In chemistry, an alcohol (),
History
The flammable nature of the exhalations of wine was already known to ancient natural philosophers such as
Aristotle (384–322 BCE),
Theophrastus (–287 BCE), and Pliny the Elder (23/24–79 CE).
[ vol. I, p. 137.] However, this did not immediately lead to the isolation of alcohol, even despite the development of more advanced distillation techniques in second- and third-century
Roman Egypt.
[.] An important recognition, first found in one of the writings attributed to Jābir ibn Ḥayyān (ninth century CE), was that by adding salt to boiling wine, which increases the wine's relative volatility, the flammability of the resulting vapors may be enhanced.
[ (same content also available on the author's website).] The distillation of wine is attested in Arabic works attributed to
Al-Kindi (–873 CE) and to
Al-Farabi (–950), and in the 28th book of
Al-Zahrawi's (Latin: Abulcasis, 936–1013)
Kitāb al-Taṣrīf (later translated into Latin as
Liber servatoris).
[ (same content also available on the author's website); cf. . Sometimes, sulfur was also added to the wine (see ).] In the twelfth century, recipes for the production of
aqua 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.
[ pp. 204–206.]
The works of Taddeo Alderotti (1223–1296) describe a method for concentrating alcohol involving repeated fractional distillation through a water-cooled still, by which an alcohol purity of 90% could be obtained.[ pp. 51–52.] The medicinal properties of ethanol were studied by Arnald of Villanova (1240–1311 CE) and John of Rupescissa (–1366), the latter of whom regarded it as a life-preserving substance able to prevent all diseases (the aqua vitae or "water of life", also called by John the quintessence of wine).[ pp. 69–71.]
Nomenclature
Etymology
The word "alcohol" derives from the Arabic
kohl (), a powder used as an eyeliner.
The first part of the word () is the Arabic definite article, equivalent to
the in English. The second part of the word () has several antecedents in Semitic languages, ultimately deriving from the Akkadian (), meaning
stibnite or
antimony.
[ Zimmern, Heinrich (1915) Akkadische Fremdwörter als Beweis für babylonischen Kultureinfluss (in German), Leipzig: A. Edelmann, page 61]
Like its antecedents in Arabic and older languages, the term alcohol was originally used for the very fine powder produced by the sublimation of the natural mineral stibnite to form antimony trisulfide . It was considered to be the essence or "spirit" of this mineral. It was used as an antiseptic, eyeliner, and cosmetic. Later the meaning of alcohol was extended to distilled substances in general, and then narrowed again to ethanol, when "spirits" was a synonym for hard liquor.
Paracelsus and Libavius both used the term alcohol to denote a fine powder, the latter speaking of an alcohol derived from antimony. At the same time Paracelsus uses the word for a volatile liquid; alcool or alcool vini occurs often in his writings.
Bartholomew Traheron, in his 1543 translation of John 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 1657 Lexicon Chymicum, by William Johnson glosses the word as "antimonium sive stibium."
The term ethanol was invented in 1892, blend word "ethane" with the "-ol" ending of "alcohol", which was generalized as a libfix.
The term alcohol originally referred to the primary alcohol ethanol (ethyl alcohol), which is used as a drug and is the main alcohol present in alcoholic drinks.
The suffix -ol appears in the International 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 prefix hydroxy- is used in its IUPAC name. The suffix -ol in non-IUPAC names (such as paracetamol or cholesterol) also typically indicates that the substance is an alcohol. However, some compounds that contain hydroxyl functional groups have trivial names that do not include the suffix -ol or the prefix hydroxy-, e.g. the sugars glucose and sucrose.
Systematic names
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 terminal
e and 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-ol for , propan-2-ol for . If a higher priority group is present (such as an
aldehyde,
ketone, or
carboxylic acid), then the prefix
hydroxy-is used,
e.g., as in 1-hydroxy-2-propanone ().
[Organic chemistry IUPAC nomenclature. Alcohols Rule C-201.] Compounds having more than one hydroxy group are called
. They are named using suffixes -diol, -triol, etc., following a list of the position numbers of the hydroxyl groups, as in
Propylene glycol for CH
3CH(OH)CH
2OH (propylene glycol).
| + Example alcohols and representations
! Structural formula
! Skeletal formula
! Preferred IUPAC name
! Other systematic names
! Common names
! Degree |
| | | 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 sp2 carbon on an aromatic ring, the molecule is classified separately as a phenols and is named using the IUPAC rules for naming phenols.[ Organic Chemistry Nomenclature Rule C-203: Phenols] Phenols have 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.,
methyl group alcohol,
ethyl group alcohol.
