Alcohol
From Encyclopædia
The familiar grain alcohol (ethanol), contained in alcoholic beverages, wood alcohol (methanol), and rubbing alcohol (isopropanol) are among the hundreds of organic compounds that form the class known as alcohols. Alcohols are characterized by a
hydroxyl group (OH) bonded to a
carbon atom. They may contain more than one
hydroxyl group per
molecule provided each group of hydrogen and
oxygen atoms is attached to a different
carbon atom. Such alcohols are called polyhydric alcohols, or polyols.Alcohols that are derived from a simple hydrocarbon are named by adding either ol or yl plus the word alcohol to the name of the parent hydrocarbon. For example, the
monohydric alcohol derived from ethane with one
hydroxyl group may be called either ethanol or ETHYL ALCOHOL. Dihydric alcohols--those with two
hydroxyl groups per
molecule--may be named as GLYCOLS, as ethylene glycol, which is the major component of automotive
antifreeze solutions. More complex alcohols, such as
cholesterol, are not named so systematically.If the alcohol's chain has three
carbon atoms, two ISOMERS (different structures) are possible; the
number of isomers increases markedly with the
number of
carbon atoms. Alcohols may be classified according to the
number of hydrogen
atoms attached to the
carbon with the
hydroxyl group. If that
carbon has two hydrogen
atoms attached, the alcohol is primary; if it has one, it is secondary; and if it has no hydrogen
atoms, it is a tertiary alcohol.Simple, low-molecular-weight alcohols are colorless, volatile, flammable liquids that are soluble in water. As the
molecular weight increases, the boiling point, melting point, and viscosity increase, while solubility in water decreases. Physical properties may be altered by the presence of other functional groups. Adding
hydroxyl groups increases the boiling point and solubility in water and often produces sweetness. Branching the
carbon chain increases solubility in water and decreases the boiling point.The important reactions of alcohols are chiefly those of the
hydroxyl group. The hydrogen
atom of the OH can be replaced by an active
metal; dehydration causes the production of unsaturated compounds or ethers; reaction with carboxylic acids forms esters, and the OH may be replaced by other functional groups. The product of oxidation depends on the class of the alcohol. Primary alcohols oxidize to aldehydes, and secondary alcohols oxidize to ketones. Tertiary alcohols do not oxidize readily, and they give products containing fewer
carbon atoms than the original compound. It is difficult to reduce alcohols to hydrocarbons.When ingested in moderate amounts, lower-molecular-weight alcohols act on the
Central nervous system and higher centers of the brain, lowering inhibitions and affecting judgment. Larger amounts may produce a loss of muscular
Control, unconsciousness, and even death. METHYL ALCOHOL (methanol) is extremely toxic, affecting the optic nerve and causing
blindness, and as little as 30 ml (1 oz) has caused death. Ethyl alcohol (ethanol) is the least toxic. Thereafter toxicity increases with
molecular weight until the compound is insoluble in water. Most of the lower-molecular-weight alcohols have commercial importance. They are used as
solvents in the preparation of dyes, pharmaceuticals,
antifreeze, esters, and other compounds; and mixed with gasoline and used as motor fuels (see
GASOHOL).Elbert H. HadleyBibliography: Monick, John A., Alcohols: Their Chemistry, Properties and Manufacture (1968); Morrison, Robert T., and Boyd, Robert N.,
organic chemistry, 5th ed. (1987); Paul, J. K., ed., Ethyl Alcohol Production and Use as a Motor Fuel (1980); Rodd, E. H., ed., Chemistry of
carbon Compounds, vols. 1A-2D, 2d ed. (1965-70); Union Carbide, Alcohols (1969); Wickson, Edward J., ed.,
monohydric alcohols: Manufacture, Applications, and Chemistry (1981).
