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 three other
hydrogen 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).

The general formula is CnH2n+1OH.

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. See ethanol
for further discussion of this type of alcohol.

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 enzymes in the body.

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; 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.