Alcohols (chemistry)

Type of physical science: Chemistry

Field of study: Chemical compounds

Alcohols are derivatives of water with wide applications in the living world. They undergo oxidation, elimination, and substitution reactions and are weakly acidic.

Overview

Alcohols (ROH) can be considered derivatives of water in which one of the hydrogens is replaced by an alkyl group, R. Alternatively, they can be conceived as hydroxyl derivatives of hydrocarbons produced by the replacement of one or more hydrogens by one or more hydroxyl (-OH) groups. Alcohols may be classified as primary (general formula R-CH₂-OH), secondary (R₂COH), or tertiary (R₃COH), depending on the number of hydrogen atoms attached to the carbon atom bearing the hydroxyl group. They are also classified as mono-, di-, tri-, or polyhydric, depending upon the number of hydroxyl groups they possess. Finally, the nature of alkyl groups can classify the alcohol as aliphatic, cyclic, heterocyclic, or unsaturated.

There are generally three ways of naming alcohols. In the first one, which follows the International Union of Pure and Applied Chemistry (IUPAC) system, the alcohol is named as a derivative of the longest carbon chain containing the hydroxyl group. The chain is then numbered from the end that gives the carbon containing the K₁OH group the lowest possible number. The -e ending of the alkane is replaced by -ol, and the substituent groups are named and numbered in the usual manner. When other functional groups, such as a carbon-carbon double bond (C=C) or triple bond (C≡C), are part of the carbon chain, the alcohol naming takes precedence. Thus, CH₂=C(CH₃)CH₂CH₂OH is called 3-methyl-3-butanol (and not 2-methyl-1-butanol). Alcohols that carry two, three, and four ;K₁₀H groups, are named diols, triols, and tetrols.

The second way of naming alcohols involves the use of the "common naming" system, which is generally used for certain simple organic compounds. Many alcohols are still referred to this way, which names the alkyl residue followed by the name "alcohol." Thus, (CH₃)₂CHCH₂CH₂OH is commonly named as isooamyl alcohol (and 3-methyl-1-butanol, according to the IUPAC).

Finally, the least popular way is used for highly substituted alcohols. The basic unit is named carbinol, and alcohols are named as substituted carbinols. Thus, C₂H₅-CH(CH₃)OH is named ethylmethylcarbinol -butanol according to the IUPAC, and sec-butanol via the common naming system).

Compounds in which the -OH group is directly attached to the benzene ring are called phenols, and their derivatives are named as phenol derivatives.

Since alcohols have the polar -OH bond, they are highly volatile liquids as a result of intermolecular hydrogen bonding, which is affected by the size and nonpolarity of the organic radical. Thus, alcohols containing up to three carbons are totally miscible with water, C₄ and C₅ alcohols are partially soluble, and higher-carbon alcohols are insoluble. As a result of hydrogen bonding, the melting and boiling points of alcohols are higher than those of ethers that have the same molecular formula, since ethers cannot undergo intermolecular hydrogen bonding.

Methanol (CH₃OH), the simplest alcohol, was once produced commercially by the destructive distillation (heating in the absence of air) of wood: hence its old name, "wood alcohol." Modern industrial processes involve the catalytic reaction of carbon monoxide and hydrogen gas (also known as synthesis gas, or syngas) under high temperature and pressure conditions, a method first used in 1925. Ethanol (C₂H₅OH) is naturally prepared by fermentation of sugars and starches. Industrially, it is produced by the acid-catalyzed hydration of ethylene. Similarly, 2-propanol, also known as rubbing alcohol, is produced by the acid-catalyzed hydration of propene. A modified version of the same reaction using aluminum alkyls as the catalyst and ethylene from the cracking of petroleum is known as the Ziegler process and is industrially important for the synthesis of high-molecular alcohols. In the oxo process, carbon monoxide and hydrogen are reacted with an alkene (using cobalt as the catalyst) under high temperature and pressure conditions to produce monohydric aliphatic alcohols.

A popular laboratory method of producing alcohols from the corresponding carbonyl compounds involves the use of reducing agents. These compounds supply two hydrogen atoms and vary in their nature and selectivity. Typical reagents are sodium borohydride (NaBH₄), lithium aluminum hydride (LiAlH₄), and hydrogen gas, with platinum or palladium as the catalyst. Formaldehyde yields methanol, any other aldehyde produces a primary alcohol, and any ketone produces a tertiary alcohol.

Phenols are prepared from the corresponding aromatic halides by treatment with sodium hydroxide under high temperature and pressure conditions.

