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Alcohol.
Alcohol
In general usage, alcohol refers almost always to ethanol, also known as grain alcohol; and sometimes to any alcoholic (alcohol-containing) beverage. This sense underlies the term alcoholism (addiction to alcohol).
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.
Table of contents showTocToggle("show","hide")
1 Structure
1.1 General formula
2 Nomenclature
3 Physical properties
4 pH
5 Methanol and ethanol
6 Toxicity of alcohols
7 Common alcohols
Structure
The functional group of an alcohol is a hydroxyl group bonded to an sp3 hybridized carbon. There are three major subsets of alcohols - 'primary', 'secondary' and 'tertiary', which is dependent upon the number of carbons the C-OH carbon is bonded to. Ethanol and methanol, shown below, are both 'primary' alcohols. The simplest secondary alcohol is propan-2-ol, and a simple tertiary alcohol is 2-methylpropan-2-ol.
General formula
The general formula is CnH2n+1OH.
Nomenclature
There are two naming systems used for alcohols: Common or trivial nomenclature, and IUPAC nomenclature. Common names usually take the name of the alkyl group and add the word "alcohol", e.g. methyl alcohol and ethyl alcohol. Under the IUPAC naming scheme, the name of the alkane chain looses the terminal "e" and adds "ol", e.g. methanol and ethanol.
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.
The IUPAC names for these place the position of the hydroxyl group between the alkane name and the "ol": propan-1-ol in the first case, propan-2-ol in the second. Sometimes, the position number proceeds the IUPAC name: 1-propanol and 2-propanol. "Hydroxy" is also sometimes used as a prefix instead: 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.
A glycol is the common name of an alcohol with two attached hydroxyl groups, 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. For three or four groups, "triol" and "tetraol" are used.
Physical properties
The hydroxyl group makes alcohols polar.
pH
Alcohols are very weakly acidic.
Methanol and ethanol
The simplest two 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", which may be produced by fermentation of fruits or grains with yeast and is one of the oldest and most widely used recreational drugs in the world, typically taken in the form of an alcoholic beverage. Ingestion in sufficient quantity results in a state known as drunkenness or intoxication. See ethanol for further discussion of this type of alcohol.
Toxicity of alcohols
Alcohols often have an odor described as 'biting' that 'hangs' in the nasal passages. All alcohols are poisonous when ingested, including ethanol at high doses. Part of the reason it is less poisonous than other alcohols is that it breaks down more readily than other alcohols. Methanol, or "wood alcohol", for instance, can cause blindness or death. It is oxidized to the poisonous formaldehyde by alcohol dehydrogenase enzymes in the liver. 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.
Common alcohols
Many alcohols can be created in uncontrolled fermentation processes.
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).
The hydroxyl groups in alcohols are capable of forming 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 highly miscible in water because the hydroxyl group predominates. 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.
See also : transesterification
The above article is adapted from from Wikipedia All Wikipedia article text is available under the terms of the GNU Free Documentation License
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