alcohols, phenols, and ethers

Alcohols, phenols, and ethers may be considered organic derivatives of water (H2O), a molecule of which consists of an oxygen atom bonded to two hydrogen atoms. In an alcohol, one of water's two hydrogen atoms is replaced by an alkyl group, which is a hydrocarbon chain, typically represented by R in organic structures; the remaining oxygen and hydrogen atoms form a hydroxyl group (OH). In ethanol, for example, the alkyl group is the ethyl group, -CH2CH3. Alcohols, which are among the most common organic compounds, are valuable intermediates in the synthesis of other compounds. Alcohols may be classified according to which carbon of the alkyl group is bonded to the hydroxyl group. In primary alcohols (RCH2OH), the oxygen of the hydroxyl group is bonded to a carbon atom bearing at least two hydrogen atoms. In secondary alcohols (R2CHOH), the hydroxyl group is bonded to a carbon joined to two other carbons, while in tertiary alcohols (R3COH), it is bonded to a carbon atom attached to three other carbon atoms. Most alcohols are colorless liquids or solids at room temperature. Primary alcohols with fewer than 12 carbon atoms are liquid, whereas those with 12 or more carbon atoms are solid. Polyhydric alcohols (those with more than one hydroxyl group (usually have the consistency of syrup. Alcohols with complex arrangements of carbon atoms, such as sterols, are usually solids. Although alcohols of low molecular weight are highly soluble in water, alcohols become less soluble with increasing molecular weight and their boiling points, densities, vapor pressures, and viscosities increase. Primary alcohols may be oxidized to aldehydes (q.v.) or further to carboxylic acids (q.v.); secondary alcohols may be reduced to ketones (q.v.). When tertiary alcohols are oxidized, they decompose, breaking carbon-carbon bonds. Alcohols generally react with carboxylic acids to produce esters—neutral organic compounds that occur in oils and fats. Alcohols can also be converted to ethers and olefinic hydrocarbons. The addition of hydroxyl groups to an alcohol without increasing the number of carbon atoms changes many of its properties. It may, in some case, enhance sweetness, and the resultant alcohols are employed as sweeteners. Alcohols with 8 to 12 carbon atoms, which have a characteristic roselike or lilylike odor, are used in making perfumes. In general, the most important industrial use of alcohols is as chemical intermediates, primarily because alcohols can be readily converted into a great many other compounds. This is achieved by simple chemical reactions to yield such products as fats and waxes—surface-active agents employed in detergents, emollients, emulsifiers, lubricants, plasticizers, and foaming agents. Some alcohols, which are among the most abundantly produced organic chemicals in industry, are utilized in great quantities, e.g., methanol and ethanol. Methanol, also known as methyl alcohol, wood alcohol, or carbinol, can be manufactured from hardwood or from hydrogen and carbon monoxide (CO). It is employed as a solvent, as a raw material for the production of formaldehyde and special resins, in special fuels, in antifreeze, and for cleaning metals. Ethanol (CH3CH2OH), also called ethyl alcohol, or grain alcohol, can be made by fermentation from the carbohydrates found in fruits, molasses, grains, and other agricultural products. It is also produced industrially from ethylene. Well known as the alcohol in alcoholic beverages, it is employed in toiletries and in pharmaceuticals and to sterilize hospital instruments. The anesthetic ether is also made from ethanol. Methanol and ethanol are good fuels for automobile engines because they have high octane ratings and low pollution emission. Their solvent properties can, however, cause problems by dissolving certain materials used in modern fuel systems. Gasohol, a solution of 10 percent ethanol in gasoline, is an alternative fuel that can be used in most automobiles without the solvency problem. Phenols are characterized by a hydroxyl (-OH) group attached to a carbon atom that is part of an aromatic ring. The term phenol, besides serving as the generic name for the entire family, is also the specific name for its simplest member, monohydroxybenzene (C6H5OH), also known as benzenol, or carbolic acid. Phenols are similar to alcohols, but because they form stronger hydrogen bonds are thus more soluble in water than are alcohols and have higher boiling points. Occurring either as colorless liquids or white solids at room temperature, many phenols have a sharp, spicy odor. Phenol smells bland and sweetish, but is highly toxic and caustic. So-called natural phenol can be made from the distillation of coal tar or crude petroleum. Other phenols of natural origin occur in essential oils, which are derived from seeds or leaves of plants. Most synthetic phenol is made by either the hydrolysis of chlorobenzene or the oxidation of isopropylbenzene; both methods use benzene as a starting material. More general syntheses, such as diazotization of arylamines, are employed in producing more complex phenols. Phenols are acidic and react with strong bases to form alkali-metal salts known as phenoxides. Phenol’s most important reaction is its condensation with formaldehyde. This produces water as a side effect and forms synthetic polymers, called phenol-formaldehyde (phenolic) resins, that are widely used plastics. Ethers are characterized by an oxygen atom attached to two carbon atoms that are part of a hydrocarbon. Although similar to alcohols, ethers are generally less dense, less soluble in water, have lower boiling points, and are relatively inert. At room temperature, ethers are pleasant smelling, colorless liquids. Ethers occur in nature as part of such substances as sugar, starch, and cellulose. Ethers may be manufactured by dehydrating alcohols, but catalytic hydration (addition of water) of olefins is the principal method of production. Although ethers are often regarded as derivatives of alcohol, they display properties that are more likely to resemble those of their parent hydrocarbons. When they are reacted with strong inorganic acids and Lewis acids (compounds capable of accepting electron pairs) they form salts called oxonium compounds. When strong acids break one of the oxygen-carbon linkages, alcohols are formed. If both linkages are broken, other organic derivatives are produced. Heat decomposes ether into olefins. The presence of oxygen causes ethers to slowly oxidize to unstable peroxides; this reaction can result in an explosion. Because ethers will form azeotropes (constant-distillation mixtures) with several different organic compounds, they are employed extensively for extraction and separation of organic chemicals. Ethers are also used as solvents for fats, oils, waxes, perfumes, resins, gums, dyes, and hydrocarbons. Vapors of certain ethers are emp0loyedas insecticides, miticides, and fumigants for soil. Ethers are also very important in medicine and pharmacology, especially for uses as anesthetics. One example, codeine, is the methyl either of morphine. Ethyl ether, a highly volatile liquid best known as anesthetic, is also used as a solvent, an extractant, and a reaction medium. Methyl ether is used as a spray propellant and a refrigerant. The ethers of ethylene glycol are employed as plasticizers and solvents.