A group of extremely reactive chemical compounds, aldehydes are placed intermediately between acids and alcohols. While aldehydes contain fewer hydrogen atoms compared to alcohols, their oxygen content is also less compared to acids.
In most cases, aldehydes are irritants and may lead to irritation even when they are watered down. Broadly, there are two varieties of aldehydes - monoterpene aldehydes and cyclic aldehydes. Monoterpene aldehydes include citronellal, geranial and citral that have a sharp and characteristic citric smell. On the other hand, cyclic aldehydes, which are also called aromatic aldehydes are much more cloying and have a sweet smell like benzaldehyde and cinnamic aldehyde. In fact, benzaldehyde and cinnamic aldehyde are cyclic aldehydes that have extensive use in perfume industry.
German scientist Justus von Liebig coined the term aldehyde, which is basically a contraction of the Latin term alcohol dehydrogenauts (translated into English, it means dehydrogenated alcohol).
In chemistry, an aldehyde (having the chemical formula RCHO) is basically a carbonyl compound wherein the carbonyl group is bonded to one alkyl group (or carbon) and one hydrogen atom. Aldehydes get their names from the names of the analogous longest aliphatic hydrocarbon chain that contains the aldehyde group by substituting the ending-e with –al.
Compared to the corresponding alkanes, the boiling point of aldehydes is higher. On the other hand, it is lower compared to that of alcohols. Aldehydes that are relatively smaller are somewhat water soluble, but their solubility decreases as the length of the carbon chain increases. This is mainly owing to the presence of partially positive charge at carbonyl carbon in the aldehydes. In fact, aldehydes undergo reactions known as nucleophilic addition reactions.
Aldehydes, however, are not common in many natural building blocks like lipids, amino acids and nucleic acids perhaps owing to the formyl group’s high reactivity. Nevertheless, nearly all the sugars are derivatives of different aldehydes. These aldehydes are present in the form of hemiacetals - a variety of covered form of the parent aldehyde. For instance, aqueous solutions contain only a minute fraction of glucose that is present in the form of aldehyde.
Aldehydes belong to a family of highly reactive organic compounds that are naturally present in products such as vanilla bean (vanillin) and cinnamon bark (cinnamaldehyde). In addition, aldehydes can also be produced in laboratories. For instance, a number of aldehyde compounds like acetaldehyde and formaldehyde are general building blocks for making other chemicals and they are widely used in perfumes, dyes, resins and organic acids.
Aldehydes have a double bond of carbon-oxygen and they are made up of one sigma (Σ) and one pi (π) bond, very similar to the double bonds of alkenes. Both, the carbon as well as oxygen is sp2hybridized. The overlapping of two sp2hybridized orbitals oxygen and carbon results in the formation of one σ bond. Similarly, of two unhybridized p orbitals of the two atoms results in the formation of the π bond. The two other orbitals of carbon bond with single hydrogen and one alkyl group each (the hydrogen bonds with carbon orbital for formation of formaldehyde). On the other hand, the unshared electron pairs bond with the two sp2hybrid oxygen orbitals.
It is interesting to note that the electron pairs that are shared by the carbonyl group's carbon and oxygen are not shared equally. In fact, they are drawn to the more electronegative oxygen atom and, in this way, they are somewhat negatively charged.
Forming aldehyde by hydroformylation is considered to be the most dominating process. In this technology, formyl group (-CHO) and hydrogen are appended with the carbon-carbon double bond. In this case, hydrogen and carbon monoxide are added to alkane to form aldehyde.
All aldehydes are basically polar compounds (compounds with distinct regions of positive and negative charge) owing to the carbonyl group's presence. In other words, the electron pairs that are shared by oxygen and carbon in the carbonyl group are drawn towards the oxygen atom which is more electronegative. Therefore, it is somewhat negatively charged. Owing to such polarity, there are intermolecular dipole-diapole attractions between the carbonyl oxygen’s partially negative charge of one molecule and the partial positive charge of another molecule's carbonyl carbon.
The boiling point of all aldehydes is much higher compared to the corresponding alkanes owing to such diapole-diapole attractions. Contrarily, since this attraction is weaker compared to hydrogen bonds, the boiling points of aldehydes are much lower compared to corresponding alcohols.
As discussed earlier, the small aldehydes are somewhat water soluble in water. However, their solubility decreases as the length of the carbon chain increases. While aldehydes are not able to bind with intermolecular hydrogen chain, they possess the ability to form hydrogen bond with water molecules. An aldehyde molecule contains a solitary pair of oxygen which has the ability to form a bond with hydrogen - with a hydrogen atom in water molecule that has a partially positive charge.
With the increase in the length of carbon chain, the aldehydes' hydrocarbon portion causes interference in the formation of hydrogen bond between water and aldehyde. As a result, the solubility of these aldehydes decreases.
As of date, for main varieties of aldehydes are known to us. These include formaldehyde, acrolein, benzaldehyde and acetaldehyde. Formaldehyde is made up of carbon molecules, carbon and oxygen. These aldehydes have different forms and they all have individual uses that not only offer benefits to the society, but are also beneficial for the industry.
Aldehydes go through a wide range of chemical reactions, which include polymerization. When they combine with various other molecules, it results in the production of the supposed aldehyde condensation polymers. These polymers have been utilized in plastics like Bakelite. In addition, they have been used in Formica, the material used to laminate tabletop. Aldehydes are also good solvents and form important ingredients in perfumes. They are also used in the form of intermediates in production of pharmaceuticals and dyes.
Specific types of aldehydes are also engaged in different physiological processes. For instance, retinal (which is a vitamin A aldehyde) is vital for the health of human eyes. It improves vision. Moreover, pyridoxal phosphate is a type of vitamin B6. In addition, glucose as well as other purported reducing sugars are also aldehydes. Similarly, many natural and synthetic hormones are also different forms of aldehydes.
Aldehydes have multiple uses, especially in the industry. They can be found in several organic compounds - in almost all things from citronella, orange rind, rose and vanilla. In addition, aldehydes can also be produce synthetically and used as ingredients in colognes and perfumes having a sweet odour. For instance, Russian perfumer Ernest Beaux created Chanel No. 5, which is basically a blend of aromatic aldehydes and it became very popular of their widespread use in various perfumes.
Aldehydes are considered to be multipurpose compounds that can be used for making dyes, resins and organic acids. In addition, they are vital ingredients in perfumes and colognes and used for making soaps and detergents. Among all types of aldehydes, formaldehyde is produced in large scale in industries. Formaldehyde is an excellent preservative and possesses anti-bacterial properties. In addition, formaldehyde chemistry comprises a wide range of products, which include composite as well as engineered wood products that are used in construction and building.