In chemistry, an alcohol is an organic compound in which the hydroxyl functional group (-OH) is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms.[1]
An important class of alcohols are the simple acyclic alcohols, the general formula for which is CnH2n+1OH. Of those, ethanol (C2H5OH) is the type of alcohol found in alcoholic beverages, and in common speech the word alcohol refers specifically to ethanol.
Other alcohols 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 IUPAC chemical name of all substances where the hydroxyl group is the functional group with the highest priority; in substances where a higher priority group is present the prefix hydroxy- will appear in the IUPAC name. The suffix -ol in non-systematic names (such as paracetamol or cholesterol) also typically indicates that the substance includes a hydroxyl functional group and, so, can be termed an alcohol. But many substances, particularly sugars (examples glucose and sucrose) contain hydroxyl functional groups without using the suffix.
Simple alcohols
The most commonly used alcohol is ethanol, C2H5OH, with the ethane backbone. Ethanol has been produced and consumed by humans for millennia, in the form of fermented and distilled alcoholic beverages. It is a clear flammable liquid that boils at 78.4 °C, which is used as an industrial solvent, car fuel, and raw material in the chemical industry. In the US and some other countries, because of legal and tax restrictions on alcohol consumption, ethanol destined for other uses often contains additives that make it unpalatable (such as Bitrex) or poisonous (such as methanol). Ethanol in this form is known generally as denatured alcohol; when methanol is used, it may be referred to as methylated spirits or "surgical spirits".
The simplest alcohol is methanol, CH3OH, which was formerly obtained by the distillation of wood and, therefore, is called "wood alcohol". It is a clear liquid resembling ethanol in smell and properties, with a slightly lower boiling point (64.7 °C), and is used mainly as a solvent, fuel, and raw material. Unlike ethanol, methanol is extremely toxic: One sip (as little as 10 ml) can cause permanent blindness by destruction of the optic nerve and 30 ml (one fluid ounce) is potentially fatal.[2]
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 can be 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.) Saccharomyces yeast are known to produce these higher alcohols at temperatures above 75 °F (24 °C). These alcohols are called fusel alcohols or fusel oils in brewing and tend to have a spicy or peppery flavor. They are considered a fault in most styles of beer.[3]
Simple alcohols, in particular, ethanol and methanol, possess denaturing and inert rendering properties, leading to their use as anti-microbial agents in medicine, pharmacy, and industry.
Systematic names
In the IUPAC system, the name of the alkane chain loses the terminal "e" and adds "ol", e.g., "methanol" and "ethanol".[4] When necessary, the position of the hydroxyl group is indicated by a number between the alkane name and the "ol": propan-1-ol for CH3CH2CH2OH, propan-2-ol for 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, ketone, or carboxylic acid), then it is necessary to use the prefix "hydroxy",[4] for example: 1-hydroxy-2-propanone (CH3COCH2OH).
The IUPAC nomenclature is used in scientific publications and where precise identification of the substance is important. 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 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.
Alcohols are classified into primary, secondary (sec), and tertiary (tert), based upon the number of carbon atoms connected to the carbon atom that bears the hydroxyl group. The primary alcohols have general formulas RCH2OH; secondary ones are RR'CHOH; and tertiary ones are RR'R"COH, where R, R', and R" stand for alkyl groups. Ethanol and n-propyl alcohol are primary alcohols; isopropyl alcohol is a secondary one. The prefixes sec- (or s-) and tert- (or t-), conventionally in italics, may be used before the alkyl group's name to distinguish secondary and tertiary alcohols, respectively, from the primary one. For example, isopropyl alcohol is occasionally called sec-propyl alcohol, and the tertiary alcohol (CH3)3COH, or 2-methylpropan-2-ol in IUPAC nomenclature is commonly known as tert-butyl alcohol or tert-butanol.
