The Aldehydes Are a class of organic compounds in which a carbon atom shares a double bond with an oxygen atom, a single bond with a hydrogen atom and a single bond with another atom or group of atoms, designated R in the chemical formulas General and structure diagrams (figure 1).
The double bond between carbon and oxygen is characteristic of all aldehydes and is known as the carbonyl group. Many aldehydes have pleasant odors, and in principle, they are derived from alcohols by dehydrogenation (removal of hydrogen), from which process comes the name aldehyde.
Figure 1: general structure of the aldehydes.
Aldehydes undergo a wide variety of chemical reactions, including polymerization. Its combination with other types of molecules produces so-called aldehyde condensation polymers, which have been used in plastics such as bakelite and table-top (Formica).
Aldehydes are also useful as solvents and perfume ingredients and as intermediates in the production of dyes and pharmaceuticals. Certain aldehydes are involved in physiological processes.
Some examples are, the retina (vitamin A: retinal aldehyde), important in human vision, and pyridoxal phosphate, one of the forms of vitamin B6.
Glucose and other so-called reducing sugars are aldehydes, as are several natural and synthetic hormones (Jerry March, 2015).
Nomenclature of aldehydes
The word aldehyde appears to have arisen from dehydrogenated alcohol. In the past, aldehydes were sometimes referred to as the corresponding alcohols, eg vinous aldehyde for acetaldehyde (Vinous is from Latin vinum = wine, the traditional source of ethanol.).
IUPAC prescribes the following nomenclature for aldehydes:
Acyclic aliphatic aldehydes are referred to as derivatives of the longer carbon chain containing the aldehyde group. Thus, HCHO is named as a derivative of methane, and CH 3 CH 2 CH 2 CHO is named as a derivative of butane.
The name is formed by changing the -ano suffix Alkane Parental to -al, so that HCHO is called methanal and CH 3 CH 2 CH 2 CHO is called butanal (New World Encyclopedia, 2016).
In other cases, such as when a -CHO group is attached to a ring, the suffix -carbaldehyde may be used. So C 6 H eleven CHO is known as cyclohexanecarbaldehyde.
If another functional group is present that the IUPAC rules prescribe should be named as a suffix, the aldehyde group is named with the prefix formil-. This prefix is preferred to the prefix metanoyl- (IvyRoses, 2003).
For example:
HCHO may be referred to as formaldehyde (methanal, methanoyl aldehyde).
CH 3 CHO may be called acetaldehyde (ethanal, ethanoyl aldehyde).
C 6 H 5 CHO may be referred to as benzaldehyde (benzanal, benzanoyl aldehyde).
Properties
Aldehydes, as well as ketones, are simple compounds containing a carbonyl group - a carbon-oxygen double bond.
They are simple in the sense that they do not have other reactive groups such as -OH or -Cl attached directly to the carbon atom in the carbonyl group -as it could be found, for example, in carboxylic acids containing -COOH.
Oxygen is much more electronegative than carbon and therefore has a strong tendency to draw electrons into a carbon-oxygen bond towards itself.
One of the two pairs of electrons that form a carbon-oxygen double bond is even easier to drag into oxygen. This makes the carbon-oxygen double bond very polar.
Figure 2: polarity of carbonyl carbon due to the difference in electronegativity.
The slightly positive carbon atom in the carbonyl group can be attacked by nucleophiles. A nucleophile is a negatively charged ion (eg, a cyanide ion, CN-), or a slightly negatively charged part of a molecule (eg the solitary pair on a nitrogen atom in ammonia, NH3).
During the reaction, the carbon-oxygen double bond is broken. The net effect of all this is that the carbonyl group undergoes addition reactions, often followed by the loss of one molecule of water. This gives a reaction known as removal of addition or condensation (Clark, 2015).
Both aldehydes and ketones contain a carbonyl group. This means that their reactions are very similar in this respect. Both molecules exist in tautomeric equilibrium (migration of a hydride ion H - ) With the enol form.
