Important aldehydes and ketones

Acetone (b.p. 56^o), the simplest ketone, is an inexpensive industrial solvent. It is synthesized industrially using the oxidation of isopropyl alcohol. It can also be formed by fermentation of sugars or starch, and is found on the breath and in the urine of many sufferers of diabetes. Acetone is a super solvent. It can dissolve most organic compounds and is also miscible with water. Casual exposure to acetone poses no health risk. Acetone is highly volatile and is also highly flammable.

Methyl and ethyl ketone (b.p. 80^o) shows many of acetone's desirable solvent properties and is used when a solvent with a higher boiling point is needed. It is approximately twice as expensive as acetone, however, in many processes the solvent is recovered and reused so that the greater initial expense is not especially important.

Acetaldehyde (b.p. 20^o) is sweet smelling but narcotic. It is formed readily by the hydration of acetylene and is used mainly as a raw material for the synthesis of other organic compounds, among them acetic acid, by reaction with oxygen. Chlorination of acetaldehyde gives chloral; chloral in the form of its hydrate is a hypnotic and is also used to prepare DDT. Acetaldehyde is present in ripe fruits, especially in apples and is a metabolite of ethanol It is less toxic than formaldehyde.

n-Butyraldehyde (b.p. 75^o), formed from oxidation of n-butyl alcohol, is widely used in the production of safety glass.

From a commercial standpoint the simplest aldehyde, formaldehyde, is also the most important. Pure formaldehyde is a gas (b.p. 21^o), which can be neither readily isolated nor handled in this state. Formaldehyde is toxic, but useful. It kills viruses, fungi and bacteria, and is used to disinfect and sterilize equipment. The largest commercial use of formaldehyde is in the preparation of phenol-formaldehyde resins for molded articles and for use in plywood. These resins are thermosetting; this means that in the course of the manufacture of an article the phenol-formaldehyde resin melts and then, under the influence of heat and pressure, resolidifies to become permanently infusible. The properties of thermosetting polymers contrast with those of thermoplastic polymers. The latter, which include most vinyl polymers, are solids at room temperature but can be molded at higher temperatures. This fusion-solidification process can be repeated numerous times.

Formaldehyde is prepared from controlled oxidation of methanol over a metal catalyst and is sold as a 37% solution in water (formalin) and also as a solid polymer, paraformaldehyde, from which it can be recovered by heating. Candles of paraformaldehyde are used as fumigating agents.

Since formaldehyde can be produced inexpensively and polymerizes readily (to paraformaldehyde), a great deal of effort has gone into investigations of its use in the preparation of commercial polymers.

The problem in using pure polyformaldehyde has been to prevent its decomposition to formaldehyde, which is poisonous. It can be shown that the decomposition begins at the ends of the chains, but when these free hydroxyl groups are converted into ester groups a stable polymer results.

Formaldehyde and ammonia react to form the cagelike compound hexamethylenetetramine, which liberates formaldehyde upon treatment with acid. Hexamethylenetetramine is medicinally useful as a urinary antiseptic (urotropine) and is also oxidized by nitric acid to the important military explosive cyclonite.