What are aldehydes?


The word aldehyde comes from the Latin dehydrogenatum which means dehydrogenated alcohol . In this sense, aldehydes are organic compounds that are characterized by having the functional group -CHO . Unlike the other functional groups that contain a carbonyl group C = O, aldehydes are only linked to a radical and through a different bond, to a hydrogen .


What are aldehydes?

Aldehydes are organic compounds that have the functional group CHO and whose function is the carbonyl group -CO and whose formula is RCHO and which are obtained from the oxidation of primary alcohols.

  • Characteristics of aldehydes
  • Classification
  • Structure
  • Formula
  • Nomenclature
  • Physical properties of aldehydes
  • Chemical properties
  • Importance
  • Obtaining
  • Reactions
  • Uses of aldehydes
  • Examples

Characteristics of aldehydes

The main characteristics of aldehydes are:

  • They have a CHO functional group .
  • They are called after the corresponding alcohols and can change their termination.
  • They are obtained from the mild oxidation of primary alcohols .
  • Its reaction is nucleophilic addition .
  • In the presence of a carbonyl they become polar compounds .
  • They are soluble in water and insoluble in organic solvents .


They are classified by family, in this way we have:

  • 1st family (Cn H2n O):  methyl or formic, ethyl or acetic, propyl, butyl, valeric, enantilic.
  • 2nd family (Cn H2n-2 O):  allelic or acrolein, protonic, isocapric.
  • 3rd family (Cn H2n-4 O):  no aldehydes of this group are known.
  • 4th family (Cn H2n-6 O):  no aldehydes of this family are known.
  • 5th family (Cn H2n-8 O):  benzyl, toluic, cuminic.
  • 6th family (Cn H2n-10 O):  cinnamic aldehyde (essence of cinnamon).
  • 7th family (Cn H2n-14 O):  isonaphthoic aldehyde.


Its structure is flat trigonal since its main carbon has hybridization with 3 sp2 atomic orbitals and 1 p orbital. These orbitals are united with the atomic orbitals of the other carbons that are 1sp2-s, with hydrogen , 1sp2-sp2 and 1p-p, both with oxygen and, finally, 1sp2-sp3, with the radical . So the aldehydes have 3 sigma bonds and one pi bond and in this way the angles that form the bonds of the carbon with the three bonded atoms is approximately 120.


The general formula of aldehydes is: CnH2n + 1CHO

where n = 0, 1, 2, 3, 4, … corresponds to the number of carbon atoms of the hydrocarbon.


With respect to the nomenclature, the ending “-al” is added to the equivalent hydrocarbon. If there are two aldehyde groups, the ending “-dial” is added.  For three or more groups the prefix «formil-« is used.  There is also a traditional nomenclature for the most common aldehydes:

  • HCHO Methanal → Formaldehyde
  • CH3-CH2-CH2-CHO Butanal → Butyraldehyde or Butyl Aldehyde

Physical properties of aldehydes

Those with few carbons have characteristic odors . For example, methanal is watery and gaseous. Most of them are liquids and the rest are solid. Their points of boiling are less than those with the respective alcohols having the same number of carbons. They have a lower density than water and the smallest ones have some solubility in water, but this decreases as the amount of carbons increases.

Acetaldehyde has a molecular weight of 44 and a boiling point of 21 ° C, while ethanol of weight 46 boils at 78 ° C.

Chemical properties

They have good reactivity. They exhibit addition , substitution, and condensation reactions .

Addition : happens when hydrogen is added and a primary alcohol is formed.

Substitution with halogens : it happens in aldehydes when they react with chlorine giving acid chlorides by substitution of the hydrogen of the carbonyl group .

They also behave as a reducing agent by oxidation of acids having the same number of carbon atoms. The reaction of aldehydes and ketones is known as nucleophilic addition .


They have a great influence on the world economy because thanks to them the perfume industry , for example, is so important. Alcohol is very important in the manufacturing of products in industries. These must be versatile in their reactions and accessible in price. Aldehydes are the main generators of simple alcohols .


They can be prepared basically by oxidation smooth the priority alcohols i os . They can also be obtained through the following processes:

  • Oxidation of alcohols : the direct oxidation of primary alcohols to carboxylic acids usually takes place through the corresponding aldehyde, which is then transformed by the reaction it has with water into an aldehyde hydrate.
  • Hydration of alkynes : two-carbon alkynes are hydrated and in this way can form corresponding aldehydes; the other alkynes are hydrated and always form ketones. This reaction proceeds the mechanism known by the name of tautomerization .
  • Acyl Halide Reduction : Acyl halides reduce with hydrogen when in the presence of palladium and vario sulfate and produce aldehydes.
  • Catalytic reduction or hydrogenation : aldehydes and ketones can undergo a direct addition with hydrogen , forming primary and secondary alcohols


The reactions that aldehydes can have are the following:

  • Oxidation : when they react with a strong oxidant in an acid medium, carboxylic acid is formed.
  • Reduction : when joined with lithium aluminum hydride a primary alcohol is produced.
  • Hydration : in an aqueous medium a hydrate can be transformed into aldehyde again
  • Acetylation: in an alcohol medium, the acetylation of the carbonyl occurs, which can be hemiacetal or diacetal and the aldehyde can be obtained by acid hydrolysis .
  • Reaction with Grignard reagents : nucleophilic addition then occurs, producing a secondary alcohol.
  • Wittig reaction : this reaction produces alkenes.
  • Aldol condensation : the hydrolysis of compounds occurs, forming unsaturated aldehydes.
  • Cyanide addition : cyanhydrins are produced.
  • Betty’s reaction : it is an organic reaction between an aldehyde, an aromatic amine and a phenol producing an α-aminobenzylphenol.
  • Formation of semicarbazones : in an acid medium it can produce semicarbazones.

Uses of aldehydes

Aldehydes can have the following uses :

  • In the manufacture of plastics , resins and acrylic products .
  • They are used in the photographic industry ; explosive and coloring.
  • They work very well as antiseptics and preservatives .
  • As a herbicide , fungicide and pesticide .
  • In accelerating vulcanization .
  • In the food and perfume industry .
  • Industry textile and pharmaceutical .
  • Feed production .
  • In 40% aqueous solution it is called formaldehyde and is used in industry to preserve wood, leather and in taxidermy.
  • Ethanal is used in the manufacture of mirrors (Tollens reaction and in the preparation of acetic acid .


Some examples of aldehydes are as follows:

  • Formaldehyde or Formol
  • Acetaldehyde or Ethanal
  • Propionaldehyde or Propanal
  • Butyraldehyde or Butanal
  • Pentanal
  • Hexanal
  • Benzaldehyde
  • Tolualdehyde
  • Salicylaldehyde
  • Phenyl acetaldehyde

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