What are Aldehydes?

Aldehydes are a class of organic compounds that contain a carbon atom bonded to a hydrogen atom and a double-bonded oxygen atom. The chemical formula for aldehydes is R—CHO, where R represents a carbon chain or substituent group. The general structure of aldehydes can be represented by the following:

    H
    |
— C=O

—CHO group is called aldehyde group or formyl group

The formyl group can be present in both aromatic and aliphatic compounds. In the absence of an aromatic ring, aldehydes are "aliphatic aldehydes". But when the carbon chain has an aromatic ring, they are an aromatic aldehyde.

Formyl group

Aldehydes are characterized by the presence of at least one formyl group. The formyl group has one carbon, one oxygen, and one hydrogen. Carbon is the central atom making three sigma bonds, one with oxygen, one with hydrogen, and one with the —R group. The pi-bond (π) exists between carbon and oxygen. Thus, carbon in the formyl group is sp2 hybridized. To know more on hybridization, read this article.

The bond angle is close to 120° and has triangular planar geometry. The angle varies depending on R.

We can also notice carbon makes a double bond with the oxygen atom, which is more electronegative. This electronegativity difference makes the formyl group polar and dictate chemical properties of aldehydes. Smaller aldehydes, like formaldehyde, acetaldehyde, are soluble in polar solvent like water, but the solubility decreases as the length and complexity of carbon chain (R) increases.

Production

One of the common methods used for production of aldehydes is oxo synthesis or oxo process. It is also called hydroformylation. The process converts alkenes to aldehydes. As the name says, we are adding a hydrogen and formyl group to the carbon-carbon double bond of alkene. This method is a century old and continues to remain a preferred method for the production of aldehydes at the industrial scale.

Chemical reactions

Aldehydes are characterized by their ability to undergo a variety of chemical reactions, including enolization, oxidation, reduction, substitution, and addition.

Because of the electronegativity difference between carbon and oxygen and the presence of a double bond, the formyl group can lose a proton and the resonance stabilizes the corresponding conjugate base. It also makes aldehydes slightly acidic.

We can convert an aldehyde to its corresponding alcohol using catalytic hydrogenation, i.e., the C=O bond converts to CH2—OH. The oxidation of aldehydes gives the corresponding carboxylic acid.

Aldehydes are also reactive with various organometallic reagents.

Some common reactions that involves aldehydes are

1. Formation of primary alcohol using catalytic hydrogenation
2. Oxidation of aldehydes using lab reagents, like potassium permanganate, nitric acid, and chromium(VI) oxide to generate the carboxylic acid
3. Nucleophilic addition reaction by adding a nucleophile (carbon, oxygen, nitrogen)
4. Converting aldehydes to diol using Pinacol coupling reaction
5. oxo-Diels–Alder reaction
6. Producing alkenes from aldehydes or ketones using Wittig reaction
7. Takai olefination converts aldehydes to alkenes using a diorganochromium reagent

Applications

Aldehydes are one of the functional-group compounds in organic chemistry. They are useful in all domains of the industry. The following list mentions major applications of aldehydes:

1. Use in the production of melamine resins. Melamine is a trimer of cyanamide, combined with formaldehyde to form the resin. Melamine resins have thermosetting characteristics and are used in decorative material, flooring, glassware, and dinnerware.
2. Methylene diphenyl diisocyanate (MDI) is prepared from aniline and formaldehyde. MDI has applications in the manufacturing of polymers (polyurethane) and adhesive.
3. Aldehydes are an important precursor in the production of many industrial intermediates, end-products, and pharmaceuticals.
4. 2-Ethylhexanol is a major industrial solvent that is prepared by the aldol condensation of n-butyraldehyde.
5. They are used in the synthesis of alcohols, carboxylic acids, and amines.
6. Acetic acid is produced at the industrial scale by the oxidation of acetaldehyde.
7. Bakelit, a widely known thermosetting resin, is produced from a condensation reaction of phenol with formaldehyde.
8. Formalin, a solution of formaldehyde and water, is commonly used in biological laboratories since it destroys most bacteria and fungi.
9. Other applications include disinfectant, fungicide, sterilizing agent, antiseptics due to anti-microbial properties of aldehydes
10. Other small-scale applications include an ingredient in perfumes and scents, cosmetic, dye, flavoring agent. 

Despite their diverse applications, aldehydes are toxic to the biological system of humans and animals in high concentrations. According to Occupational Safety and Health Administration (OSHA), they are hazardous substances to us and direct contact should be avoided. The direct contact with the skin and eyes may result in irritation. If inhaled, they can have adverse effects on the respiratory system. Proper safety protocols and precautionary measures are must when handling and working with aldehydes.

Aldehydes are a reacting compound to a wide range of industrial and everyday products. Their unique chemical properties make them an important and irreplaceable constituent in chemistry. They also contribute to the global economy. Common aldehydes, such as formaldehyde and butyraldehyde are produced in million metric tons.

IUPAC nomenclature

The IUPAC nomenclature adds the "-al" suffix to the name of aldehydes. For example, one-carbon aldehyde is methanal, two-carbon is ethanal, three-carbon is propanal, four-carbon is butanal, five-carbon is pentanal, and so forth.

Common Aldehydes

One of the most common and popular aldehydes is formaldehyde (IUPAC: methanal). It is the simplest aldehyde with chemical formula H—CHO and characterized by a strong, pungent smell. It is a colorless gas and can be dissolved in water to form formalin. As of 2022, it is the largest-scale produced aldehyde with installed capacity exceeding six million tons per annum. 

Formaldehyde is precursor to a variety of industrial products and intermediates, that include bakelite, polymers, resins, plastics, adhesive, additives, plasticizers. It also has disinfectant and preservative properties. 

Another important aldehyde is acetaldehyde; it is a colorless liquid that boils near room temperature and naturally occurs in fruits and coffee.

Acetic acid can be produced by the oxidation of acetaldehyde. Its use includes industrial solvent, flavoring agents, a main precursor in the production of polyvinyl acetate.

Other common aldehydes include propanal, butanal, phenylmethanal, vanillin, furfural, citral,  glycolaldehyde, benzaldehyde, cinnamaldehyde, and retinaldehyde.

Aldehydes are also parts of biological systems in plants and animals. Many biological chemicals contain the formyl group.