Alcohol (chemistry)

This article is about the class of chemical compounds. For the alcohol found in alcoholic drinks, seeAlcohol (drug).For other uses, seeAlcohol (disambiguation).

Ball-and-stick modelof an alcohol molecule (R₃COH). The red and white balls represent the hydroxyl group (−OH). The three "R" s stand for carbonsubstituentsor hydrogen atoms.

Inchemistry,analcoholis a type oforganic compoundthat carries at least onehydroxyl(−OH)functional groupbound to carbon.Alcohols range from the simple, likemethanolandethanol,to complex, likesugarsandcholesterol.The presence of an OH group strongly modifies the properties of hydrocarbons, conferring hydrophilic (water-loving) properties. The OH group provides a site at which many reactions can occur.

History

The flammable nature of the exhalations of wine was already known to ancient natural philosophers such asAristotle(384–322 BCE),Theophrastus(c. 371–287 BCE), andPliny the Elder(23/24–79 CE).However, this did not immediately lead to the isolation of alcohol, even despite the development of more advanced distillation techniques in second- and third-centuryRoman Egypt.An important recognition, first found in one of the writings attributed toJābir ibn Ḥayyān(ninth century CE), was that byadding saltto boiling wine, which increases the wine'srelative volatility,the flammability of the resulting vapors may be enhanced.The distillation of wine is attested in Arabic works attributed toal-Kindī(c. 801–873 CE) and toal-Fārābī(c. 872–950), and in the 28th book ofal-Zahrāwī's (Latin: Abulcasis, 936–1013)Kitāb al-Taṣrīf(later translated into Latin asLiber servatoris).In the twelfth century, recipes for the production ofaqua ardens( "burning water", i.e., alcohol) by distilling wine with salt started to appear in a number of Latin works, and by the end of the thirteenth century, it had become a widely known substance among Western European chemists.

The works ofTaddeo Alderotti(1223–1296) describe a method for concentrating alcohol involving repeatedfractional distillationthrough a water-cooled still, by which an alcohol purity of 90% could be obtained.The medicinal properties of ethanol were studied byArnald of Villanova(1240–1311 CE) andJohn of Rupescissa(c. 1310–1366), the latter of whom regarded it as a life-preserving substance able to prevent all diseases (theaqua vitaeor "water of life", also called by John thequintessenceof wine).

Nomenclature

Etymology

The word "alcohol" derives from the Arabickohl(Arabic:الكحل,romanized:al-kuḥl), a powder used as an eyeliner.The first part of the word (al-) is the Arabicdefinite article,equivalent tothein English. The second part of the word (kuḥl) has several antecedents inSemitic languages,ultimately deriving from theAkkadian𒎎𒋆𒁉𒍣𒁕(guḫlum), meaningstibniteorantimony.

Like its antecedents in Arabic and older languages, the termalcoholwas originally used for the very fine powder produced by thesublimationof the natural mineralstibniteto formantimony trisulfideSb₂S₃.It was considered to be the essence or "spirit" of this mineral. It was used as anantiseptic,eyeliner, andcosmetic.Later the meaning of alcohol was extended to distilled substances in general, and then narrowed again to ethanol, when "spirits" was a synonym forhard liquor.

ParacelsusandLibaviusboth used the termalcoholto denote a fine powder, the latter speaking of analcoholderived from antimony. At the same time Paracelsus uses the word for a volatile liquid;alcooloralcool vinioccurs often in his writings.

Bartholomew Traheron,in his 1543 translation ofJohn of Vigo,introduces the word as a term used by "barbarous" authors for "fine powder." Vigo wrote: "the barbarous auctours use alcohol, or (as I fynde it sometymes wryten) alcofoll, for moost fine poudre."

The 1657Lexicon Chymicum,by William Johnson glosses the word as "antimonium sive stibium."By extension, the word came to refer to any fluid obtained by distillation, including "alcohol of wine," the distilled essence of wine.LibaviusinAlchymia(1594) refers to "vini alcohol vel vinum alcalisatum".Johnson (1657) glossesalcohol vinias "quando omnis superfluitas vini a vino separatur, ita ut accensum ardeat donec totum consumatur, nihilque fæcum aut phlegmatis in fundo remaneat."The word's meaning became restricted to" spirit of wine "(the chemical known today asethanol) in the 18th century and was extended to the class of substances so-called as "alcohols" in modern chemistry after 1850.

