Alcohol

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Alcohol

Alcohol is any carbon-hydrogen compound with at least one hydroxyl group (symbolized as–OH) in its molecular structure. Categorized by the number and placement of the–OH groups, and the size and shape of the attached carbon molecule, alcohols are fundamental to organic chemical synthesis. The chemical industry produces and uses many different kinds of alcohols.

History

Techniques for producing beer and wine, the first alcoholic beverages, were developed millennia ago by various Middle Eastern and Far Eastern cultures. Even the word alcohol is of Arabic derivation. Ancient Egyptian papyrus scrolls gave directions for making beer from dates and other plant foods. These beverages contained ethyl alcohol, since pure alcohol could not be made at that time; it was always mixed with the water, flavorings, and plant residue from the original fermentation. Almost all ethanol produced was used for drinking.

Fermentation, the oldest kind of alcohol production and possibly the oldest chemical technology in the world, is the action of yeast on sugars in solution. When fruits, vegetables, honey, molasses, or grains such as barley are mashed up in water, the yeast’s metabolism of the sugars produces ethanol as a byproduct. Wood and starches can also be fermented, although their complex molecules must be broken down somewhat first. They always give small amounts of other larger alcohols (collectively called fusel oil) in addition to ethanol.

The process of distillation was discovered sometime after the first century AD. Purer ethanol distilled from crude fermentation mixtures was then available for consumption, medicinal, and chemical uses. In 1923 the first industrial process for generating methanol was developed; since then many different alcohols have been made by direct chemical synthesis (the production of complex chemical compounds from simpler ones).

Names, properties, and uses

The formal name of an alcohol tells the number of hydroxyl (–OH) groups and the number of carbon atoms in the molecule, the names of any other atoms, and the attachment of the atoms in the molecule. Most simple alcohols end with the —ol suffix, added to the name of the molecule that had the same number of

carbon atoms, but with a hydrogen atom in place of the hydroxyl group. Methanol, for example, has one carbon atom (like methane) and one hydroxyl group. Ethanol has two carbon atoms (like ethane) and one hydroxyl group. Two isomers of propanol exist (1-propanol and 2-propanol) because the hydroxyl group can be attached either at the end or in the middle of the three-carbon atom chain. Rubbing alcohol (also called isopropanol or isopropyl alcohol) contains mostly 2-propanol.

As the number of carbon atoms increases, the alcohol’s solubility in water decreases. But as the number of hydroxyl groups grows, the solubility increases, as does its boiling point. Alcohols with two hydroxyl groups on adjacent carbon atoms are called glycols or diols. Ethylene glycol’s properties of high water solubility, high boiling point, and low freezing point make it a good antifreeze for cars. These characteristics are due to the hydrogen bonding making the glycol associate firmly with water. Alcohols with more than three hydroxyl groups are called polyols. The sweetener sorbitol, often found in packaged baked goods and sugar-free gum, is a polyol with six carbon atoms and six hydroxyl groups in its molecules.

Most alcohols burn in air, a quality that makes ethanol a fuel additive. Methanol can sometimes be used as a fuel, though it evaporates too quickly for regular use in cars. Isopropyl alcohol is widely used in industry as a paint solvent and in chemical processes. In addition to its presence in alcoholic beverages, ethanol also is used as a solvent for food extracts such as vanilla, perfumes, and some types of paints and lacquers. Some alcohols can be ingested, although methanol is extremely toxic, and even ethanol is poisonous in large quantities. Glycerol, an alcohol with three carbon atoms and three hydroxyl groups in its molecules, has very low toxicity. It also possesses good moisturizing properties; about 50% of the amount produced goes into foods and cosmetics.

Almost any organic chemical can form with the smaller alcohols, so both methanol and ethanol are in the top 50 industrial chemicals in the United States. Methanol is particularly important in the synthesis of organic chemicals such as formaldehyde (used to make various polymers), methyl-tert-butyl ether (the octane enhancer that replaced lead in gasoline), acetic acid, and synthetic gasoline, though this use has not yet become common.

