ALDEHYDES: STRUCTURE, PROPERTIES, USES AND EXAMPLES - CHEMISTRY - 2025

The aldehydes are organic compounds having the general formula RCHO. R represents an aliphatic or aromatic chain; C to carbon; O to oxygen and H to hydrogen. They are characterized by having a carbonyl group like ketones and carboxylic acids, which is why aldehydes are also called carbonyl compounds.

The carbonyl group gives the aldehyde many of its properties. They are compounds that are easily oxidized and are very reactive to nucleophilic additions. The double bond of the carbonyl group (C = O) has two atoms that have differences in their avidity for electrons (electronegativity).

General structure of aldehydes

Oxygen attracts electrons with greater force than carbon, so the electron cloud moves towards it, making the double bond between carbon and oxygen polar in nature, with a significant dipole moment. This makes the aldehydes polar compounds.

The polarity of the aldehydes will influence their physical properties. The boiling point and solubility of aldehydes in water are higher than nonpolar chemical compounds with similar molecular weights, such is the case of hydrocarbons.

Aldehydes with less than five carbon atoms are soluble in water, since hydrogen bonds are formed between the oxygen of the carbonyl group and the water molecule. However, increasing the number of carbons in the hydrocarbon chain produces an increase in the non-polar portion of the aldehyde, making it less soluble in water.

But what are they like and where do they come from? Although its nature depends essentially on the carbonyl group, the rest of the molecular structure also contributes a lot to the whole. Thus, they can be of any size, small or large, or even a macromolecule can have regions where the character of aldehydes predominates.

Thus, as with all chemical compounds, there are "pleasant" aldehydes and other bitter ones. They can be found in natural sources or synthesized on a large scale. Examples of aldehydes are vanillin, very present in ice cream (top image) and acetaldehyde, which adds flavor to alcoholic beverages.

Chemical structure

Source: By NEUROtiker, from Wikimedia Commons

Aldehydes consist of a carbonyl (C = O) to which a hydrogen atom is directly attached. This differentiates it from other organic compounds such as ketones (R ₂ C = O) and carboxylic acids (RCOOH).

The upper image shows the molecular structure around –CHO, a formyl group. The formyl group is flat because carbon and oxygen have sp ² hybridization. This planarity makes it susceptible to attack by nucleophilic species, and therefore it is easily oxidized.

What does this oxidation refer to? To the formation of a bond with any other atom more electronegative than carbon; and in the case of aldehydes it is an oxygen. Thus, the aldehyde is oxidized to a carboxylic acid, –COOH. What if the aldehyde was reduced? A primary alcohol, ROH, would form in its place.

Aldehydes are only produced from primary alcohols: those where the OH group is at the end of a chain. In the same way, the formyl group is always at the end of a chain or protrudes from it or the ring as a substituent (in the case of other more important groups, such as –COOH).

Physical and chemical properties

Being polar compounds, their melting points are higher than those of nonpolar compounds. Aldehyde molecules are not capable of hydrogen bonding intermolecularly, as they only have carbon atoms attached to hydrogen atoms.

Due to the above, aldehydes have lower boiling points than alcohols and carboxylic acids.

Melting points

Formaldehyde -92; Acetaldehyde -121; Propionaldehyde -81; n-Butyraldehyde -99; n-Valeraldehyde -91; Caproaldehyde -; Heptaldehyde - 42; Phenylacetaldehyde -; Benzaldehyde -26.

Boiling points

Formaldehyde -21; Acetaldehyde 20; Propionaldehyde 49; n-Butyraldehyde 76; n-Valeraldehyde 103; Caproaldehyde 131; Heptaldehyde 155; Phenylacetaldehyde 194; Benzaldehyde 178.

Solubility in water expressed in g / 100 g of H

Formaldehyde, very soluble; Acetaldehyde, infinite; Propionaldehyde, 16; n-Butyraldehyde, 7; n-Valeraldehyde, slightly soluble; caproaldehyde, slightly soluble; Slightly soluble phenylacetaldehyde; Benzaldehyde, 0.3.

The boiling points of aldehydes tend to increase directly with molecular weight. On the contrary, there is a tendency to decrease the solubility of aldehydes in water as their molecular weight increases. This is reflected in the physical constants of the aldehydes just mentioned.