The termethanolwas invented in 1892,blending"ethane"with the" -ol "ending of" alcohol ", which was generalized as alibfix.

The termalcoholoriginally referred to the primary alcoholethanol(ethyl alcohol), which isused as a drugand is the main alcohol present inalcoholic drinks.

The suffix-olappears in theInternational Union of Pure and Applied Chemistry(IUPAC)chemical nameof all substances where the hydroxyl group is the functional group with the highest priority. When a higher priority group is present in the compound, the prefixhydroxy-is used in its IUPAC name. The suffix-olin non-IUPAC names (such asparacetamolorcholesterol) also typically indicates that the substance is an alcohol. However, some compounds that contain hydroxyl functional groups havetrivial namesthat do not include the suffix-olor the prefixhydroxy-,e.g. the sugarsglucoseandsucrose.

Systematic names

IUPAC nomenclatureis used in scientific publications, and in writings where precise identification of the substance is important. In naming simple alcohols, the name of the alkane chain loses the terminaleand adds the suffix-ol,e.g.,as in "ethanol" from the alkane chain name "ethane".When necessary, the position of the hydroxyl group is indicated by a number between the alkane name and the-ol:propan-1-olforCH₃CH₂CH₂OH,propan-2-olforCH₃CH(OH)CH₃.If a higher priority group is present (such as analdehyde,ketone,orcarboxylic acid), then the prefixhydroxy-is used,e.g., as in1-hydroxy-2-propanone(CH₃C(O)CH₂OH).Compounds having more than one hydroxy group are calledpolyols.They are named using suffixes -diol, -triol, etc., following a list of the position numbers of the hydroxyl groups, as inpropane-1,2-diolfor CH₃CH(OH)CH₂OH (propylene glycol).

Example alcohols and representations

Structural formula	Skeletal formula	Preferred IUPAC name	Other systematic names	Common names	Degree
CH3−CH2−CH2−OH		propan-1-ol	1-propanol; n-propyl alcohol	propanol	primary
		propan-2-ol	2-propanol	isopropyl alcohol; isopropanol	secondary
		cyclohexanol			secondary
		2-methylpropan-1-ol	2-methyl-1-propanol	isobutyl alcohol; isobutanol	primary
		tert-amyl alcohol	2-methylbutan-2-ol; 2-methyl-2-butanol	TAA	tertiary

In cases where the hydroxy group is bonded to an sp²carbon on anaromatic ring,the molecule is classified separately as aphenoland is named using the IUPAC rules for naming phenols.Phenolshave distinct properties and are not classified as alcohols.

Common names

In other less formal contexts, an alcohol is often called with the name of the corresponding alkyl group followed by the word "alcohol", e.g.,methylalcohol,ethylalcohol.Propylalcohol may ben-propyl alcoholorisopropyl alcohol,depending on whether the hydroxyl group is bonded to the end or middle carbon on the straightpropanechain. As described under systematic naming, if another group on the molecule takes priority, the alcohol moiety is often indicated using the "hydroxy-" prefix.

In archaic nomenclature, alcohols can be named as derivatives of methanol using "-carbinol" as the ending. For instance,(CH₃)₃COHcan be namedtrimethylcarbinol.

Primary, secondary, and tertiary

Alcohols are then classified into primary, secondary (sec-,s-), and tertiary (tert-,t-), based upon the number of carbon atoms connected to the carbon atom that bears thehydroxylfunctional group.The respective numeric shorthands 1°, 2°, and 3° are sometimes used in informal settings.The primary alcohols have general formulasRCH₂OH.The simplest primary alcohol is methanol (CH₃OH), for which R = H, and the next is ethanol, for whichR = CH₃,themethyl group.Secondary alcohols are those of the form RR'CHOH, the simplest of which is 2-propanol (R = R' = CH₃). For the tertiary alcohols, the general form is RR'R "COH. The simplest example istert-butanol(2-methylpropan-2-ol), for which each of R, R', and R "isCH₃.In these shorthands, R, R', and R "representsubstituents,alkyl or other attached, generally organic groups.