The reactions of alcohols can be generally classified into two main classes: reactions in which the O-H is broken, and reactions in which the C-O bond is cleaved.

Because of their high acidity, phenols, unlike alcohols, will form salts with aqueous alkalies. The free phenol can be regenerated by treatment of the salt with acids such as hydrochloric acid. Although aliphatic alcohols do not form salts with aqueous alkalies, both alcohols and phenols will form salts with active metals such as sodium, potassium, and magnesium, among others. These reactions are similar to the reaction of water with active metals to give an alkali metal hydroxide and hydrogen. It is worth noting that the salts of alcohols (called alkoxides) are strong bases when used in a nonaqueous solvent. Just as the hydroxyl anion (OH-) is a strong inorganic base, alkoxide ions (RO-) are strong organic bases. In aqueous solution, the alkoxides are hydrolyzed back to the alcohol.

In another reaction involving the O-H bond, alcohols will react with carboxylic acid in an acid medium to form an ester, as shown: RCOOH (carboxylic acid) + R'OH (alcohol) → RCOOR' (ester) + H₂O (water).

Inorganic acids also form esters with alcohols. For example, glycerol, known also as propane-1,2,3 triol, is produced by the esterification of glycerol with nitric acid, to produce glyceryl nitrate (nitroglycerin). Similarly, phosphate esters, which are of extreme importance in biochemistry, can also be produced by the esterification of alcohols with phosphoric acid.

In a reaction involving the C-O bond, the hydroxyl group of alcohols can be replaced with a halogen atom by several types of reagents. With hydrogen chloride, the reaction is generally carried out using zinc chloride as a catalyst, and the reactivity, generally, follows the trend of tertiary to secondary to primary. In this case, the reaction is better known as the Lucas reaction, and it is often used in the qualitative analysis of alcohols. For synthetic purposes, the reaction with phosphorus trichloride finds a greater use, especially with alcohols in which rearrangement is not possible. Phenols do not undergo this reaction, but treatment with bromine leads to bromophenols, compounds that are ring-brominated.

The above reaction produces alkenes upon modification of experimental conditions such as solvents and heat. The method is known as dehydration, is acid-catalyzed, and is used extensively in the preparation of olefins, which are quickly removed by distillation from the reaction pot as soon as they are formed.

The most significant reaction of the alcohols, however, is the reaction with a mild oxidizing agent, such as potassium or sodium dichromate in dimethyl sulfoxide, or chromium(VI) oxide in pyridine. This is the reverse of the reaction that involves a reducing agent. Primary alcohols yield aldehydes, while secondary alcohols produce ketones. During the reaction, the orange color of dichromate changes to the green of chromium(III). Under normal conditions, tertiary alcohols do not react, and the orange color persists. This reaction, known also as the Jones oxidation, is used qualitatively to distinguish between a primary or secondary and a tertiary alcohol. Similar results are obtained when alkaline potassium permanganate is used. In this case, however, because of the stronger oxidizing ability of the permanganate anion, the aldehyde that is produced from the primary alcohol is further oxidized to the corresponding carboxylic acid. On the other hand, both secondary and tertiary alcohols react toward permanganate the same way they would react toward the dichromate, producing a secondary and a tertiary alcohol, respectively.

Alcohols of the general formula CH₃CH(OH)R react positively toward the iodoform reagent, which is a basic solution of iodine in potassium iodide, to produce a carboxylic acid and the yellow precipitate of iodoform (CHI₃). The mechanism involves the initial conversion of the alcohol to the carbonyl compound.

Applications

Aliphatic alcohols are widely used in the laboratory, the clinic, and industry.

Isopropanol, a colorless liquid, is the common "rubbing alcohol" used as an antiseptic for small cuts, as a muscle relaxant for massages, and for lowering body temperatures in cases of fever. It is also found as a base in perfumes, creams, lotions, and cosmetics. It is often used as an industrial solvent or in the production of acetone. Ethylene glycol is used extensively in antifreezes because of its high boiling point (197 degrees Celsius) and freezing point (-17 degrees Celsius) and great solubility in water. A mixture of 50 percent ethylene glycol and 50 percent water freezes even lower (-36 degrees Celsius). It also has been used since the early 1970's in the manufacture of Dacron. Glycerol, a viscous, water-soluble liquid, is used both as a lubricant (in suppositories) and a moistening agent (in cosmetics and drugs). It is also a component of many fats and oils and is the starting material for the production of nitroglycerin and dynamite.

