Lesson 4 Alcohols，Aldehydes and Carboxylic Acids

4.1 Alcohols

4.1.1 Sources of Alcohols

Until the 1920s，the major source of methanol was as a by product in the production of charcoal from wood，hence，was named wood alcohol. Now，most of the more than 10 billion pounds of methanol used annually in the United States is synthetic，prepared directly from carbon monoxide and hydrogen.

Almost half of this methanol is converted to formaldehyde as a starting material for various resins and plastics. Methanol is also used as a solvent，as an antifreeze，and as a convenient clean-burning liquid fuel. This last property makes it a candidate as a fuel for automobiles，methanol is already used to power Indianapolis-class race cars，but extensive emissions tests remain to be done before it can be approved as a gasoline substitute. Methanol is a colorless liquid，boiling at 65℃，and is miscible with water in all proportions. It is poisonous，drinking as little as 30mL has been fatal. Ingestion of sublethal amounts can lead to blindness.

When vegetable matter ferments，its carbohydrates are converted to ethanol and carbon dioxide by enzymes present in yeast. Fermentation of barley produces beer; grapes give wine. The maximum ethanol content is on the order of 15 percent，because higher concentrations inactivate the enzymes，halting fermentation.Since ethanol boils at 78℃ and water at 100℃，distillation of the fermentation broth can be used to give “distilled spirits” of increased ethanol content. Whiskey is the aged distillate of fermented grain and contains slightly less than 50% ethanol. Brandy and cognac are made by aging the distilled spirits from fermented grapes and other fruits. The characteristic flavors，odors，and colors of the various alcoholic beverages depend on both their origin and the way they are aged.

4.1.2 Conversion of Alcohols to Ethers

Primary alcohols are converted to ethers on heating in the presence of an acid catalyst，usually sulfuric acid.

This kind of reaction is said to be a condensation. A condensation is a reaction in which two molecules combine to form a larger one while librating a small molecule. In this case two alcohol molecules combine to give an ether and water. When applied to the synthesis of ethers，the reaction is effective only with primary alcohols. Elimination to form alkenes predominates with secondary and tertiary alcohols.

4.1.3 Esterification of Alcohols

Acid-catalyzed condensation of an alcohol and a carboxylic acid yields an ester and water and is known as the Fischer esterification.

Fischer esterification is reversible，and the position of equilibrium lies slightly to the side of products when the reactants are simple alcohols and carboxylic acids. When the Fischer esterification is used for preparative purposes，the position of equilibrium can be made more favorable by using either the alcohol or the carboxylic acid in excess. Another way to shift the position of equilibrium to favor the formation of ester is to remove the other product （water） from the reaction mixture as it is formed. This can be accomplished by adding benzene as a cosolvent and distilling the azeotropic mixture of benzene and water^①.