Examples

Type	Formula	IUPACName	Common name
Monohydric alcohols	CH3OH	Methanol	Wood alcohol
	C2H5OH	Ethanol	Alcohol
	C3H7OH	Propan-2-ol	Isopropyl alcohol, Rubbing alcohol
	C4H9OH	Butan-1-ol	Butanol, Butyl alcohol
	C5H11OH	Pentan-1-ol	Pentanol, Amyl alcohol
	C16H33OH	Hexadecan-1-ol	Cetyl alcohol
Polyhydric alcohols (sugar alcohols)	C2H4(OH)2	Ethane-1,2-diol	Ethylene glycol
	C3H6(OH)2	Propane-1,2-diol	Propylene glycol
	C3H5(OH)3	Propane-1,2,3-triol	Glycerol
	C4H6(OH)4	Butane-1,2,3,4-tetraol	Erythritol, Threitol
	C5H7(OH)5	Pentane-1,2,3,4,5-pentol	Xylitol
	C6H8(OH)6	hexane-1,2,3,4,5,6-hexol	Mannitol, Sorbitol
	C7H9(OH)7	Heptane-1,2,3,4,5,6,7-heptol	Volemitol
Unsaturated aliphaticalcohols	C3H5OH	Prop-2-ene-1-ol	Allyl alcohol
	C10H17OH	3,7-Dimethylocta-2,6-dien-1-ol	Geraniol
	C3H3OH	Prop-2-yn-1-ol	Propargyl alcohol
Alicyclicalcohols	C6H6(OH)6	Cyclohexane-1,2,3,4,5,6-hexol	Inositol
	C10H19OH	5-Methyl-2-(propan-2-yl)cyclohexan-1-ol	Menthol

Applications

Total recordedalcohol per capita consumption(15+), in litres of pure ethanol

Alcohols have a long history of myriad uses. For simple mono-alcohols, which is the focus on this article, the following are most important industrial alcohols:

• methanol, mainly for the production offormaldehydeand as afuel additive
• ethanol, mainly for alcoholic beverages, fuel additive, solvent
• 1-propanol, 1-butanol, and isobutyl alcohol for use as a solvent and precursor to solvents
• C6–C11 alcohols used forplasticizers,e.g. inpolyvinylchloride
• fatty alcohol (C12–C18), precursors todetergents

Methanol is the most common industrial alcohol, with about 12 million tons/y produced in 1980. The combined capacity of the other alcohols is about the same, distributed roughly equally.

Toxicity

With respect to acute toxicity, simple alcohols have low acutetoxicities.Doses of several milliliters are tolerated. Forpentanols,hexanols,octanols,and longer alcohols,LD50range from 2–5 g/kg (rats, oral). Ethanol is less acutely toxic.All alcohols are mild skin irritants.

Methanol and ethylene glycol are more toxic than other simple alcohols. Their metabolism is affected by the presence of ethanol, which has a higher affinity forliver alcohol dehydrogenase.In this way,methanolwill be excreted intact in urine.

Physical properties

In general, thehydroxyl groupmakes alcoholspolar.Those groups can formhydrogen bondsto one another and to most other compounds. Owing to the presence of the polar OH alcohols are more water-soluble than simple hydrocarbons. Methanol, ethanol, and propanol aremisciblein water.1-Butanol,with a four-carbon chain, is moderately soluble.

Because ofhydrogen bonding,alcohols tend to have higher boiling points than comparablehydrocarbonsandethers.The boiling point of the alcohol ethanol is 78.29 °C, compared to 69 °C for the hydrocarbonhexane,and 34.6 °C fordiethyl ether.

Occurrence in nature

Alcohols occur widely in nature, as derivatives ofglucosesuch ascelluloseandhemicellulose,and inphenolsand their derivatives such aslignin.Starting frombiomass,180 billion tons/y of complex carbohydrates (sugar polymers) are produced commercially (as of 2014).Many other alcohols are pervasive in organisms, as manifested in other sugars such asfructoseandsucrose,in polyols such asglycerol,and in someamino acidssuch asserine.Simple alcohols like methanol, ethanol, and propanol occur in modest quantities in nature, and are industrially synthesized in large quantities for use as chemical precursors, fuels, and solvents.

