In general usage, alcohol (from Arabic al-khwl الكحول, or al-ghawl الغول) refers almost always to ethanol, also known as grain alcohol, and often to any beverage that contains ethanol (see alcoholic beverage). This sense underlies the term alcoholism (addiction to alcohol). Other forms of alcohol are usually described with a clarifying adjective, as in isopropyl alcohol or by the suffix -ol, as in isopropanol.
In chemistry, alcohol is a more general term, applied to any organic compound in which a hydroxyl group (_OH) is bound to a carbon atom, which in turn is bound to other hydrogen and/or carbon atoms.
The functional group of an alcohol is a hydroxyl group bonded to an sp3 hybridized carbon.
The general formula is CnH2n+1OH.
Methanol and ethanol
The simplest and most commonly used alcohols are methanol and ethanol (common names methyl alcohol and ethyl alcohol, respectively), which have the following structures:
H H H | | | H-C-O-H H-C-C-O-H | | | H H H methanol ethanol
In common usage, "alcohol" often refers simply to ethanol or "grain alcohol".
Other common alcohols
- isopropyl alcohol (sec-propyl alcohol, propan-2-ol, 2-propanol) H3C-CH(OH)-CH3, or "rubbing alcohol"
- ethylene glycol (ethane-1,2-diol) HO-CH2-CH2-OH, which is the primary component in antifreeze
- glycerin (or glycerol, propane-1,2,3-triol) HO-CH2-CH(OH)-CH2-OH bound in natural fats and oils, which are triglycerides (triacylglycerols)
- phenol, an alcohol where the hydroxyl group is bound to a benzene ring
These are derived from natural fats and oils. Those with common names include:
- erucyl alcohol
- ricinolyl alcohol
- arachidyl alcohol
- capryl alcohol
- capric alcohol
- behenyl alcohol
- lauryl alcohol (1-dodecanol) -- 12 carbon atoms
- myristyl alcohol (1-tetradecanol) -- 14 carbon atoms
- cetyl (or palmityl) alcohol (1-hexadecanol) -- 16 carbon atoms
- stearyl alcohol (1-octadecanol) -- 18 carbon atoms
- isostearyl alcohol
- oleyl alcohol (cis-9-octadecen-1-ol) -- 18 carbon atoms, unsaturated
- palmitoleyl alcohol
- linoleyl alcohol (9Z, 12Z-octadecadien-1-ol) -- 18 carbon atoms, polyunsaturated
- elaidyl alcohol (9E-octadecen-1-ol)
- elaidolinoleyl alcohol (9E, 12E-octadecadien-1-ol)
- linolenyl alcohol (9Z, 12Z, 15Z-octadecatrien-1-ol)
- elaidolinolenyl alcohol (9E, 12E, 15-E-octadecatrien-1-ol)
The smaller molecules are used in cosmetics and food, and as industrial solvents. Some of the larger molecules are important as biofuels.
Alcohols are in wide use in industry and science as reagents, solvents, and fuels. State-of-the-art engineering has achieved replacement of gasoline (and other hydrocarbons which produce toxic fumes) with forms of alcohol such as ethanol or methanol (which burn more cleanly). Because of its low toxicity and ability to dissolve non-polar substances, ethanol is often used as a solvent in medical drugs, perfumes, and vegetable essences such as vanilla.
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, oil, or coal feedstocks.
Common names for alcohols usually take the name of the corresponding alkyl group and add the word "alcohol", e.g. methyl alcohol and ethyl alcohol. For more complex alcohols, the common name depends on whether the alcohol is primary, secondary or tertiary. Propyl alcohol may be named n-propyl alcohol or sec-propyl alcohol depending on whether the hydroxyl group is bonded to the 1st or 2nd carbon on the propane chain. Secondary propyl alcohol is also called isopropyl alcohol.
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 in the first case, propan-2-ol in the second. Sometimes, the position number is written before the IUPAC name: 1-propanol and 2-propanol. Another system uses the prefix "hydroxy" with the alkane's name: 1-hydroxypropane, 2-hydroxypropane.