History and etymology
The first alcohol (today known as ethyl alcohol) was discovered by the tenth-century Persian alchemist al-Razi.
kuḥl was originally the name given to the very fine powder, produced by the sublimation of the natural mineral stibnite to form antimony sulfide Sb2S3 (hence the essence or "spirit" of the substance), which was used as an antiseptic and eyeliner. Bartholomew Traheron in his 1543 translation of John of Vigo introduces the word as a term used by "barbarous" (Moorish) authors for "fine powder":
the barbarous auctours use alcohol, or (as I fynde it sometymes wryten) alcofoll, for moost fine poudre.
The word alcohol appears in English, as a term for a very fine powder, in the 16th century, loaned via French from medical Latin, ultimately from the Arabic الكحل (al-kuḥl, "the kohl, a powder used as an eyeliner"). ال al is Arabic for the definitive article, the in English.
William Johnson in his 1657 Lexicon Chymicum 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. Libavius in Alchymia (1594) has vini alcohol vel vinum alcalisatum. Johnson (1657) glosses alcohol 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 as ethanol) in the 18th century, and was extended to the class of substances so-called as "alcohols" in modern chemistry, after 1850.
The current Arabic name for alcohol is الكحول al-kuḥūl, re-introduced into that language from western usage.
Physical and chemical properties
Alcohols have an odor that is often described as “biting” and as “hanging” in the nasal passages. Ethanol has a slightly sweeter (or more fruit-like) odor than the other alcohols.
In general, the hydroxyl group makes the alcohol molecule polar. Those groups can form hydrogen bonds to one another and to other compounds (except in certain large molecules where the hydroxyl is protected by steric hindrance of adjacent groups[6]). 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 the tendency 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 -OH group. With a pKa of around 16-19, they are, in general, 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:
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.
As one moves from primary to secondary to tertiary alcohols with the same backbone, the hydrogen bond strength, the boiling point, and the acidity typically decrease.
Toxicity
Ethanol in alcoholic beverages has been consumed by humans since prehistoric times for a variety of hygienic, dietary, medicinal, religious, and recreational reasons. The consumption of large doses of ethanol causes drunkenness (intoxication), which may lead to a hangover as its effects wear off. Depending upon the dose and the regularity of its consumption, ethanol can cause acute respiratory failure or death. Because ethanol impairs judgment in humans, it can be a catalyst for reckless or irresponsible behavior. The LD50 of ethanol in rats is 10.3 g/kg.[9]
Ethanol's toxicity is largely caused by its primary metabolite; acetaldehyde[10] and secondary metabolite; acetic acid.[11][12][13][14] All primary alcohols are broken down into aldehydes then to carboxylic acids, and whose toxicities are similar to acetaldehyde and acetic acid.
Some secondary and tertiary alcohols are less poisonous than ethanol because the liver is unable to metabolise them into these toxic by-products. This makes them more suitable for recreational[15][16] and medicinal[17] use as the chronic harms are lower. Ethchlorvynol is a good example of a tertiary alcohol which saw both medicinal and recreational use.
Other alcohols are substantially more poisonous than ethanol, partly because they take much longer to be metabolized and partly because their metabolism produces substances that are even more toxic. Methanol (wood alcohol), for instance, is oxidized to formaldehyde and then to the poisonous formic acid in the liver by alcohol dehydrogenase and formaldehyde dehydrogenase enzymes, respectively; accumulation of formic acid can lead to blindness or death.[18] Likewise, poisoning due to other alcohols such as ethylene glycol or diethylene glycol are due to their metabolites, which are also produced by alcohol dehydrogenase.[19][20] An effective treatment to prevent toxicity after methanol or ethylene glycol 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.[18][21][22]
Methanol itself, while poisonous, has a much weaker sedative effect than ethanol. Some longer-chain alcohols such as n-propanol, isopropanol, n-butanol and t-butanol do, however, have stronger sedative effects, but also have higher toxicity than ethanol.[23][24] These longer chain alcohols are found as contaminants in some alcoholic beverages and are known as fusel alcohols,[25][26] and are reputed to cause severe hangovers although it is unclear if the fusel alcohols are actually responsible.[27] Many longer chain alcohols are used in industry as solvents and are occasionally abused by alcoholics,[28][29] leading to a range of adverse health effects.
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