Propyl alcohol may be
propanol or isopropyl alcohol, depending on whether the hydroxyl group is bonded to the end or middle carbon on the straight
propane chain. 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, can be named trimethylcarbinol.
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 the
hydroxyl functional group. The respective numeric shorthands 1°, 2°, and 3° are sometimes used in informal settings.
The primary alcohols have general formulas . The simplest primary alcohol is methanol (), for which R = H, and the next is ethanol, for which , the
methyl group. Secondary alcohols are those of the form RR'CHOH, the simplest of which is 2-propanol (). For the tertiary alcohols, the general form is RR'R"COH. The simplest example is
tert-butanol (2-methylpropan-2-ol), for which each of R, R', and R" is . In these shorthands, R, R', and R" represent
substituents, alkyl or other attached, generally organic groups.
Examples
|
Monohydric
alcohols | | Methanol | Wood alcohol |
| | Ethanol | Alcohol, Rubbing alcohol |
| | Propan-2-ol | Isopropyl alcohol,
Rubbing alcohol |
| | Butan-1-ol | Butanol,
Butyl alcohol |
| | Pentan-1-ol | Pentanol,
Amyl alcohol |
| | Hexadecan-1-ol | Cetyl alcohol |
| Sugar alcohol | | Ethane-1,2-diol | Ethylene glycol |
| | Propane-1,2-diol | Propylene glycol |
| | Propane-1,2,3-triol | Glycerol |
| | Butane-1,2,3,4-tetraol | Erythritol,
Threitol |
| | Pentane-1,2,3,4,5-pentol | Xylitol |
| | hexane-1,2,3,4,5,6-hexol | Mannitol,
Sorbitol |
| | Heptane-1,2,3,4,5,6,7-heptol | Volemitol |
Unsaturated
aliphatic alcohols | | Prop-2-ene-1-ol | Allyl alcohol |
| | 3,7-Dimethylocta-2,6-dien-1-ol | Geraniol |
| | Prop-2-yn-1-ol | Propargyl alcohol |
| Alicyclic alcohols | | Cyclohexane-1,2,3,4,5,6-hexol | Inositol |
| | 5-Methyl-2-(propan-2-yl)cyclohexan-1-ol | Menthol |
Applications
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 of formaldehyde and as a fuel additive
-
ethanol, mainly for alcoholic beverages, fuel additive, solvent, and to sterilize hospital instruments.
-
1-propanol, 1-butanol, and isobutyl alcohol for use as a solvent and precursor to solvents
-
C6–C11 alcohols used for , e.g. in polyvinylchloride
-
fatty alcohol (C12–C18), precursors to
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 acute
. Doses of several milliliters are tolerated. For
,
,
, and longer alcohols, LD50 range from 2–5 g/kg (rats, oral). Ethanol is less acutely toxic.
[Ethanol toxicity] 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 for liver alcohol dehydrogenase. In this way, methanol will be excreted intact in urine.
Physical properties
In general, the
hydroxyl group makes alcohols
polar molecule. Those groups can form
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 are
miscible in water. 1-Butanol, with a four-carbon chain, is moderately soluble.
Because of hydrogen bonding, alcohols tend to have higher boiling points than comparable and . The boiling point of the alcohol ethanol is 78.29 °C, compared to 69 °C for the hydrocarbon hexane, and 34.6 °C for diethyl ether.
Occurrence in nature
Alcohols occur widely in nature, as derivatives of
glucose such as
cellulose and
hemicellulose, and in
phenols and their derivatives such as
lignin.
Starting from
biomass, 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 as
fructose and
sucrose, in polyols such as
glycerol, and in some
such as
serine. 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 by
hydroxylation, i.e., the installation of a hydroxy group using oxygen or a related oxidant. Hydroxylation is the means by which the body processes many
xenobiotic, converting lipophilic compounds into hydrophilic derivatives that are more readily excreted. Enzymes called
and
facilitate these conversions.
Many industrial alcohols, such as cyclohexanol for the production of nylon, are produced by hydroxylation.
Ziegler and oxo processes
In the
Ziegler process, linear alcohols are produced from ethylene and triethylaluminium followed by oxidation and hydrolysis.
An idealized synthesis of 1-octanol is shown:
The process generates a range of alcohols that are separated by distillation.
