Alcohols - Chemistry

Alcohols are among the most common and widely used chemicals. Methanol (methyl alcohol), tne simplest, has long been used as a solvent and a fuel. Another well-known alcohol is ethanol, or ethyl alcohol—the intoxicant in alcoholic beverages for thousands of years. Ethanol is also important as a solvent and a reaction medium, acting on other substances to form new substances.

Alcohol (ethanol) has long been made by fermentation. During this process, enzymes from yeast cells (shown highly magnified, right) bring about the breakdown of starch or sugar into alcohol and carbon dioxide. The process is still the basis of the brewing industry for beer and lager and for the production of spirits such as whiskey.

Structure

All alcohols consist of chemically bonded atoms of carbon, hydrogen, and oxygen. All alcohol molecules possess at least one hydroxyl group in their molecular makeup. (A hydroxyl group is an arrangement of atoms in which a hydrogen atom is bonded to an oxygen atom.) All simple alcohols are liquids. Besides their physical uses as solvents and reaction media, alcohols are of great value to the synthetic chemist because they are versatile in chemical reactions and can be converted into almost every other kind of aliphatic compound. They form a whole host of important chemical intermediates and can be converted easily into alkyl halides, alkoxides, ethers, esters, and alkenes. Also, alcohols can be oxidized (combined with oxygen) to form aldehydes, ketones, and carboxylic acids.

The boiling points of primary alcohols (red line) are higher than the boiling points of their isomeric counterparts. The boiling points of secondary alcohols lie along the purple line. The boiling points of tertiary alcohols lie along the orange line. These secondary and tertiary alcohols have the same molecular weight and lie on the same vertical line on the diagram as their primary alcohols.

Methanol and ethanol

Methanol, the simplest of all alcohols, boils at 148.5° F. (64.7° C). Known as wood alcohol, it was formerly obtained by heating wood to about 900° F. (about 500° C) in the absence of air and distilling off (purifying) the liquid formed. Today, the chief source of methanol is synthesis gas, a mixture of carbon monoxide and hydrogen. Nearly half the methanol manufactured is converted into methanal (formaldehyde), a starting material for phenolic resins. These synthetic resins harden and become permanently shaped under the continual application of heat. Other derivatives include ethanoic acid (acetic acid) and chloromethane (methyl chloride).

Two key reactions of alcohols are esterification and saponification. Esterification (A) is the reaction between an alcohol and a carboxylic acid to form an ester and water. It is analogous to the neutralization of an acid by a base. Saponification (B) can be considered the reverse reaction. The action of a base on an ester followed by the addition of excess acid regenerates the original alcohol and acid.

With a boiling point of 172° F. (78.3° C), ethanol is a colorless liquid at room temperature. Its preparation by fermentation is an ancient process used for producing alcoholic beverages. Before the modern petrochemical industry, ethanol was prepared by fermenting plant-derived carbohydrates such as starch, sugar, or cellulose. Today, most industrial ethanol is prepared synthetically. About 45 per cent of industrial ethanol is used in the production of ethanal (acetaldehyde); as a source of acetic acid; as a solid fuel (canned heat); and in the synthesis of many other compounds. About 30 per cent of the ethanol produced annually is used as a solvent, and the remainder in various chemical processes.