more details see the #Chemistry of alcoholssection below.

Toxicity

Alcohols often have an odor described as 'biting' that 'hangs' in the nasal passages. Ethanolin the form of alcoholic beverageshas 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 drunkennessor 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 dehydrogenaseenzymesin 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 acidand excreted before it causes damage.

Preparation of alcohols

Laboratory

Several methods exist for the preparation of alcohols in the laboratory.

• Primary Alkyl halidesreact with aqueous NaOHor KOHmainly to primary alcohols in nucleophilic aliphatic substitution. (Secondary and especially tertiary alkyl halides will give the elimination (alkene) product instead).
• Aldehydesor ketonesare reducedwith sodium borohydrideor lithium aluminium hydride. (after an acidic workup)
• Alkenesengage in a acidcatalysed hydration reactionusing concentrated sulfuric acidas a catalyst which gives usually secondary or tertiary alcohols.
• The hydroboration-oxidationand oxymercuration-reductionof alkenes are more reliable in organic synthesis.
• Grignard reagentsreact with carbonylgroups to secondary and tertiary alcohols

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

Image:Alcohol prep.gif

Industrial

Industrially alcohols are produced in several ways.

• by fermentationusing glucoseproduced from sugar from the hydrolysisof starch, in the presence of yeast and temperature of less than 37°C to produce ethanol. For instance the conversion of invertaseto glucoseand fructoseor the conversion of glucoseto zymaseand ethanol.
• By direct hydration: using etheneor other alkenes from crackingof fractions of distilled crude oil. Uses a catalyst of phosphoric acidunder high temperature and pressure.
• Methanolis producted from water gas: It is manufactured from synthesis gas, where carbon monoxideand 2 equivalents of hydrogen gas are combined to produce methanolusing a copper, zinc oxideand aluminium oxidecatalyst 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 alkoxidesalt is either performed with a strong base such as sodium hydrideor 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⁺

e.g. 2 CH₃CH₂-OH+ 2 Na → 2 CH₃-CH₂-O⁻Na⁺

Water is similar in pK_ato many alcohols, so with sodium hydroxidethere 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.

Nucleophilic substitution

The OHgroup is not a good leaving groupin nucleophilic substitutionreactions, 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 acidto produce tertiary alkyl halides, where the hydroxyl groupis replaced by a chlorineatom. If primary or secondary alcohols are to be reacted with hydrochloric acid, an activator such as zinc chlorideis needed. Alternatively the conversion may be performed directly using thionyl chloride.^[1]

Image:Alcohol reaction examples.gif

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

3 R-OH + PBr₃ → 3 RBr + H₃PO₃

In the Barton-McCombie deoxygenationan alcohol is deoxygenated to an alkanewith tributyltin hydrideor a trimethylborane-water complex in a radical substitutionreaction.

Dehydration

Alcohols are themselves nucleophilic, so R−OH₂⁺ can react with ROH to produce ethersand water in a dehydration reaction, although this reaction is rarely used except in the manufacture of diethyl ether.

More useful is the E1 elimination reactionof 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 requre a higher temperature.

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

Image:DehydrationOfAlcoholWithH-.png

Esterification

To form an esterfrom an alcohol and a carboxylic acidthe reaction, known as Fischer esterification, is usually performed at refluxwith a catalystof concentrated sulfuric acid:

R-OH + R'-COOH <math>\Leftrightarrow</math> R'-COOR + H₂O

In order to drive the equilibrium to the right and produce a good yieldof 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 chloridein 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-toluenesulfonylchloride in pyridine.

Oxidation

Primary alcohols generally give aldehydesor carboxylic acidsupon oxidation, while secondary alcohols give ketones. Traditionally strong oxidantssuch as the dichromateion or potassium permanganateare 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 aldehydepreparations 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, TPAPor methods such as Swern oxidationare now preferred.

Alcohols with a methylgroup attached to the alcohol carbon can also undergo a haloform reaction(such as the iodoform reaction) in the presence of the halogenand 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

• alcohol as a fuel
• alcoholic beverage
• effects of alcohol on the body
• transesterification
• sugar alcohols
• fatty alcohols

External links

• What Is Alcohol, Anyway?Interesting information about alcohols.ar:????? ( ?????? ????? )

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It uses material from the http://en.wikipedia.org/wiki/Alcohol Wikipedia article Alcohol.