Tertiary alcohols take tert before their common names: (CH3)3COH is tert-butyl alcohol, or 2-methylpropan-2-ol under IUPAC rules, indicating a propane chain with methyl and hydroxyl groups both attached to the middle (2) carbon.
An alcohol with two hydroxyl groups is commonly called a "glycol", e.g. HOCH2CH2OH is ethylene glycol. The IUPAC name is ethane-1,2-diol, "diol" indicating two hydroxyl groups, and 1,2 indicating their bonding positions. Vicinal glycols (with the two hydroxyls on the same carbon atom), such as ethane-1,1-diol, are generally unstable. For three or four groups, "triol" and "tetraol" are used.
The word "alcohol" almost certainly comes from the Arabic language (the "al-" prefix being the Arabic definite article); however, the precise origin is unclear. It 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 alchemists.
A popular theory, found in many dictionaries, is that it comes from الكحل = ALKHL = al-kuhul, 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 = ALKHWL = al???, does not derive from al-kuhul. The Quran in verse 37:47 uses the word الغول = ALGhWL = al-ghawl — properly meaning "spirit" ("spiritual being") or "demon" — with the sense "the thing that gives the wine its headiness". The word al_ghawl also originated 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. (Incidentally, the etymology "alcohol" = "the devil" was used in the 1930s by the U.S. Temperance Movement for propaganda purposes.)
According to the second theory, the popular etymology and the spelling "alcohol" would not be due to generalization of the meaning of ALKHL, but rather to Western alchemists and authors confusing the two words ALKHL and ALGhWL, which have indeed been transliterated in many different and overlapping ways. The fact that stibnite is also mentioned in the Hebrew Bible under the name כהל = kohel = "" can only have contributed to the confusion.
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. 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 predominates. Because of the high chemical bond strength in alcohols primary structure it has very high burning temperatues. Methanol has an exceedingly high burning Butanol is moderately soluble because of a balance between the two trends. Pentanol and branched butanols are effectively insoluble because of the hydrocarbon chain's dominance. Because of hydrogen bonding, alcohols tend to have higher boiling points than comparable hydrocarbons and ethers. All simple alcohols are miscible in organic solvents.
Alcohols are so_called "protic" solvents. They can lose the proton H+ of the hydroxyl group and are very weak acids: weaker than water (except for methanol), but still stronger than ammonia or acetylene.
One important class of reactions undergone by alcohols is nucleophilic substitution, where one nucleophilic group attached to a carbon atom is replaced by another. So, for instance, alcohols react with hydrochloric acid to produce alkyl halides, where the hydroxyl group is replaced by a chlorine atom. The equilibrium lies to the right, since chlorine is a stronger nucleophile, but can be driven to the left using an alkaline medium, which is one way of synthesizing alcohols.
Alcohols are themselves nucleophilic, so can react with one another to produce ethers and water. They also react with hydroxy acids (or acid halides) to produce compounds called esters, of which the esters of organic acids are the most important. At high temperatures, alcohols can undergo an elimination reaction to produce alkenes. The reverse of this, the addition of water to an alkene to produce an alcohol, is catalyzed by acids but is of limited use for synthesis because it generally results in mixtures. Some other techniques exist to convert alkenes to alcohols more reliably.
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. While infrequent consumption of ethanol in small quantities may be harmless or even beneficial, larger doses result in a state known as drunkenness or intoxication and, depending on the dose and regularity of use, can cause acute respiratory failure or death and with chronic use can cause severe health problems, such as liver and brain damage.
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. Interestingly, an effective treatment to prevent formaldehyde toxicity after methanol ingestion is to administer ethanol. This will bind to alcohol dehydrogenase, preventing methanol from binding and thus its acting as a substrate.
- Alcohol and the Distillation of Wine in Arabic Sources (http://www.gabarin.com/ayh/alcohol.htm) by Ahmad Y. al-Hassan.