Alcohol

I

Introduction

Alcohol, term applied to members of a group of chemical compounds that contain the OH group. Also commonly used to refer to the specific compound ethyl alcohol, or ethanol. The word is derived from the Arabic al-kuhl, or kohl, a fine powder of antimony used as an eye makeup. The word alcohol originally denoted any fine powder, but the alchemists of medieval Europe later applied it to essences obtained by distillation, and this led to the current usage.

Alcohols have one, two, or three hydroxyl, -OH, groups attached to their molecules and are thus classified as monohydric, dihydric, or trihydric, respectively. Methanol and ethanol are monohydric alcohols. Alcohols are further classified as primary, secondary, or tertiary, according to whether one, two, or three other carbon atoms are bound to the carbon atom to which the hydroxyl group is bound. Alcohols are characterized by many common reactions, the most important of which is the reaction with acids to form substances called esters, which are analogous to inorganic salts. Alcohols are normal by-products of digestion and chemical processes within cells, and are found in the tissues and fluids of animals and plants.

II

Wood Alcohol

Methyl alcohol, or methanol, CH₃ OH, is the simplest of all the alcohols. It was formerly made by the destructive distillation of wood, however, almost all of the methanol produced today is synthetic, made from hydrogen and carbon monoxide. Methanol is used as a denaturant for grain alcohol, as an antifreeze, as a solvent for gums and lacquers, and in the synthesis of many organic compounds, particularly methanal (formaldehyde). When taken internally, by either drinking the liquid or inhaling the vapours, methanol is extremely poisonous. Methanol melts at -97.8° C -144.0° F), boils at 64.7° C (148.5° F), and has a relative density of 0.7915 at 20° C (68° F).

III

Grain Alcohol

Ethyl alcohol, or ethanol, C₂ H₅OH, is a clear, colourless liquid, with a burning taste and characteristic, agreeable odour. Ethanol is the alcohol in such beverages as beer, wine, and brandy. Because of its low freezing point, it has been used as the fluid in thermometers for temperatures below the freezing point of mercury -39° C (-38° F), and for other special low-temperature purposes, such as for antifreeze in car radiators.

Ethanol is normally concentrated by distillation of dilute solutions. Commercial ethanol contains 95 per cent by volume of ethanol and 5 per cent of water. Dehydrating agents remove the remaining water and produce absolute ethanol. Ethanol melts at -114.1° C (-173.4° F), boils at 78.5° C (173.3° F), and has a relative density of 0.789 at 20° C (68° F).

Ethanol has been made since ancient times by the fermentation of sugars. All beverage ethanol and more than half of industrial ethanol is still made by this process. Starch from potatoes, maize, or other cereals can be the raw material. The yeast enzyme, zymase, changes the simple sugars into ethanol and carbon dioxide. The fermentation reaction, represented by the simple equation

C6H12O6 → 2C2 H5OH + 2CO2

is actually very complex because impure cultures of yeast produce varying amounts of other substances, including fusel oil, glycerine, and various organic acids. The fermented liquid, containing from 7 to 12 per cent ethanol, is concentrated to 95 per cent by a series of distillations. In the production of beverages such as whisky and brandy, some of the impurities, which supply the flavour, are of great value. Most ethanol not intended for drinking is now made synthetically, either from ethanal (acetaldehyde) made from ethyne (acetylene), or from ethene made from petroleum. A small amount is made from wood pulp.

Ethanol can be oxidized to form first ethanal and then ethanoic (acetic) acid. It can be dehydrated to form diethyl ether. Other products made from ethanol include butadiene, used in making synthetic rubber; cloroethane, used as a local anaesthetic; and many other organic chemicals. Ethanol is miscible (mixable) with water and with most organic solvents. It is an excellent solvent for many substances and is used in making such products as perfumes, lacquer, celluloid, and explosives. Alcoholic solutions of non-volatile substances are called tinctures; if the solute is volatile, the solution is called a spirit.

IV

Higher Alcohols

Higher alcohols, those of greater molecular weight than ethanol, have many specific and general uses. Propanol is used extensively as a rubbing alcohol, butanol is a base for perfumes and fixatives, and others are important flavouring agents and perfumes.