The enol form is defined as a Alkene With a hydroxyl group attached to one of the carbon atoms that make up the double bond.
Figure 3: aldo enol balance.
The aldo and keto forms predominate in equilibrium for most aldehydes and ketones. However, the enol form is important for some reactions because the deprotonated desolate form is a strong nucleophile.
The equilibrium is strongly driven thermodynamically, and at room temperature the aldo and keto forms are favored. Interconversion can be catalyzed by the presence of an acid or a base (Boundless, S.F.).
Applications
Low molecular weight aldehydes, for example formaldehyde and acetaldehyde, have acute and unpleasant odors. Higher molecular weight aldehydes, eg benzaldehyde and furfural, have pleasant odors, often floral, and are found in the essential oils of certain plants.
Aldehydes are used for the manufacture of synthetic resins. Bakelite and for the manufacture of dyes, flavors, perfumes and other chemicals. Some are used as preservatives and disinfectants.
Perfumes that use aldehydes can be floral, fruity or citrus in nature. Fatty aldehydes contain long chains of carbon atoms attached to an aldehyde group.
They have 8-13 carbon atoms in their molecular formula. Fatty aldehydes have a very pleasant odor, with a fruity or floral aroma and can be detected in very low concentrations (Ali, S.F.).
Examples
Formaldehyde
Highly reactive aldehyde gas formed by oxidation or incomplete combustion of hydrocarbons.
In solution, it has a wide range of uses: in the manufacture of resins and textiles, as a disinfectant, and as a laboratory fixative or preservative (The Metabolomics Innovation Center, 2017).
Acetaldehyde
It is a colorless and flammable liquid used in the manufacture of acetic acid, perfumes and flavors. It is also an intermediate in the metabolism of alcohol .
It has a general narcotic action and also causes irritation of the mucous membranes. Large doses can cause the death of respiratory paralysis (National Center for Biotechnology Information., 2004).
Propionaldehyde
Propionaldehyde or propanal, is a clear colorless liquid with a strong fruity odor. It is used in the manufacture of plastics, in the synthesis of rubber chemicals, and as a disinfectant and preservative.
Butyraldehyde
It is a flavoring agent and cosmetic fragrance. It has a chocolate smell and a cocoa flavor.
Glucose
Glucose is a carbohydrate found in the body. Its degradation produces energy in the form of ATP and its presence activates the segregation of insulin which activates enzymes of catabolism.
Benzaldehyde
It is an aromatic aldehyde bearing a single formyl group with an almond smell. Benzaldehyde can be derived from natural sources and is widely used by the chemical industry in the preparation of various aniline dyes, perfumes, flavorings and pharmaceuticals.
Cinnamaldehyde
It is the aldehyde that gives cinnamon its taste and smell. Cinnamaldehyde occurs naturally in the bark of cinnamon trees and other species of the genus Cinnamomum such as camphor and cassia (Burnham, S.F.).
References
- The Metabolomics Innovation Center. (2017, May 19). Formaldehyde . Taken from drugbank: drugbank.ca.
- Ali, N. S. (S.F.). Aldehydes . Taken from fragrantica: fragrantica.com.
- (S.F.). Aldehydes and Ketones . Taken from boundless: boundless.com.
- Burnham, P. M. (S.F.). CINNAMALDEHYDE . Taken from chm.bris.ac.uk: chm.bris.ac.uk.
- Clark, J. (2015, November). INTRODUCING ALDEHYDES AND KETONES . Taken from chemguide.co.uk: chemguide.co.uk.
- (2003). Naming Aldehydes . Taken from ivyroses: ivyroses.com.
- Jerry March, W. H. (2015, December 31). Aldehyde . Taken from britannica: britannica.com.
- National Center for Biotechnology Information. (2004, September 16). PubChem Compound Database; CID = 177 . Taken from PubChem: pubchem.ncbi.nlm.nih.gov.
- New World Encyclopedia. (2016, February 25). Aldehyde . Taken from newworldencyclopedia: newworldencyclopedia.org.