4.2 Aldehydes

4.2.1 Sources of Aldehydes and Ketones

Low-molecular-weight aldehydes and ketones are important industrial chemicals. While specialized procedures have been developed for making many of them，most can be prepared by oxidation （or dehydrogenation） of the corresponding alcohol. Formaldehyde，a starting material for a number of plastics，is prepared by oxidation of methanol over a silver or iron oxide—molybdenum oxide catalyst at elevated temperature.

Similar processes are used to convert ethanol to acetaldehyde and isopropyl alcohol to acetone. Acetaldehyde can be prepared by hydration of acetylene，but a more economical procedure is air oxidation of ethylene in the presence of palladium chloride and copper （Ⅱ） chloride as catalysts:

This is known as the Wacker process. An organopalladium compound is an intermediate.

4.2.2 Constructure and Physical Properties

Aldehydes and ketones are characterized by the presence of an acyl group （）bonded either to hydrogen or to another carbon.

In general，aldehydes and ketones have higher boiling points than hydrocarbon analogs because they are more polar and the dipole-dipole attractive forces between molecules are stronger. They have lower boiling points than alcohols，because unlike alcohols，two carbonyl groups cannot form hydrogen bonds to each other.

Aldehydes and ketones can form hydrogen bonds with the protons of water and are more soluble than hydrocarbons，but less soluble than alcohols in water.