The slightly positive carbon atom in the carbonyl group can be attacked by nucleophiles. A nucleophile is a negatively charged ion (for example, a cyanide ion, CN-), or a slightly negatively charged part of a molecule (for example, the lone pair on a nitrogen atom in ammonia, NH3).
During the reaction, the carbon-oxygen double bond gets broken. The net effect of all this is that the carbonyl group undergoes addition reactions, often followed by the loss of a water molecule. This gives a reaction known as addition-elimination or condensation. You will find examples of simple addition reactions and addition-elimination if you explore the aldehydes and ketones menu (link at the bottom of the page).
Both aldehydes and ketones contain a carbonyl group. That means that their reactions are very similar in this respect.
Where aldehydes and ketones differ
An aldehyde differs from a ketone by having a hydrogen atom attached to the carbonyl group. This makes the aldehydes very easy to oxidise.
For example, ethanal, CH3CHO, is very easily oxidised to either ethanoic acid, CH3COOH, or ethanoate ions, CH3COO-.
Ketones don't have that hydrogen atom and are resistant to oxidation. They are only oxidised by powerful oxidising agents which have the ability to break carbon-carbon bonds.
You will find the oxidation of aldehydes and ketones discussed if you follow a link from the aldehydes and ketones menu (see the bottom of this page).
Physical properties
Boiling points
Methanal is a gas (boiling point -21°C), and ethanal has a boiling point of +21°C. That means that ethanal boils at close to room temperature.
The other aldehydes and the ketones are liquids, with boiling points rising as the molecules get bigger.
The size of the boiling point is governed by the strengths of the intermolecular forces.
van der Waals dispersion forces
These attractions get stronger as the molecules get longer and have more electrons. That increases the sizes of the temporary dipoles that are set up. This is why the boiling points increase as the number of carbon atoms in the chains increases - irrespective of whether you are talking about aldehydes or ketones.
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By William H. BrownSee All Last Updated: Oct 14, 2022 Edit History
Table of Contentsoxidation of alcohols
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Related Topics:furfural formaldehyde benzaldehyde citral acetaldehyde...(Show more)
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Summary
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aldehyde, any of 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 general chemical formulas and structure diagrams). The double bond between carbon and oxygen is characteristic of all aldehydes and is known as the carbonyl group. Many aldehydes have pleasant odours, and in principle, they are derived from alcohols by dehydrogenation (removal of hydrogen), from which process came the name aldehyde.
Aldehydes undergo a wide variety of chemical reactions, including polymerization. Their combination with other types of molecules produces the so-called aldehyde condensation polymers, which have been used in plastics such as Bakelite and in the laminate tabletop material 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. Examples are retinal (vitamin A 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.
Structure of aldehydes
In formaldehyde, the simplest aldehyde, the carbonyl group is bonded to two hydrogen atoms. In all other aldehydes, the carbonyl group is bonded to one hydrogen and one carbon group. In condensed structural formulas, the carbonyl group of an aldehyde is commonly represented as ―CHO. Using this convention, the formula of formaldehyde is HCHO and that of acetaldehyde is CH3CHO.
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Nomenclature of aldehydes
There are two general ways of naming aldehydes. The first method is based on the system used by the International Union of Pure and Applied Chemistry (IUPAC) and is often referred to as systematic nomenclature. This method assumes the longest chain of carbon atoms that contains the carbonyl group as the parent alkane. The aldehyde is shown by changing the suffix -e to -al. Because the carbonyl group of an aldehyde can only be on the end of the parent chain and, therefore, must be carbon 1, there is no need to use a number to locate it.
In the compound named 4-methylpentanal, the longest carbon chain contains five carbon atoms, and so the parent name is pentane; the suffix -al is added to indicate the presence of the aldehyde group, and the chain is numbered beginning at the carbonyl group. The methyl group is given the number 4, because it is bonded to the fourth carbon of the chain.
The other method of nomenclature for aldehydes, referred to as common nomenclature, is to name them after the common name of the corresponding carboxylic acid; i.e., the carboxylic acid with the same structure as the aldehyde except that ―COOH appears instead of ―CHO. The acids are usually given a name ending in -ic acid. Aldehydes are given the same name but with the suffix -ic acid replaced by -aldehyde. Two examples are formaldehyde and benzaldehyde.
As another example, the common name of CH2=CHCHO, for which the IUPAC name is 2-propenal, is acrolein, a name derived from that of acrylic acid, the parent carboxylic acid.