The most important of the alcohols is ethanol, also known as "grain alcohol," which is produced via yeast fermentation of sugar. It is found in alcoholic beverages, first discovered by early man. The fermentation process used to prepare wine has been attempted with practically every fruit or vegetable, including corn, wheat, rye, rice, grapes, cherries, peaches, potatoes, grapes, and even cactus pulp. Ethanol is industrially produced from the fermentation of corn starch and cane sugar. The fermentation process produces, however, only a 12 percent solution of ethanol, since concentrations higher than this kill the bacteria or yeasts responsible for the fermentation. Thus, in order to make whiskey (40 to 60 percent alcohol), the crude fermentation product must undergo a very careful distillation process to concentrate the more volatile ethanol.

Ethanol is an inexpensive industrial chemical with hundreds of industrial uses. The price of liquor is, generally, higher than its cost because of taxes imposed by the government on the basis of the alcoholic content (proof), which is defined as twice the alcohol content. Thus, a 180-proof alcoholic beverage contains 90 percent ethanol. Ethanol is used in hospitals as a disinfectant and has a hypnotic effect.

To protect its revenues, the United States government demands that ethanol for industrial use must be denatured. Denatured alcohol contains an added substance to make it toxic. Often, the denaturant is methanol (methyl alcohol), which is very toxic--to the extent that 10 milliliters can cause permanent blindness, and 30 milliliters, death. Undenatured, pure ethanol is used in laboratories under controlled supervision and is known as "absolute alcohol." It is usually prepared by azeotropic distillation with benzene, or by treatment with calcium oxide.

Another popular form of ethyl alcohol in the laboratory is 95 percent ethanol, the remaining 5 percent being water.

Phenol was once used as an antiseptic in hospitals, but its use has been discontinued because it is readily absorbed through the skin, causing burns, and because of its toxicity. Several of its derivatives, however, are in full use in everyday life. Butylated hydroxy toluene is widely used as a food preservative. It is usually put in food wrappers to avoid the inclusion of their antiseptic flavor in the food. Another derivative, thymol, is used as an antiseptic in mouthwash preparations. The most important industrial application of phenol is its reaction product with formaldehyde. The product is a resin called Bakelite, after Leo Hendrik Baekeland who, in 1909, synthesized it for the first time and received a U.S. patent. The polymer is formed by driving off water via heat. It is an extremely complex molecule that exists as a huge, three-dimensional network somewhat like the framework of a giant building. It is an example of thermosetting plastics, which are fusible at some stage of their production but become permanently hard under the influence of heat and pressure. As a result, they cannot be softened and remolded.

The lower-alcohol (C₁ to C₅) esters are employed extensively as solvents for paints, varnishes, inks, and adhesives. They are also used in food as substituents for natural flavors, such as fruit flavors. The esters of many plasticizer-range alcohols (C₆ to C₁₁) are the plasticizers and the lubricants of high-speed applications such as jet engines. Finally, the sulfate and ethoxysulfate esters of the higher alcohols are used in the detergent and surfactant industry.

Context

Alcohols have been significant for centuries. The fermentation of sugars and starches to produce alcoholic beverages has always been an integral part of human civilization. Industrially, the use of highly specialized yeasts on carbohydrates produces not only ethanol but also 1-butanol and acetone. Nevertheless, fermentation is still the main source of ethanol worldwide.

As the sources of hydrocarbons based on petroleum continue to be depleted, fermentation will become more significant. The use of ethanol and methanol as motor fuel is, thus far, limited by economic considerations. The use of gasohol, a blend of fermented agricultural products and gasoline, has become more and more significant. In Brazil, where sugar cane is an important crop, the production of ethanol is promoted, and its use as an automotive fuel supply is continuously increasing.

Methanol as fuel is another alternative for the future. The synthetic process in which solid feedstock is gasified to hydrogen, carbon monoxide, and carbon dioxide is well established, and the overall energy conversion from the raw material is in the range of 50 to 65 percent.

Methanol also has some great advantages over gasoline products. It is less flammable, fires can be easily extinguished with water, and it yields fewer pollutants as by-products.

Alcoholism is a serious problem for society. Its excessive consumption causes an impairment of the sensitivity of taste and smell, increased fear and anxiety, suppressive effects on the nervous system, and rapid deterioration of the liver. In the mid 1980's, there were about 10 million problem drinkers in the United States, with an estimated 200,000 alcohol-related deaths per year. Many organizations have been established for the care of alcoholics and much money has been spent for the cure of their problem.

