Dehydration

As noted in Figure 14.4 “Reactions of Alcohols”, an alcohol undergoes dehydration in the presence of a catalyst to form an alkene and water. The reaction removes the OH group from the alcohol carbon atom and a hydrogen atom from an adjacent carbon atom in the same molecule:

Under the proper conditions, it is possible for the dehydration to occur between two alcohol molecules. The entire OH group of one molecule and only the hydrogen atom of the OH group of the second molecule are removed. The two ethyl groups attached to an oxygen atom form an ether molecule.

(Ethers are discussed in Section 14.4 “Reactions That Form Alcohols”.) Thus, depending on conditions, one can prepare either alkenes or ethers by the dehydration of alcohols.

Note

Both dehydration and hydration reactions occur continuously in cellular metabolism, with enzymes serving as catalysts and at a temperature of about 37°C. (For more information about hydration reactions, see Chapter 13 “Unsaturated and Aromatic Hydrocarbons”, Section 13.4 “Chemical Properties of Alkenes”.) The following reaction occurs in the Embden–Meyerhof pathway. (For more information about metabolic reactions, see Chapter 20 “Energy Metabolism”.)

Although the participating compounds are complex, the reaction is the same: elimination of water from the starting material. The idea is that if you know the chemistry of a particular functional group, you know the chemistry of hundreds of different compounds.

Oxidation

Primary and secondary alcohols are readily oxidized. We saw earlier how methanol and ethanol are oxidized by liver enzymes to form aldehydes. Because a variety of oxidizing agents can bring about oxidation, we can indicate an oxidizing agent without specifying a particular one by writing an equation with the symbol [O] above the arrow. For example, we write the oxidation of ethanol—a primary alcohol—to form acetaldehyde—an aldehyde—as follows:

We shall see (in Section 14.9 “Aldehydes and Ketones: Structure and Names”) that aldehydes are even more easily oxidized than alcohols and yield carboxylic acids.

Secondary alcohols are oxidized to ketones. The oxidation of isopropyl alcohol by potassium dichromate (K₂Cr₂O₇) gives acetone, the simplest ketone:

Unlike aldehydes, ketones are relatively resistant to further oxidation (Section 14.9 “Aldehydes and Ketones: Structure and Names”), so no special precautions are required to isolate them as they form.

Note that in oxidation of both primary (RCH₂OH) and secondary (R₂CHOH) alcohols, two hydrogen atoms are removed from the alcohol molecule, one from the OH group and other from the carbon atom that bears the OH group.

Note

These reactions can also be carried out in the laboratory with chemical oxidizing agents. One such oxidizing agent is potassium dichromate. The balanced equation (showing only the species involved in the reaction) in this case is as follows:

Alcohol oxidation is important in living organisms. Enzyme-controlled oxidation reactions provide the energy cells need to do useful work. One step in the metabolism of carbohydrates involves the oxidation of the secondary alcohol group in isocitric acid to a ketone group:

Note that the overall type of reaction is the same as that in the conversion of isopropyl alcohol to acetone. (For more information on metabolic reactions, see Chapter 20 “Energy Metabolism”.)

Tertiary alcohols (R₃COH) are resistant to oxidation because the carbon atom that carries the OH group does not have a hydrogen atom attached but is instead bonded to other carbon atoms. The oxidation reactions we have described involve the formation of a carbon-to-oxygen double bond. Thus, the carbon atom bearing the OH group must be able to release one of its attached atoms to form the double bond. The carbon-to-hydrogen bonding is easily broken under oxidative conditions, but carbon-to-carbon bonds are not. Therefore tertiary alcohols are not easily oxidized.