Club Chemistry: Alcohol, More Than Just a Drink

The Importance in Chemistry of Alcohols and Related Compounds

Ethyl alcohol is one of the few drugs that can be made, bought and sold without fear of criminal prosecution, as long as certain clearly defined rules are adhered to.

Whether it is made as wine or beer, its method of production is essentially the same. A carbohydrate, such as starch or sugar, is broken down into glucose, which is then mixed with yeast and allowed to ferment for weeks or even months. Catalysts within the yeast convert the glucose into alcohol and carbon dioxide, releasing the energy that the yeast needs in order to survive and grow,

Yeast can normally tolerate a concentration of up to about 12% of what is, in effect, a waste product of its energy generation. This puts an upper limit on the alcohol content of wine or beer. To increase this, the fermented liquor needs to be distilled. As the boiling point of alcohol is 78 degrees Celsius, compared with 100 °C for water, the liquid that distils contains a higher proportion of alcohol than the original wine or beer. This is how spirits such as whiskey, brandy, vodka etc. are made.

Proof as a Measure of Alcoholic Strength

The alcohol content of a drink is usually given as a percentage, but sometimes in terms of an indefinite measure known as ‘proof’. This concept goes back to a time when sailors, suspicious of innkeepers who might have watered down their rum, discovered that a small quantity of gunpowder soaked in the drink would ignite only if the alcohol content were high enough. Ignition was accepted as proof that the drink had not been diluted. 100 degrees proof means approximately 40% alcohol. Drinks containing less than this would not allow gunpowder to ignite.

Molecular Structure of Alcohols

In chemistry, the word ‘alcohol’ has a much broader meaning than that in relation to alcoholic drinks. It refers to a very large number of distinct compounds, only one of which is present in drinks. In their molecular structure, the alcohols bear a similarity to water. Indeed, water could be thought of as the simplest of the alcohols.

A water molecule consists of two hydrogen atoms joined to one atom of oxygen. In the alcohols, the place of one of these hydrogens is taken by a collection of carbon and hydrogen atoms known as a hydrocarbon group. The difference between one alcohol and another lies in the number of carbon atoms in this group.

A molecule of the drinking alcohol contains two carbon atoms, and is specifically named ethanol, or ethyl alcohol. Methanol (methyl alcohol), which contains only one carbon atom, is much more toxic than ethanol, and must not be drunk. It is obtained by heating wood chippings and is often referred to as wood alcohol. Even small quantities can cause blindness. Together with colouring agents, it is added to pure ethanol to make the latter undrinkable, the mixture formed being known as methylated spirits.

Uses of Some Alcohols

Alcohols are widely used as solvents, being able to dissolve many substances that cannot be dissolved in water. Ethanediol (ethylene glycol), an alcohol, is used as anti-freeze, to prevent water from freezing in car radiators. The sweet tasting glycerol (glycerine) is an alcohol, made as a by-product of the manufacture of soap from fats or oils. Ethanol has a boiling point similar to that of petrol. It burns very well with a clean flame, and can therefore be used as a fuel.

Oxidation of Alcohols

If the stopper is left off a bottle of wine or spirits, for a few days, the drink turns sour. This is caused by oxygen in the air, which oxidises the ethanol to ethanoic, or acetic acid, more commonly known as vinegar.

Oxidation of alcohols to organic acids like vinegar is a two-stage process, the intermediate compound being an aldehyde. Methanol, for example, is oxidised to methanal (formaldehyde), which is used to preserve dead biological specimens and organs for use in scientific research. The oxidation of methanal produces methanoic (formic) acid, which is responsible for the stings of ants and nettles.

Esters and the Flavours of Fruits and Wines

Alcohols react with organic acids to form substances called esters, which are found in all living organisms. Animal fats and vegetable oils are examples of such esters. Many esters are sweet-smelling chemicals, widely distributed among fruits. It is these that give a fruit its characteristic smell and flavour. Many esters are manufactured for use as food flavourings, while others are used as solvents. Ethyl ethanoate (acetate), for example, made from ethanol and ethanoic acid, is a common solvent for paints, glues and nail varnish.

In the many different wines, while the main constituent remains the ethanol, it is the mixture of aldehydes and esters, carried through from the original fruits, that give each individual wine, even the most exotic, its own unique taste and aroma.

The copyright of the article Alcohol, More Than Just a Drink in Chemistry is owned by Anthony Toole. Permission to republish Alcohol, More Than Just a Drink in print or online must be granted by the author in writing.

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