What Is an Aldehyde?

A compound containing -CHO (aldehyde group) in the molecule is called an aldehyde, and the general formula is RCHO. R- may not be a hydrocarbon group, for example R- of hydroxyacetaldehyde is HOCH2-; R- may also be a hydrocarbon group, such as alkyl, alkenyl, aromatic or cycloalkyl. According to the number of aldehyde groups, they can be divided into mono- and poly-aldehydes. Lower aldehydes are liquid, higher aldehydes are solid, and only formaldehyde is a gas. The chemical properties of aldehydes are very active, they can react with sodium bisulfite, hydrogen, ammonia, etc., and are easily oxidized by weak oxidants to the corresponding carboxylic acids. Aldehydes are widely used. Formaldehyde vapor can sterilize the air. Formaldehyde solutions can be used for the preservation of biological specimens. Fatty aldehydes generally have anesthetic and hypnotic effects. For example, chloral hydrate is an early synthetic hypnotic drug. ^[1]

At room temperature, in addition to formaldehyde as a gas, fatty aldehydes with less than 12 carbon atoms in the molecule are liquid, higher aldehydes are solid; and aromatic aldehydes are liquid or solid. Low-grade fatty aldehydes have a strong pungent odor. The aldehydes containing 9 carbon atoms in the molecule and the aldehydes containing 10 carbon atoms in the molecule have a flower and fruit aroma.

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Aldehydes have high reactivity and participate in many reactions. From an industrial point of view, the most important reactions are condensation reactions, such as the preparation of plasticizers and polyhydroxy compounds, and the reduction reaction to produce alcohols (especially carbonyl alcohols). From a biological point of view, the important reactions mainly include: reactions for preparing imines, that is, nucleophilic addition reactions of formyl groups, such as: oxidative deamine reactions, hemiacetal structure (aldose)

Important aldehydes and related compounds. From left to right: formaldehyde and paraformaldehyde, acetaldehyde and its enol form, glucose (pyranose), edible flavor cinnamaldehyde and vitamin B6.

Aldehydes in natural products

Many trace amounts of aldehydes are found in essential oils due to their aromatic odors, such as cinnamaldehyde, carboxaldehyde, and vanillin. Perhaps due to the high activity of formyl, aldehyde groups are rare in natural products (amino acids, nucleic acids, oils). Most sugars are derivatives of aldehydes. These "aldoses" are generally in the form of hemiacetals, and a few are in the form of aldehydes. For example, a small part of the glucose in aqueous solution is in the form of aldehydes.

After undergoing "smoke-type" and "photochemical smog-type" pollution, urban residents have entered the third period of pollution characterized by "indoor air pollution", whose protagonist is formaldehyde. Formaldehyde is a colorless and soluble irritating gas. Prolonged inhalation of formaldehyde gas can damage the liver, kidneys, blood system, digestive system, respiratory system, central nervous system and immune system. Women and pregnant women are exposed to low concentrations of formaldehyde for a long time. Gas can cause menstrual disorders, fetal malformations, decreased newborn immunity, decreased physical fitness, and impaired intellectual development. In addition, formaldehyde has teratogenic and carcinogenic effects. People with long-term exposure to formaldehyde can cause chromosomal mutations, cancers of the nasal cavity, mouth, nasopharynx, throat, skin and digestive tract, and even inhibit bone marrow hematopoietic function, causing aplastic anemia. ^[2]