Many higher alcohols are produced by hydroformylation of alkenes followed by hydrogenation. When applied to a terminal alkene, as is common, one typically obtains a linear alcohol:
Such processes give , 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, and
tert-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 the
sulfate ester, which is subsequently hydrolyzed. The direct hydration uses
ethylene (ethylene hydration)
or other alkenes from cracking of fractions of distilled
crude oil.
Hydration is also used industrially to produce the diol ethylene glycol from ethylene oxide.
Fermentation
Ethanol is obtained by fermentation of
glucose (which is often obtained from
starch) in the presence of yeast. Carbon dioxide is cogenerated. Like ethanol,
n-Butanol can be produced by fermentation processes.
Saccharomyces yeast are known to produce these higher alcohols at temperatures above . The bacterium
Clostridium acetobutylicum can feed on
cellulose (also an alcohol) to produce butanol on an industrial scale.
Substitution
Primary
react with aqueous
Sodium hydroxide or KOH to give alcohols in nucleophilic aliphatic substitution. Secondary and especially tertiary alkyl halides will give the elimination (alkene) product instead.
react with
carbonyl groups to give secondary and tertiary alcohols. Related reactions are the
Barbier reaction and the Nozaki–Hiyama–Kishi reaction.
Reduction
Aldehydes or
are
redox with sodium borohydride or lithium aluminium hydride (after an acidic workup). Another reduction using aluminium isopropoxide is the Meerwein–Ponndorf–Verley reduction. Noyori asymmetric hydrogenation is the asymmetric reduction of β-keto-esters.
Hydrolysis
Alkenes engage in an acid catalyzed hydration reaction using concentrated sulfuric acid as a catalyst that gives usually secondary or tertiary alcohols. Formation of a secondary alcohol via alkene reduction and hydration is shown:
The hydroboration-oxidation and oxymercuration-reduction of alkenes are more reliable in organic synthesis. Alkenes react with N-bromosuccinimide and water in halohydrin formation reaction. can be converted to , which are then hydrolyzed.
Reactions
Deprotonation
With aqueous
pKa values of around 16–19, alcohols are, in general, slightly weaker
than
water. With strong bases such as
sodium hydride or
sodium they form salts called
, with the general formula (where R is an
alkyl and M is a
metal).
The acidity of alcohols is strongly affected by solvation. In the gas phase, alcohols are more acidic than in water.
Nucleophilic substitution
Tertiary alcohols react with hydrochloric acid to produce tertiary
alkyl chloride. Primary and secondary alcohols are converted to the corresponding chlorides using
thionyl chloride and various phosphorus chloride reagents.
Primary and secondary alcohols, likewise, convert to using phosphorus tribromide, for example:
In the Barton–McCombie deoxygenation an alcohol is deoxygenated to an alkane with tributyltin hydride or a organoborane-water complex in a radical substitution reaction.
Dehydration
Meanwhile, the oxygen atom has
of nonbonded electrons that render it weakly basic in the presence of strong acids such as
sulfuric acid. For example, with methanol:
Upon treatment with strong acids, alcohols undergo the E1 elimination reaction to produce . The reaction, in general, obeys Zaytsev'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 produce ethylene:
A more controlled elimination reaction requires the formation of the xanthate ester.
Protonolysis
Tertiary alcohols react with strong acids to generate carbocations. The reaction is related to their dehydration, e.g.
isobutylene from
tert-butyl alcohol. A special kind of dehydration reaction involves triphenylmethanol and especially its amine-substituted derivatives. When treated with acid, these alcohols lose water to give stable carbocations, which are commercial dyes.
Esterification
Alcohol and
react in the so-called Fischer esterification. The reaction usually requires a
catalyst, such as concentrated sulfuric acid:
Other types of ester are prepared in a similar manner−for example,
tosyl (tosylate) esters are made by reaction of the alcohol with 4-toluenesulfonyl chloride in
pyridine.
Oxidation
Primary alcohols () can be oxidized either to
() or to
(). The oxidation of secondary alcohols () normally terminates at the
ketone () stage. Tertiary alcohols () are resistant to oxidation.
The direct oxidation of primary alcohols to carboxylic acids normally proceeds via the corresponding aldehyde, which is transformed via an aldehyde hydrate () by reaction with water before it can be further oxidized to the carboxylic acid.
Reagents useful for the transformation of primary alcohols to aldehydes are normally also suitable for the oxidation of secondary alcohols to ketones. These include Collins reagent and Dess–Martin periodinane. The direct oxidation of primary alcohols to carboxylic acids can be carried out using potassium permanganate or the Jones reagent.
See also
Notes
Citations
General references
