What is Aldehyde?

Lucy Bell-Young

by Lucy Bell-Young

1st September 2021

Aldehydes are primarily derivatives of alcohols. Like many other chemicals, aldehydes are identified based on their functional group. They are organic compounds that have a carbonyl group attached to a hydrocarbon group (i.e. an alkyl substituent) and hydrogen atom. This is structurally and chemically similar to ketones, so it’s not surprising that some of the properties of aldehydes are similar to ketones. 

How Are Aldehydes Produced?

Oxidative routes are commonly used to produce aldehydes on an industrial scale. For example, aldehydes can be synthesised by removing hydrogen from alcohol. Below is an example of an aldehyde derived from alcohol. As you can see, hydrogen is removed from 1-propanol to produce propionaldehyde, or propanal:Chemical formula showing the removal of hydrogen from alcohol to produce an aldehyde

Other chemical routes, such as the ozonolysis of alkenes, are also implemented in the synthesis of aldehydes. Here are some examples:

  • Organic reduction: In this method, an ester is reduced into an aldehyde using diisobutylaluminium hydride or sodium aluminium hydride
  • Rosenmund reaction: This involves the systematic and selective reduction of acyl chlorides into aldehydes. The most common and effective reagent used for the process is lithium tri-t-butoxyaluminum hydride
  • Formylation reactions: Here, nucleophilic arenes are used as substrates. An organic compound is functionalised with a formyl group, which is a pathway to aldehydes
  • Zincke reaction: Pyridines are used as a substrate, allowing them to react with two equivalents of secondary amines. A Zincke aldehyde is formed in the process
  • Hofmann rearrangement: During this process, an intermediate carbamate is hydrolysed into aldehydes

Formaldehyde, or formalin, is the most useful and most abundant aldehyde produced worldwide each year. Aside from preserving organic tissues, such as in embalming, formaldehyde is very useful as a chemical precursor and reagent for a wide range of synthetic chemicals.

Formaldehyde vector illustration

What is the Formula for Aldehyde?

The formula for aldehyde varies depending on the type of aldehyde. Here are some examples of specific aldehydes and their chemical formulas and molecular structures:

  • Formaldehyde (CH2O): A naturally occurring substance present in trace amounts in organisms as a waste product of cellular metabolism. It can also be found in food, but only in small amounts. Industrially, it can be produced through the catalytic oxidation of methanol.
Chemical structure of formaldehyde
  • Ethanal (C2H4O): This is one of the most important aldehydes in nature. It’s naturally found in coffee and ripe fruits as plant metabolites. It’s industrially manufactured through the catalytic process of oxidising ethene or ethanol.
Chemical structure of ethanal
  • Propanal (C3H6O): Otherwise known as propionaldehyde, this aldehyde is mainly used in the manufacture of plastics, as well as being a disinfectant.

Chemical structure of propanal

  • Butanal (C4H8O): Butanal is also known as butyraldehyde. It’s a biomarker and a metabolite found in many organisms, including E. coli and mice.

Chemical structure of butanal

  • Pentanal (C5H10O): This aldehyde is a metabolite produced by eukaryotic organisms, which include plants like conifers and gymnosperms. It has several commercial uses, such as in food flavouring and resin manufacturing. It’s also useful as a rubber accelerator.
Chemical structure of pentanal

By now, you may have noticed a pattern emerging in the chemical formulas: the proportion of hydrogen is double the amount of carbon in an aldehyde, and the number of oxygen is always one. 

This same pattern is the basis for the systematic IUPAC nomenclature, or naming of aldehydes, which is much more precise than the common names we use. 

The basic IUPAC rules in naming aldehydes are the following:

  1. Remove the –e at the end of the name of the parent alkane chains and replace it with the –al
  2. The functional group of an aldehyde always has the lowest possible location number, which is one. Therefore, it’s not included in the name
  3. Add the suffix –carbaldehyde if the aldehyde functional group is attached to a ring
  4. Include the names of other functional groups, if there are any, in alphabetical order. Watch this video to find some examples

How to Identify an Aldehyde

While there are many different types of aldehydes and even though their respective chemical formulas vary, there is a generalised formula for all aldehydes, as shown below, which can be used to identify them. This formula also illustrates the generic molecular structure of an aldehyde molecule:
The generalised chemical structure of aldehydes

An aldehyde has a central carbonyl functional group, which is a carbon atom that’s double-bonded with an oxygen atom. Attached to this central functional group is a generic alkyl substituent (the R group) and a hydrogen atom.

Experimentally, several methods can be used to test for the presence of an aldehyde in a mixture. Tollens’ reagent is a classic and commonly-used method for detecting the presence of aldehyde. Here, the reagent is a mixture of silver(I) ions that are dissolved in a dilute ammonia aqueous solution. An aldehyde is oxidised and the silver(I) ions precipitate as silver metal.

However, identifying the specific composition of an unknown aldehyde sample requires more advanced techniques, such as gas chromatography–mass spectrometry (GC-MS). Careful analytical techniques are also applied in identifying the composition of an unknown sample of aldehyde.

What is an Aldehyde Functional Group?

The complete aldehyde functional group has the carbonyl group, which is a carbon double-bonded to oxygen, at the centre. Attached to this is a generic alkyl chain, or side chain R group, on one side, and a hydrogen atom on the other side.

Other functional groups, such as amines and hydroxyl groups, may also displace one or more hydrogen atoms in the hydrocarbon chain in more complex aldehyde compounds. In naming compounds with more than one functional group, the aldehyde functional group is usually given priority.

For example, both aldehyde and ketone groups can be present in the same molecule, such as in the case of 2-methyl-3-oxo-butanal, as shown in the illustration below:Chemical structure of 2-methyl-3-oxo-butanal

What Are the Uses of Aldehydes?

Various types of aldehydes have biological, industrial, and commercial uses. Some aldehydes have important roles in metabolic and physiological processes. For example, retinal (C20H28O), otherwise known as vitamin A aldehyde or retinaldehyde, has an important role in vision. It’s part of the retina as a conjugated chromophore that detects specific wavelengths of light.

Another important micronutrient aldehyde in the human body is pyridoxal phosphate (C8H10NO6P), which is a type of vitamin B6. It functions as a coenzyme in many biochemical reactions, such as the biosynthesis of serotonin.

Some aldehydes have industrial applications and commercial value. For example, formaldehyde is commonly dissolved in water as a 37% solution, known as formalin. This has several uses, such as in embalming, tanning, and disinfecting. It’s also used in manufacturing plastics and resins.

Structural chemical formula and molecule model of retinaldehyde

Structural chemical formula and molecule model of retinaldehyde

Is Aldehyde Hydrophobic or Hydrophilic?

Aldehydes are virtually identical to hydrocarbons in terms of composition and structure. The main difference is the carbonyl functional group, which makes it slightly polar. This part of the molecule is hydrophilic and makes some aldehydes soluble in water.

However, the solubility or hydrophilic properties of aldehydes are limited to five carbon atoms. If an aldehyde has more than five carbon atoms, the hydrophobic nature of the hydrocarbon substituent takes over, making the substance hydrophobic.


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