Organic compound with at least one hydroxyl (–OH) group
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 model of an alcohol molecule (R3COH). The red and white balls represent the hydroxyl group (−OH). The three "R"s stand for carbonsubstituents or hydrogen atoms.[1]
Inchemistry, analcohol (from Arabic al-kuḥl'thekohl'),[2] is a type oforganic compound that carries at least onehydroxyl (−OH)functional group bound to asaturated carbon atom.[3][4] Alcohols range from the simple, likemethanol andethanol, to complex, likesugar alcohols andcholesterol. 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.
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).[5] 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.[6] An important recognition, first found in one of the writings attributed toJābir ibn Ḥayyān (ninth century CE), was that byadding salt to boiling wine, which increases the wine'srelative volatility, the flammability of the resulting vapors may be enhanced.[7] 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).[8] 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.[9]
The word "alcohol" derives from the Arabickohl (Arabic:الكحل,romanized: al-kuḥl), a powder used as an eyeliner.[12] The first part of the word (al-) is the Arabicdefinite article, equivalent tothe in English. The second part of the word (kuḥl) has several antecedents inSemitic languages, ultimately deriving from theAkkadian𒎎𒋆𒁉𒍣𒁕 (guḫlum), meaningstibnite orantimony.[13]
Like its antecedents in Arabic and older languages, the termalcohol was originally used for the very fine powder produced by thesublimation of the natural mineralstibnite to 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.[14]
Paracelsus andLibavius both used the termalcohol to denote a fine powder, the latter speaking of analcohol derived from antimony. At the same time Paracelsus uses the word for a volatile liquid;alcool oralcool vini occurs often in his writings.[15]
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."[16]
The 1657Lexicon Chymicum, by William Johnson glosses the word as "antimonium sive stibium."[17] By extension, the word came to refer to any fluid obtained by distillation, including "alcohol of wine," the distilled essence of wine.Libavius inAlchymia (1594) refers to "vini alcohol vel vinum alcalisatum". Johnson (1657) glossesalcohol vini as "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.[16]
The termethanol was invented in 1892,blending "ethane" with the "-ol" ending of "alcohol", which was generalized as alibfix.[18]
The suffix-ol appears in theInternational Union of Pure and Applied Chemistry (IUPAC)chemical name of 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-ol in non-IUPAC names (such asparacetamol orcholesterol) also typically indicates that the substance is an alcohol. However, some compounds that contain hydroxyl functional groups havetrivial names that do not include the suffix-ol or the prefixhydroxy-, e.g. the sugarsglucose andsucrose.
IUPAC nomenclature is 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 terminale and adds the suffix-ol,e.g., as in "ethanol" from the alkane chain name "ethane".[19] When necessary, the position of the hydroxyl group is indicated by a number between the alkane name and the-ol:propan-1-ol forCH3CH2CH2OH,propan-2-ol forCH3CH(OH)CH3. If a higher priority group is present (such as analdehyde,ketone, orcarboxylic acid), then the prefixhydroxy-is used,[19] e.g., as in1-hydroxy-2-propanone (CH3C(O)CH2OH).[20] 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-diol for CH3CH(OH)CH2OH (propylene glycol).
In cases where the hydroxy group is bonded to an sp2 carbon on anaromatic ring, the molecule is classified separately as aphenol and is named using the IUPAC rules for naming phenols.[21]Phenols have distinct properties and are not classified as alcohols.
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.,methyl alcohol,ethyl alcohol.Propyl alcohol may ben-propyl alcohol orisopropyl alcohol, depending on whether the hydroxyl group is bonded to the end or middle carbon on the straightpropane chain. As described under systematic naming, if another group on the molecule takes priority, the alcohol moiety is often indicated using the "hydroxy-" prefix.[22]
In archaic nomenclature, alcohols can be named as derivatives of methanol using "-carbinol" as the ending. For instance,(CH3)3COH can be namedtrimethylcarbinol.
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.[23] 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.
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:[25]
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.[25]
With respect to acute toxicity, simple alcohols have low acutetoxicities. Doses of several milliliters are tolerated. Forpentanols,hexanols,octanols, and longer alcohols,LD50 range from 2–5 g/kg (rats, oral). Ethanol is less acutely toxic.[27] All alcohols are mild skin irritants.[25]
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,methanol will be excreted intact in urine.[28][29][30]
In general, thehydroxyl group makes alcoholspolar. Those groups can formhydrogen bonds to 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 aremiscible in water.1-Butanol, with a four-carbon chain, is moderately soluble.
Because ofhydrogen bonding, alcohols tend to have higher boiling points than comparablehydrocarbons andethers. The boiling point of the alcohol ethanol is 78.29 °C, compared to 69 °C for the hydrocarbonhexane, and 34.6 °C fordiethyl ether.