Reactivity

Oxidation reaction

Aldehydes can be oxidized to the corresponding carboxylic acid in the presence of any of these compounds: Ag (NH ₃) ₂, KMnO _4, or K ₂ Cr ₂ O ₇.

Reduction to alcohols

They can be hydrogenated with the aid of nickel, platinum or palladium catalysts. Thus, the C = O becomes C-OH.

Reduction to Hydrocarbons

In the presence of Zn (Hg), concentrated HCl or NH ₂ NH _2, the aldehydes lose the carbonyl group and become hydrocarbons.

Nucleophilic addition

There are several compounds that are added to the carbonyl group, among them are: Grignard reagents, cyanide, derivatives of ammonia and alcohols.

Nomenclature

Source: Gabriel Bolívar

Four aldehydes are illustrated in the image above. How are they named?

Since they are oxidized primary alcohols, the name of the alcohol is changed from -ol to -al. Thus, methanol (CH ₃ OH) if oxidized to CH ₃ CHO is called methanal (formaldehyde); CH ₃ CH ₂ CHO ethanal (acetaldehyde); CH ₃ CH ₂ CH ₂ CHO propanal and CH ₃ CH ₂ CH ₂ CH ₂ CHO butanal.

All the newly named aldehydes have the group -CHO at the end of the chain. When it is found at both ends, as in A, the ending -to is prefixed with di-. As A has six carbons (counting those of both formyl groups), it derives from 1-hexanol and its name is therefore: dial hexane.

When there is a substituent, such as an alkyl radical, a double or triple bond, or a halogen, the carbons of the main chain are listed giving -CHO the number 1. Thus, aldehyde B is called: 3-iodohexanal.

However, in aldehydes C and D the group -CHO does not have priority to identify these compounds from others. C is a cycloalkane, while D a benzene, both with one of their Hs substituted by a formyl group.

In them, as the main structure is cyclic, the formyl group is named carbaldehyde. Thus, C is cyclohexanecarbaldehyde, and D is benzenecarbaldehyde (better known as benzaldehyde).

Applications

Aldehydes exist in nature capable of conferring pleasant flavors, such is the case of cinnamaldehyde, responsible for the characteristic flavor of cinnamon. That is why they are often used as artificial flavorings in many products such as candy or food.

Formaldehyde

Formaldehyde is the most industrially produced aldehyde. The formaldehyde obtained by the oxidation of methanol is used in a 37% solution of the gas in water, under the name of formalin. This is used in the tanning of hides and in the preservation and embalming of corpses.

Also, formaldehyde is used as a germicide, fungicide and insecticide for plants and vegetables. However, its greatest utility is the contribution to the production of polymeric material. The plastic called Bakelite is synthesized by the reaction between formaldehyde and phenol.

Bakelite

Bakelite is a highly hard polymer with a three-dimensional structure used in many household utensils such as handles for pots, pans, coffee makers, knives, etc.

Bakelite-like polymers are made from formaldehyde in combination with the compounds urea and melamine. These polymers are used not only as plastics, but also as adhesive glues and coating material.

Plywood

Plywood is the trade name of a material made up of thin sheets of wood, joined by polymers produced from formaldehyde. The Formica and Melmac brands are manufactured with the latter's participation. Formica is a plastic material used to coat furniture.

Melmac plastic is used to make plates, glasses, cups, etc. Formaldehyde is the raw material for the synthesis of the compound methylene-diphenyl-diisocyanate (MDI), a precursor to polyurethane.

Polyurethane

Polyurethane is used as an insulator in refrigerators and freezers, upholstery for furniture, mattresses, coatings, adhesives, soles, etc.

Butyraldehyde

Butyraldehyde is the main precursor for the synthesis of 2-ethylhexanol, which is used as a plasticizer. It has a pleasant apple aroma that allows its use in food as a flavoring.

It is also used for the production of rubber accelerators. It intervenes as an intermediate reagent in the manufacture of solvents.

Acetaldehyde

Acetaldehyde was used in the production of acetic acid. But this function of acetaldehyde has diminished in importance, as it has been displaced by the carbonylation process of methanol.

