The aldehydes have the radical group -CHO in the molecule, and because of their ease of oxidation are important reducing agents. They are also used in the manufacture of synthetic resins and many other chemicals. Aldehydes occur in animal tissues and in the odorous parts of plants.

Acetaldehyde is a water-white flammable liquid with an aromatic penetrating odor, used as a reducing agent, preservative, and for silvering mirrors, and in the manufacture of synthetic resins, dyestuifs, and explosives. Also called ethanal, it has the composition of CH₃·CHO, and is made by the oxidation of alcohol, by catalytic hydration of acetylene, or from ethylene. The specific gravity is 0.801 and the boiling point is 20.8°C. It is soluble in water, alcohol, and naphtha. Paraldehyde, (CH₃·CHO)₃, may be used instead of acetaldehyde in resin manufacture, has a higher boiling point,
124°C, and a higher flash point, but is not as reactive and will not reduce silver solutions to form a mirror. It is used for fulling leather. Propion aldehyde, or propanal, CH₃CH₂CHO, is made in the same way by oxidation of propyl alcohol. It has a boiling point at 48.8°C, and has reactions similar to acetaldehyde.

When acetaldehyde is condensed by reaction with a dilute alkali, it forms acetaldol, also called aldol, a viscous
pale-yellow liquid of the composition CH₃·CH(OH):CH₂CHO, with a specific gravity of about 1.10, soluble in hot water and in alcohol. It is used to replace formaldehyde for synthetic resins, and for cadmium plating baths and dye baths, and for making butadiene rubber. Paraldol, the double molecule of aldol, is a white crystalline material melting at 82°C. When crude aldol is slightly acidified with acetic acid and heated, it yields croton aldehyde, also called crotonic aldehyde and propylene aldehyde, CH₃-CH:CH ·CHO, with a specific gravity of about 0.855 and a boiling point of 99 to 104°C. It is soluble in water, alcohol, and hydrocarbons, and is used as a solvent for resins, gums, and rubber, and in tanning leather. It has a pungent, suffocating odor, and is used in tear gases. Small quantities are sometimes used in city gas mains as a warning agent on the escape of poisonous fuel gas,as even tiny quantities will awaken a sleeping person.
 

Acrolein is acrylic aldehyde, CH₂:CH·CHO, a colorless volatile liquid of specific gravity 0.8389, boiling at 52.7°C. The vapor is irritating to the eyes and nose, and the unpleasant effect of scorching fat is due to the acrolein formed. Acrolein is made by oxidation of propylene with a catalyst. It polymerizes easily, and can be copolymerized with ethylene, styrene, epoxies, and other resins to form various types of plastics. Its reactive double bond and carbonyl group make it a useful material for chemical synthesis. Acrolein cyanohydrin, CH₂:CHCH(OH)CN, a water-soluble liquid, boiling at 165°C, is also used to modify synthetic resins by introducing a nitrile group and a free hydroxyl into the molecular chain. It will copolymerize with ethylene and with acrylonitrile.