Production

Alcohols are produced industrially from petroleum, coal, or other natural products. The “cracking” of crude petroleum yields many lower-molecule-weight chemical compounds, including some starting materials for alcohols such as ethylene and propylene. Ethylene reacts with hot steam over a catalyst to yield ethanol directly. A process known as hydration produces isopropyl alcohol when water is chemically added to propylene.

When “gasified,” (heated to a high temperature with very little oxygen) coal and other carbon-based compounds yield a mixture of hydrogen gas and carbon monoxide called synthesis gas, an important starting material for a variety of low- or high-molecular-weight alcohols. Methanol is now made almost entirely from synthesis gas. Before direct chemical synthesis was available, methanol was produced by the destructive distillation of wood (hence its older name, wood alcohol). Wood heated to a high temperature without air does not burn in the regular sense but decomposes into a large number of different

KEY TERMS

Alcohol— Any of the large number of molecules containing a hydroxyl (–OH) group bonded to a carbon atom to which only other carbon atoms or hydrogen atoms are bonded.

Azeotrope— A liquid mixture of substances that distill together at the same boiling temperature, instead of separately.

Destructive distillation— An antiquated process for obtaining small amounts of alcohols, particularly methanol, from wood. The wood, heated to a high temperature in the absence of air, gradually decomposes into a large number of chemicals.

Distillation— Collecting and condensing the vapor from a boiling solution. Each distinct volatile chemical compound boils off individually at a specific temperature, so distillation is a way of purifying the volatile compounds in a mixture.

Ester— A molecule with a carbon both bonded to an ether linkage (carbon-oxygen-carbon), and double bonded to an oxygen.

Fermentation— The action of yeast metabolism on sugar solutions, which produces ethanol and carbon dioxide.

Glycol— An alcohol with two hydroxyl groups bonded to adjacent carbons in the molecule. Also called a diol.

Hydroxyl group— The–OH group attached to a carbon atom in a molecule. If the carbon atom itself is attached to only other carbon atoms or hydrogen atoms, the molecule is an alcohol.

Polyol— An alcohol with many hydroxyl groups bonded to the carbon atom backbone of the molecule.

Synthesis gas— A mixture of carbon monoxide and hydrogen gases, obtainable both from coal and natural gas, and used widely for the synthesis of alcohols and other organic compounds in the chemical industry.

chemicals, of which methanol and some other alcohols are a small fraction.

Distilling the ethanol from fermentation products gives a mixture of 95% ethanol and 5% water, resulting in an azeotrope: a liquid composed of two chemicals that distill together instead of separating at their different boiling temperatures. Most industrial ethanol is 95% alcohol unless there is specific need for very dry ethanol.

Reactions

The plastics industry is a major consumer of all types of alcohols, because they are intermediates in a large variety of polymer syntheses. The hydroxyl group makes the alcohol molecule relatively reactive and thus useful in synthesis. Important esters made from ethanol include the insecticide malathion, the fragrance compound ethyl cinnamate, and the polymer building blocks ethyl acrylate and ethyl methacry-late. Esters made from methanol include methyl salicylate (oil of wintergreen), the perfume ingredients methyl paraben and methyl benzoate, and the polymer starting material methyl acrylate. High-molecule-weight alcohols converted into esters are widely used in the polymer industry as plasticizers, and very high-molecule-weight alcohols with 12-18 carbon atoms are used to make biodegradable surfactants (detergents).

Alcohols can also be oxidized. If the hydroxyl group is at the end of a carbon atom chain, an oxidation reaction produces either a carboxylic acid or an aldehyde. If the hydroxyl group is in the middle of a straight carbon atom chain, it produces a ketone. An alcohol whose hydroxyl group is attached to a carbon atom that also has three other carbon branches attached to it cannot be oxidized.

Alcohols can also be dehydrated to form double bonds in hydrocarbons. Adding acid to the alcohol removes not only the hydroxyl group, but a hydrogen atom from an adjacent carbon atom as well. The reaction is called a dehydration, because H–O–H (water) is removed from the molecule and a double bond forms between the two carbon atoms.