Alcohol

Alcohol is commonly thought of as either rubbing alcohol, the active ingredient in an alcoholic drink, or the additive that makes gasoline into gasohol. Chemists generalize its meaning to include almost 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 the first alcoholic beverages, beer and wine, were developed millennia ago by various Middle Eastern and Far Eastern cultures. The word alcohol is of Arabic derivation. Ancient Egyptian papyrus scrolls exist with directions for making beer from dates and other plant foods. The alcohol in these drinks was ethanol (ethyl alcohol). 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 the ethanol produced was used for drinking, consumed as is perhaps after filtering or allowing the sediment to settle.

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 and undergo fermentation, 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. Fermentation has been developed to an advanced technology and after distillation provides much of the fuel ethanol for addition to gasoline.

The process of distillation was discovered sometime after the first century a.d. Purer ethanol distilled from crude fermentation mixtures was then available for consumption, medicinal, and chemical uses. Eventually people learned to make methanol (wood alcohol) by destructively distilling wood but ethanol and methanol were the only alcohols available before the modern era.

The discovery and understanding of the alcohols as a group of chemical compounds has only happened in the last century. In 1926 the first industrial process for generating methanol was developed and since then many different alcohols have been made by direct chemical synthesis (the making of chemical compounds from simpler ones). The industrial processes that generate and consume alcohols change to keep up with modern technology and discoveries, but the alcohols continue to occupy a central place in the science of chemical synthesis.

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, although many older names are still used. 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 contains mostly 2-propanol (also called isopropanol or isopropyl alcohol).

As the number of carbon atoms increases, the alcohol's solubility in water decreases. But as the number of hydroxyl groups grows, the solubility of the alcohol in water 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; ethanol has been frequently used as fuel. 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 solvent for paints and 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, so about 50% of the amount produced goes into foods and cosmetics.

Almost any organic chemical can be made by starting with the smaller alcohols, so both methanol and ethanol are in the top 50 industrial chemicals in the United States. Methanol occupies a particularly important place in industrial organic synthesis: it is used 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 unleaded 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," coal yields 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 very large number of different 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. This is when two chemicals 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 is the part of an alcohol that makes the molecule relatively reactive and thus very useful in synthesis. Dozens of reactions are possible. Important esters made from ethanol include the insecticide malathion, the fragrance compound ethyl cinnamate, and the polymer building blocks ethyl acrylate and ethyl methacrylate. Examples of 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 as plasticizers in the polymer industry, 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 alcohol's 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 attached in the middle of a straight carbon atom chain, an oxidation reaction 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.

The formation of double bonds in hydrocarbons can be accomplished by the dehydration of alcohols. Acid added to the alcohol removes not only the hydroxyl group, but also a hydrogen atom from an adjacent carbon atom. The reaction is called a dehydration, because H-OH (water) is removed from the molecule and a double bond forms between the two carbon atoms.

Resources

books

Bailey, James E. Ullmann's Encyclopedia of Industrial Chemistry. New York: VCH, 2003.

Bruice. Paula. Organic Chemistry. 3rd ed. Englewood Cliffs, NJ: Prentice-Hall, 2001.

Meyers, Robert A. Encyclopedia of Analytical Chemistry: Applications, Theory and Instrumentation. New York: John Wiley & Sons, 2000.

Szmant, H. H. Organic Building Blocks of the Chemical Industry. New York: Wiley, 1989.

Gail B. C. Marsella

KEY TERMS

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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 mixture of certain 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 is heated to a high temperature in the absence of air, and 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, resulting in the production of ethanol and carbon dioxide.

Glycol

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

Grignard synthesis

—A classic laboratory method of preparing alcohols. An alkyl halide is first reacted with magnesium, and then the addition of an aldehyde or ketone results in the formation of the alcohol.

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.

Intermediate

—In a chemical synthesis, any compound that is generated only to be used in the next step of the process.

Polyol

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

Primary alcohol

—An alcohol with the hydroxyl group at one end of the chain of carbon atoms.

Secondary alcohol

—An alcohol with the hydroxyl group in the middle of a straight chain of carbon atoms.

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.

Tertiary alcohol

—An alcohol with the hydroxyl group in the middle of a branched chain of carbon atoms.