Production

Hydroxylation

Many alcohols are produced byhydroxylation,i.e., the installation of a hydroxy group using oxygen or a related oxidant. Hydroxylation is the means by which the body processes manypoisons,converting lipophilic compounds into hydrophilic derivatives that are more readily excreted. Enzymes calledhydroxylasesandoxidasesfacilitate these conversions.

Many industrial alcohols, such ascyclohexanolfor the production ofnylon,are produced by hydroxylation.

Ziegler and oxo processes

In theZiegler process,linear alcohols are produced from ethylene andtriethylaluminiumfollowed by oxidation and hydrolysis.An idealized synthesis of1-octanolis shown:

${\ce {Al(C2H5)3 + 9 C2H4 -> Al(C8H17)3}}$

${\ce {Al(C8H17)3 + 3O + 3 H2O -> 3 HOC8H17 + Al(OH)3}}$

The process generates a range of alcohols that are separated bydistillation.

Many higher alcohols are produced byhydroformylationof alkenes followed by hydrogenation. When applied to aterminal alkene,as is common, one typically obtains a linear alcohol:

${\ce {RCH=CH2 + H2 + CO -> RCH2CH2CHO}}$

${\ce {RCH2CH2CHO + 3 H2 -> RCH2CH2CH2OH}}$

Such processes givefatty alcohols,which are useful for detergents.

Hydration reactions

Some low molecular weight alcohols of industrial importance are produced by the addition of water to alkenes. Ethanol, isopropanol, 2-butanol, andtert-butanol are produced by this general method. Two implementations are employed, the direct and indirect methods. The direct method avoids the formation of stable intermediates, typically using acid catalysts. In the indirect method, the alkene is converted to thesulfate ester,which is subsequently hydrolyzed. The directhydrationusesethylene(ethylene hydration)or other alkenes fromcrackingof fractions of distilledcrude oil.

Hydration is also used industrially to produce the diolethylene glycolfromethylene oxide.

Fermentation

Ethanol is obtained byfermentationofglucose(which is often obtained fromstarch) in the presence of yeast. Carbon dioxide is cogenerated. Like ethanol,butanolcan be produced by fermentation processes.Saccharomycesyeast are known to produce these higher alcohols at temperatures above 75 °F (24 °C). The bacteriumClostridium acetobutylicumcan feed oncellulose(also an alcohol) to produce butanol on an industrial scale.

Substitution

Primaryalkyl halidesreact with aqueousNaOHorKOHto give alcohols innucleophilic aliphatic substitution.Secondary and especially tertiary alkyl halides will give the elimination (alkene) product instead.Grignard reagentsreact withcarbonylgroups to give secondary and tertiary alcohols. Related reactions are theBarbier reactionand theNozaki-Hiyama reaction.

Reduction

Aldehydesorketonesarereducedwithsodium borohydrideorlithium aluminium hydride(after an acidic workup). Another reduction usingaluminium isopropoxideis theMeerwein-Ponndorf-Verley reduction.Noyori asymmetric hydrogenationis the asymmetric reduction of β-keto-esters.

Hydrolysis

Alkenesengage in an acid catalyzedhydration reactionusing concentrated sulfuric acid as a catalyst that gives usually secondary or tertiary alcohols. Formation of a secondary alcohol viaalkene reduction and hydrationis shown:

Thehydroboration-oxidationandoxymercuration-reductionof alkenes are more reliable in organic synthesis. Alkenes react withN-bromosuccinimideand water inhalohydrin formation reaction.Aminescan be converted todiazonium salts,which are then hydrolyzed.

Reactions

Deprotonation

With aqueouspK_avalues of around 16–19, alcohols are, in general, slightly weakeracidsthanwater.With strong bases such assodium hydrideorsodiumthey formsaltscalledalkoxides,with the general formulaRO⁻M⁺(where R is analkyland M is ametal).

