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
nH
2n+1OH.
Generally, the word ''alcohol'' usually refers to
ethanol, also known as ''grain alcohol'' or (older) ''spirits of wine''. Ethanol is a very strong smelling, colorless, volatile liquid formed by the
fermentation of sugars. It also often refers to any beverage that contains ethanol (see ''
alcoholic beverage''). It is the most widely used
depressant in the world, and has been for thousands of years. This sense underlies the term
alcoholism (
addiction to alcohol).
Other forms of alcohol are usually described with a clarifying adjective, as in ''
isopropyl alcohol'' (''propan-2-ol'') or ''wood alcohol'' (''methyl alcohol'', or
methanol). The suffix ''-ol'' appears in the "official"
IUPAC chemical name of all 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' . The simplest secondary alcohol is
isopropyl alcohol (propan-2-ol), and a simple tertiary alcohol is
''tert''-butyl alcohol (2-methylpropan-2-ol).
The
phenols with parent compound
phenol have a hydroxyl group (attached to a
benzene ring) just like alcohols but differ sufficiently in properties as to warrant a separate treatment.
Carbohydrates (sugars) and
sugar alcohols are an important class of compounds containing multiple alcohol functional groups. For example,
sucrose (common sugar) contains eight hydroxyl groups per molecule and
sorbitol has six. Most of the attributes of these
polyols, from nomenclature, to occurance, use and toxicity, are sufficiciently different from simple aliphatic alcohols as to require 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 in the chart. 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" or "denatured alcohol", 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.
Propanol and butanol
Two other alcohols whose uses are relatively widespread (though not so much as those of methanol and ethanol) are propanol and butanol. Like ethanol, they are produced by fermentation processes. (However, the fermenting agent is a bacterium, ''
Clostridium acetobutylicum'', that feeds on
cellulose, not sugars like the Saccharomyces yeast that produces ethanol.)
Sources
Many alcohols can be created by
fermentation of
fruits or
grains with
yeast, but only ethanol is commercially produced this way — chiefly for
fuel and
drink. Other alcohols are generally produced by synthetic routes from
natural gas,
petroleum, or
coal feed stocks; for example, via acid catalyzed
hydration of
alkenes. For more details see
Preparation of alcohols.
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
3CH
2CH
2OH,
propan-2-ol for CH
3CH(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
3COCH
2OH).
Some examples of simple alcohols and how to name them:
Examples of alcohols & their names
Common names for alcohols usually takes 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)
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.
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. The
Persian physician and scientist
Rhazes discovered this substance, but because he wanted his book to be published in most of the then-known world, he used the Arabic language instead of Persian (although he made copies in Persian). The word 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 and
Persian alchemists [1].
A popular theory, found in many dictionaries, is that it comes from الكحل ''al-kuḥl'', originally the name of very finely powdered
antimony sulfide Sb2S3 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 '', does not derive from ''. The
Qur'an, in verse 37:47, uses the word الغول '' — properly meaning "
spirit" or "
demon" — with the sense "the thing that gives the wine its headiness". The word '' 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.
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. This hydrogen bonding means that alcohols can be used as
protic solvents. 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 in water because of the hydrocarbon chain's dominance. All simple alcohols are miscible in organic solvents.
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.
Alcohols, like water, can show either acidic or basic properties at the O-H group. With a
pKa 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. The lone pairs of electrons on the oxygen of the hydroxyl group also makes alcohols nucleophiles. For more details see the
reactions of alcohols section below.
Uses
Automotive
Main articles: Alcohol as a fuel
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 used as an
antifreeze in automobile
radiators. To add to an
internal combustion engine's performance, methanol may be injected into turbocharged and supercharged engines. This cools the air intake charge, providing a denser air charge.
Scientific, medical, and industrial
Alcohols have applications in industry and science as reagents or
solvents. Because of its low toxicity and ability to dissolve
non-polar substances, ethanol can be used as a solvent in medical drugs,
perfumes, and vegetable essences such as
vanilla. In
organic synthesis, alcohols serve as versatile intermediates.