Alcohols occur widely in nature, as derivatives ofglucose such ascellulose andhemicellulose, and inphenols and their derivatives such aslignin.[31] Starting frombiomass, 180 billion tons/y of complex carbohydrates (sugar polymers) are produced commercially (as of 2014).[32] Many other alcohols are pervasive in organisms, as manifested in other sugars such asfructose andsucrose, in polyols such asglycerol, and in someamino acids such 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.
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 calledhydroxylases andoxidases facilitate these conversions.
Many industrial alcohols, such ascyclohexanol for the production ofnylon, are produced by hydroxylation.
The process generates a range of alcohols that are separated bydistillation.
Many higher alcohols are produced byhydroformylation of alkenes followed by hydrogenation. When applied to aterminal alkene, as is common, one typically obtains a linear alcohol:[25]
RCH=CH2 + H2 + CO → RCH2CH2CHO
RCH2CH2CHO + 3 H2 → RCH2CH2CH2OH
Such processes givefatty alcohols, which are useful for detergents.
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 directhydration usesethylene (ethylene hydration)[33] or other alkenes fromcracking of fractions of distilledcrude oil.
Ethanol is obtained byfermentation ofglucose (which is often obtained fromstarch) in the presence of yeast. Carbon dioxide is cogenerated. Like ethanol,butanol can be produced by fermentation processes.Saccharomyces yeast are known to produce these higher alcohols at temperatures above 75 °F (24 °C). The bacteriumClostridium acetobutylicum can feed oncellulose (also an alcohol) to produce butanol on an industrial scale.[34]
Alkenes engage in an acid catalyzedhydration reaction using concentrated sulfuric acid as a catalyst that gives usually secondary or tertiary alcohols. Formation of a secondary alcohol viaalkene reduction and hydration is shown:
With aqueouspKa values of around 16–19, alcohols are, in general, slightly weakeracids thanwater. With strong bases such assodium hydride orsodium they formsalts[a] calledalkoxides, with the general formulaRO−M+ (where R is analkyl and M is ametal).
R−OH + NaH → R−O−Na+ + H2
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.[35] InDMSO, alcohols (and water) have a pKa of 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).[36][37]
Tertiary alcohols react withhydrochloric acid to produce tertiaryalkyl chloride. Primary and secondary alcohols are converted to the corresponding chlorides usingthionyl chloride and various phosphorus chloride reagents.[38]
Meanwhile, the oxygen atom haslone pairs of nonbonded electrons that render it weaklybasic in the presence of strong acids such assulfuric acid. For example, with methanol:
Upon treatment with strong acids, alcohols undergo the E1elimination reaction to producealkenes. The reaction, in general, obeysZaytsev'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.
Tertiary alcohols react with strong acids to generate carbocations. The reaction is related to their dehydration, e.g.isobutylene fromtert-butyl alcohol. A special kind of dehydration reaction involvestriphenylmethanol and especially its amine-substituted derivatives. When treated with acid, these alcohols lose water to give stable carbocations, which are commercial dyes.[39]
Preparation ofcrystal violet by protonolysis of the tertiary alcohol.
Other types of ester are prepared in a similar manner−for example,tosyl (tosylate) esters are made by reaction of the alcohol with4-toluenesulfonyl chloride inpyridine.
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 reagent andDess–Martin periodinane. The direct oxidation of primary alcohols to carboxylic acids can be carried out usingpotassium permanganate or theJones reagent.
^Although commonly described as "salts", alkali metal alkoxides are actually better described structurally as oligomeric clusters or polymeric chains. For instance, potassiumtert-butoxide consists of a cubane-like tetramer,[t-BuOK]4, that persists even in polar solvents like THF.
^al-Hassan AY (2009). "Alcohol and the Distillation of Wine in Arabic Sources from the 8th Century".Studies in al-Kimya': Critical Issues in Latin and Arabic Alchemy and Chemistry. Hildesheim: Georg Olms Verlag. pp. 283–298. (same content also available onthe author's website).
^Lodgsdon J.E. (1994). "Ethanol". In Kroschwitz J.I. (ed.).Encyclopedia of Chemical Technology. Vol. 9 (4th ed.). New York: John Wiley & Sons. p. 820.ISBN978-0-471-52677-3.
^Zverlov W, Berezina O, Velikodvorskaya GA, Schwarz WH (August 2006). "Bacterial acetone and butanol production by industrial fermentation in the Soviet Union: use of hydrolyzed agricultural waste for biorefinery".Applied Microbiology and Biotechnology.71 (5):587–97.doi:10.1007/s00253-006-0445-z.PMID16685494.S2CID24074264.