${\ce {2 R-OH + 2 NaH -> 2 R-O-Na + 2 H2}}$

${\ce {2 R-OH + 2 Na -> 2 R-O-Na + H2}}$

The acidity of alcohols is strongly affected bysolvation.In the gas phase, alcohols are more acidic than in water.InDMSO,alcohols (and water) have a pK_aof around 29–32. As a consequence, alkoxides (and hydroxide) are powerful bases and nucleophiles (e.g., for theWilliamson ether synthesis) in this solvent. In particular,RO⁻orHO⁻in DMSO can be used to generate significant equilibrium concentrations of acetylide ions through the deprotonation of alkynes (seeFavorskii reaction).

Nucleophilic substitution

Tertiary alcohols react withhydrochloric acidto produce tertiaryalkyl chloride.Primary and secondary alcohols are converted to the corresponding chlorides usingthionyl chlorideand various phosphorus chloride reagents.

Primary and secondary alcohols, likewise, convert toalkyl bromidesusingphosphorus tribromide,for example:

${\ce {3 R-OH + PBr3 -> 3 RBr + H3PO3}}$

In theBarton-McCombie deoxygenationan alcohol is deoxygenated to analkanewithtributyltin hydrideor atrimethylborane-water complex in aradical substitutionreaction.

Dehydration

Meanwhile, the oxygen atom haslone pairsof nonbonded electrons that render it weaklybasicin the presence of strong acids such assulfuric acid.For example, with methanol:

Upon treatment with strong acids, alcohols undergo the E1elimination reactionto producealkenes.The reaction, in general, obeysZaitsev's Rule,which states that the most stable (usually the most substituted) alkene is formed. Tertiary alcohols are eliminated easily at just above room temperature, but primary alcohols require a higher temperature.

This is a diagram of acid catalyzed dehydration of ethanol to produceethylene:

A more controlled elimination reaction requires the formation of thexanthate ester.

Protonolysis

Tertiary alcohols react with strong acids to generate carbocations. The reaction is related to their dehydration, e.g.isobutylenefromtert-butyl alcohol. A special kind of dehydration reaction involvestriphenylmethanoland especially its amine-substituted derivatives. When treated with acid, these alcohols lose water to give stable carbocations, which are commercial dyes.

Preparation ofcrystal violetby protonolysis of the tertiary alcohol.

Esterification

Alcohol andcarboxylic acidsreact in the so-calledFischer esterification.The reaction usually requires acatalyst,such as concentrated sulfuric acid:

${\ce {R-OH + R'-CO2H -> R'-CO2R + H2O}}$

Other types of ester are prepared in a similar manner−for example,tosyl(tosylate) esters are made by reaction of the alcohol with4-toluenesulfonyl chlorideinpyridine.

Oxidation

Main article:Alcohol oxidation

Primary alcohols (R−CH₂OH) can be oxidized either toaldehydes(R−CHO) or tocarboxylic acids(R−CO₂H). The oxidation of secondary alcohols (R¹R²CH−OH) normally terminates at theketone(R¹R²C=O) stage. Tertiary alcohols (R¹R²R³C−OH) are resistant to oxidation.

The direct oxidation of primary alcohols to carboxylic acids normally proceeds via the corresponding aldehyde, which is transformed via analdehyde hydrate(R−CH(OH)₂) by reaction with water before it can be further oxidized to the carboxylic acid.

Mechanism of oxidation of primary alcohols to carboxylic acids via aldehydes and aldehyde hydrates

Reagents useful for the transformation of primary alcohols to aldehydes are normally also suitable for the oxidation of secondary alcohols to ketones. These includeCollins reagentandDess-Martin periodinane.The direct oxidation of primary alcohols to carboxylic acids can be carried out usingpotassium permanganateor theJones reagent.

See also

• Beer chemistry
• Enol
• Ethanol fuel
• Fatty alcohol
• Index of alcohol-related articles
• List of alcohols
• Lucas test
• Polyol
• Rubbing alcohol
• Sugar alcohol
• Transesterification
• Wine chemistry

General references

• Metcalf AA (1999).The World in So Many Words.Houghton Mifflin.ISBN0-395-95920-9.