Ethanol can be used as an antiseptic to disinfect the skin before injections are given, often along with iodine. Ethanol-based soaps are becoming common in restaurants and are convenient because they do not require drying due to the volatility of the compound. Alcohol is also used as a
preservative for
specimens.
Toxicity
Alcohols often have an odor 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. The consumption of large doses result in
drunkenness or intoxication (which may lead to a
hangover as the effect wears off) and, depending on the dose and regularity of use, can cause acute respiratory failure or death and with chronic use has medical repercussions. Because alcohol impairs judgment, it can often be a catalyst for reckless or irresponsible behavior. The
LD50 of ethanol in rats is 11,300 mg/kg.
[2] 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
ethane or other alkenes from
cracking of fractions of distilled
crude oil. Uses a catalyst of
phosphoric acid under high temperature and pressure of 50-120.
★
Methanol is produced 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
aluminum 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
+ +
H2↑
: 2 R-OH + 2
Na → 2R-O
−Na + H
2
: E.g. 2
CH3CH2-OH + 2 Na → 2 CH
3-CH
2-O
−Na + H
2
Water is similar in
pKa 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
2O (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.
[1]
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
3PO
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
2O
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
2SO
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 (R-CH
2-OH) can be oxidized either to
aldehydes (R-CHO) or to
carboxylic acids (R-CO
2H), while the oxidation of secondary alcohols (R
1R²CH-OH) normally terminates at the
ketone (R
1R²C=O) stage. Tertiary alcohols (R
1R²R³C-OH) 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 (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
Often it is possible to interrupt the oxidation of a primary alcohol at the aldehyde level by performing the reaction in absence of water, so that no aldehyde hydrate can be formed.
Reagents useful for the transformation of primary alcohols to aldehydes are normally also suitable for the oxidation of secondary alcohols to ketones. These include:
★ Chromium-based reagents, such as
Collins reagent (CrO
3·Py
2),
PDC or
PCC.
★ Activated
DMSO, resulting from reaction of DMSO with
electrophiles, such as
oxalyl chloride (
Swern oxidation), a
carbodiimide (
Pfitzner-Moffatt oxidation) or the complex SO
3·Py (
Parikh-Doering oxidation).
★ Hypervalent iodine compounds, such as
Dess-Martin periodinane or
2-Iodoxybenzoic acid.
★ Catalytic
TPAP in presence of excess of
NMO (
Ley oxidation).
★ Catalytic
TEMPO in presence of excess
bleach (
NaOCl) (
Anelli’s oxidation).
Oxidation of alcohols to aldehydes and ketones
Allylic and benzylic alcohols can be oxidized in presence of other alcohols using certain selective oxidants such as
manganese dioxide (MnO
2).
Reagents useful for the oxidation of secondary alcohols to ketones, but normally inefficient for oxidation of primary alcohols to aldehydes, include
chromium trioxide (CrO
3) in a mixture of
sulfuric acid and
acetone (
Jones oxidation) and certain ketones, such as
cyclohexanone, in the presence of
aluminium isopropoxide (
Oppenauer oxidation).
The direct oxidation of primary alcohols to carboxylic acids can be carried out using:
★
Potassium permanganate (KMnO
4).
★
Jones oxidation.
★
PDC in
DMF.
★
Heyns oxidation.
★
Ruthenium tetroxide (RuO
4).
★
TEMPO.
Oxidation of primary alcohols to carboxylic acids
Alcohols possessing two
hydroxy groups located on adjacent carbons —that is, 1,2-diols— suffer oxidative breakage at a carbon-carbon bond with some oxidants such as
sodium periodate (NaIO
4) or
lead tetraacetate (Pb(OAc)
4), resulting in generation of two
carbonyl groups.
Oxidative breakage of carbon-carbon bond in 1,2-diols
See also
★
Alcohol fuel
★
Alcoholic beverage
★
Blood alcohol content
★
Breathalyzer
★
Effects of alcohol on the body
★
Fatty alcohol
★
Fetal alcohol syndrome
★
Oxidation of primary alcohols to carboxylic acids
★
Rubbing alcohol
★
Sugar alcohol
★
Transesterification
References
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