Synthesis

Other aldehydes are precursors of oxoalcohols, which are used in the production of detergents. So-called oxoalcohols are prepared by adding carbon monoxide and hydrogen to an olefin to obtain an aldehyde. And finally the aldehyde is hydrogenated to obtain the alcohol.

Some aldehydes are used in the manufacture of perfumes, as is the case with Chanel No. 5. Many aldehydes of natural origin have pleasant odors, for example: heptanal has a smell of green grass; the octanal an orange smell; the nonanal a smell of roses and the citral a smell of lime.

Examples of aldehydes

Glutaraldehyde

Source: By Jynto, from Wikimedia Commons

Glutaraldehyde has two formyl groups in its structure at both ends.

Marketed under the name Cidex or Glutaral, it is used as a disinfectant to sterilize surgical instruments. It is used in the treatment of warts on the feet, applied as a liquid. It is also used as a tissue fixing agent in histology and pathology laboratories.

Benzaldehyde

It is the simplest aromatic aldehyde which is formed by a benzene ring where a formyl group is linked.

It is found in almond oil, hence its characteristic smell that allows it to be used as a food flavoring. In addition, it is used in the synthesis of organic compounds related to the manufacture of drugs and in the manufacture of plastics.

Glyceraldehyde

It is an aldotriose, a sugar made up of three carbon atoms. It has two isomers called D and L enantiomers. Glyceraldehyde is the first monosaccharide to be obtained in photosynthesis during the dark phase (Calvin cycle).

Glyceraldehyde-3-phosphate

Source: Jynto and Ben Mills

The upper image illustrates the structure of glyceraldehyde-3-phosphate. The red spheres next to the yellow one correspond to the phosphate group, while the black ones the carbon skeleton. The red sphere linked to the white one is the OH group, but when it is linked to the black sphere and the latter to the white one, then it is the CHO group.

Glyceraldehyde-3-phosphate is involved in glycolysis, a metabolic process in which glucose is degraded to pyruvic acid with the production of ATP, an energy reservoir of living beings. In addition to the production of NADH, a biological reducing agent.

In glycolysis, glyceraldehyde-3-phosphate and dihydroacetone phosphate originate from the cleavage of D-fructose-1-6-bisphosphate

Glyceraldehyde-3-phosphate is involved in the metabolic process known as the pentose cycle. In this, NADPH, an important biological reducer, is generated.

11-cis-Retinal

Source: Pixabay.

Β-carotene is a natural pigment present in various vegetables, especially carrots. It undergoes an oxidative breakdown in the liver, thus transforming into the alcohol retinol or vitamin A. The oxidation of vitamin A and the subsequent isomerization of one of its double bonds, forms the 11-cis-retinal aldehyde.

Pyridoxal phosphate (Vitamin B6)

Source: Jynto and Ben Mills.

It is a prosthetic group linked to several enzymes, which is the active form of vitamin B6 and participates in the synthesis process of the inhibitory neurotransmitter GABA.

Where is the formyl group in your structure? Note that this differs from the rest of the groups attached to the aromatic ring.

Salicylaldehyde

It is a raw material for the synthesis of acetylsalicylic acid, an analgesic and antipyretic drug known as Aspirin.

References

1. Robert C. Neuman, Jr. Chapter 13, Carbonyl Compounds: Ketones, Aldehydes, Carboxylic Acids.. Taken from: chem.ucr.edu
2. Germán Fernández. (September 14, 2009). Nomenclature of aldehydes. Taken from: quimicaorganica.net
3. TW Graham Solomons, Craigh B. Fryhle. Organic Chemistry. (Tenth Edition, p 729-731) Wiley Plus.
4. Jerry March and William H. Brown. (December 31, 2015). Aldehyde. Taken from: britannica.com
5. Wikipedia. (2018). Aldehydes. Taken from:
6. Morrison, RT and Boyd, RN (1990). Organic Chemistry Fifth Edition. Editorial Addison-Wesley Iberoamericana.
7. Carey, FA (2006). Organic Chemistry Sixth Edition. Editorial Mc Graw Hill.
8. Mathews, Ch. K., Van Holde, KE and Athern, KG (2002). Biochemistry. Third edition. Publishing Pearson Adisson Wesley.