Aldehydes are organic compounds that contain a carbonyl group in their molecule, which binds to a hydrogen atom and a radical (as can be seen in the image). The simplest aldehyde is formaldehyde, formed by the oxidation of methane. The name aldehyde comes from the words dehydrogenated alcohol and refers to one of the methods of obtaining it, namely the dehydrogenation of primary alcohols.



According to the IUPAC rules, the nomenclature is established as follows:
-Establishing the chain with the most carbon atoms.
-Adding the suffix -al to the name of the respective alkane: H-CHO-methanal, CH3-CHO ethanal, etc.
-In the case of aromatic aldehydes, they are named by adding the name of the aldehyde to the aromatic phenylmethane radical, or adding the name of the aldehyde radical to the hydrocarbon formylbenzene (rarely used nomenclature)

Other nomenclatures

More often used are those formed by the name of the acid with the same structure: H-CHO-formaldehyde, C6H5-CHO -benzaldehyde


-Dehydrogenation or oxidation of primary alcohols
-Swern Oxidation
Alcohols are oxidized to aldehydes via sulfoxides.
-Hydrolysis of halogenated compounds
-Addition of water according to Markovnikov's rule to alkynes
-Decomposition of calcium salts
-Reduction of esters
-Ozonolysis of alkenes or polyenes

Physical-chemical properties

Physical properties

With the exception of formaldehyde, which is a gas, the other aldehydes are liquids, having a characteristic smell. They can be soluble in water, but only the aldehydes with 5 carbon atoms, the rest not being able to form hydrogen bonds with water. This is also why they have points lower boiling points compared to the corresponding alcohols.

Spectral characterization

Spectre UV

The carbonyl group is a chromophore, due to the 2 non-participant electrons in the ps orbitals, more precisely 2ps. When n→π* type transitions (low energy transitions) occur, the absorption maxima in the UV spectra appear at wavelengths λ=270 -290nm. When the electron transition takes place from the π→π* orbitals, the absorption energies are higher, at wavelengths λ=190-200nm.

Spectre IR

Aldehydes (generally carbonyl compounds) show an absorption band 1700±40cm, a band that is greatly influenced by the physical state, by the (non)formation of hydrogen bonds.

Chemical properties

They largely depend on the hybridization of the atoms in the carbonyl group. Both the carbon atom and the oxygen atom have the same sp2 hybridization state, the angle between the 2 carbon substituents being 120


The sp2 hybridization is also found in alkenes, but unlike these, the O atom of the carbonyl group has non-participating electrons, in sp2 orbitals. This is due to this fact, as well as the large difference in electronegativity between carbon and oxygen. the C=O group has a very strong polar character, which allows it to participate in addition reactions of some polar compounds (water, organomagnesium compounds).

Addition reactions

Adding water:
The addition of alcohols takes place in an acidic environment following a type 1 SN1 nucleophilic substitution mechanism, the final product being called a hemiacetal
The addition of hydrogen leads to different compounds depending on the catalyst:
catalyst Ni Raney, at a temperature of about 500C, the formation of the corresponding primary or secondary alcohol takes place (in the case of the ketone):
catalyst TiCl3, when alkenes are formed (McMurry reaction)
Addition of hydracids HX.
The hydracids, especially the hydrochloric and hydrobromic ones, are added to the carbonyl group, giving rise to halohydrins, unstable compounds. They can react with the alcohols that are added to the reaction medium, thus forming α-halogenated ethers
The addition of hydrocyanic acid HCN leads to the formation of cyanohydrins
The reaction takes place by a nucleophilic addition mechanism, in basic catalysis with catalyst regeneration.
Addition of ammonia
The product is called ammonia-aldehyde adduct, an unstable compound that can polymerize.
Addition of sodium bisulfite
The addition of sodium bisulfite leads to the formation of bisulfite compounds, crystallized, insoluble in organic solvents, used for the isolation of carbonyl compounds:
Addition of chloroform
It takes place in the presence of KOH, following a nucleophilic mechanism:
Addition of organomagnesium compounds

Condensation reactions

-Condensation with aldehyde
Aldehydes can add another aldehyde molecule, in a basic environment forming a compound called aldol (aldehyde-alcohol)
Condensation with hydrocarbons:
Cyclopentadiene The condensation compounds are called fulvenes.