The production of thermosetting plastics is not declining. Methods were developed in the early 1980's for the recycling of thermosetting polymers; they involve breaking down the polymer into the individual monomers after collection and separation from other, similar polymers. The process, although energy-consuming, is extremely important for reducing environmental pollution. Similarly, other synthetic polymers involving alcohols have contributed extensively to litter and have provided breeding grounds for disease-carrying insects and rodents, and they pose a severe threat for the extinction of marine life. An increasing surge in the synthesis of biodegradable polymers to replace the resistant plastics has been in effect since the early 1970's.

Principal terms

ALDEHYDE: any organic compound of the general formula RCHO

ALKOXIDE: the reaction product of an alcohol with a metal, such as sodium or potassium

CARBOXYLIC ACID: an organic compound of the general formula RCOOH

DENATURED ALCOHOL: ethanol to which a poisonous substance has been added to make it unfit to drink

ESTER: any organic compound of the general formula RCOOR, where R and R are organic radicals

HYDROGEN BOND: the force of attraction between the electropositive charge on a hydrogen atom bonded by a covalent bond to an oxygen, nitrogen, sulfur, or fluorine atom, and the electronegative charge on a nearby oxygen, nitrogen, sulfur, or fluorine atom

KETONE: any organic compound of the general formula RCOR

OXIDATION: the complete or partial loss of electrons in a substance; the species that initiates this change is called an oxidizing agent (the reagent itself gets reduced); examples include the dichromate and permanganate anions

POLYMER: any macromolecule with a repeating structural unit

REDUCTION: the complete or partial gain of electrons by a substance; the species that initiates this change is called a reducing agent (the reagent itself gets oxidized); examples include sodium borohydride and hydrogen

Bibliography

Hill, John W. CHEMISTRY FOR CHANGING TIMES. 4th ed. New York: Macmillan, 1984. A textbook written for the lay person, with excellent sections on various subjects taken from everyday life. Chapter 11 deals extensively with alcohols (methyl alcohol, ethyl alcohol, and others), and chapter 18 includes a section on alcoholic beverages.

Holum, John R. ELEMENTS OF GENERAL AND BIOLOGICAL CHEMISTRY. 6th ed. New York: Wiley, 1983. An introductory text for health sciences students. Chapter 12 is dedicated to alcohols and their derivatives (thioalcohols, ethers, and amines). Discusses the nomenclature, and physical and chemical properties. Also includes a section on the most important individual alcohols.

Loudon, G. Marc. ORGANIC CHEMISTRY. Reading, Mass.: Addison-Wesley, 1984. An undergraduate text in organic chemistry that gives a mechanistic approach to the different organic families. Chapters 17 and 18 discuss the properties and reactions of alcohols, thiols, and phenols, and relate them to biochemical subjects (for example, biological oxidation).

Matta, Michael S., and A. C. Wilbraham. GENERAL, ORGANIC, AND BIOLOGICAL CHEMISTRY. 2d ed. Menlo Park, Calif.: Benjamin/Cummings, 1986. An excellent introductory textbook, with several sections titled "Closer Look," in which applications of phenomena and laws in everyday events are highlighted, Chapter 12 covers halocarbons, alcohols, and ethers and their reactions. Section 12.3 gives an account of the uses of alcohols.

Solomons, T. W. Graham. FUNDAMENTALS OF ORGANIC CHEMISTRY. 3d ed. New York: Wiley, 1990. An excellent text for health sciences students that gives a fundamental background of organic chemistry. Discusses alcohols and ethers, their reactions, and their synthesis.

Spangler, Charles W. ORGANIC CHEMISTRY: A BRIEF CONTEMPORARY PERSPECTIVE. Englewood Cliffs, N.J.: Prentice-Hall, 1980. An introductory textbook for the health sciences student. Chapter 8 is dedicated to alcohols and ethers and discusses the nomenclature, synthesis, and reactions.

Stacy, Gardner W., and C. C. Wamser. ORGANIC CHEMISTRY: A BACKGROUND FOR THE LIFE SCIENCES. Dubuque, Iowa: Kendall/Hunt, 1985. A simple and easy-to-read text of basic organic chemistry for the health sciences courses. Sections 10.2 through 10.4 discuss alcohols, phenols, and thiols. A section at the end of chapter 10 gives a "Highlight on Catalysis" as related to alcohols.

Weininger, Stephen J., and F. R. Stermitz. ORGANIC CHEMISTRY. Orlando, Fla.: Academic Press, 1984. An organic chemistry text with several sections on alcohols and phenols. A portion of chapter 24 discusses the biosynthesis of phenols.

Alcohols: some common and IUPAC names