Alcohol

In chemistry, an alcohol is an 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; in other words, alcohol is characterized by one or more hydroxyl (OH) groups attached to a carbon atom of an alkyl group (hydrocarbon chain).

In a non-chemistry context, alcohol refers almost always to ethanol, also known as grain alcohol, which gives alcoholic beverages their alcohol content.

Table of contents

1 General formula 2 Methanol and ethanol 3 Toxicity of alcohols 4 Notable alcohols

General formula

The general formula is C_nH_2n+1OH.

Methanol and ethanol

The simplest two alcohols are methanol and ethanol (also called 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.

Notable alcohols

By convention, the names of alcohols typically end in "-ol". Notable alcohols: isopropyl alcohol (2-propanol) H₃C-CH(OH)-CH₃, or "rubbing alcohol"; ethylene glycol HO-CH₂-CH₂-OH, which is the primary component in antifreeze; glycerin (or glycerol) HO-CH₂-CH(OH)-CH₂-OH bound in natural fats and oils, which are triglycerides (triacylglycerols); Phenol is an alcohol where the hydroxyl group is bound to a benzene ring. 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