The Carbonyl Group

Aldehydes and ketones both contain the carbonyl group.

The carbonyl group is the functional group responsible for the properties of ketones and aldehydes and consists of an atom of carbon double-bonded to an atom of oxygen.

The difference between the two classes of compound is the location of the carbonyl group within the molecule.

aldehyde and ketone structure | carbonyl functional group

In an aldehyde, the carbonyl group is bonded to at least 1 hydrogen atom and located at the end of the parent chain of the molecule.

A ketone is an organic compound whose molecules have a carbonyl group bonded to 2 carbon atoms in the carbon chain.

Naming Aldehydes and Ketones

For an aldehyde, replace the final -e from the name of the parent alkane with the suffix -al.

To name a ketone, the -e of the parent alkane is replaced with the suffix -one.

Properties of Aldehydes and Ketones

The chemical and physical properties of aldehydes and ketones are affected by the carbonyl group.

The molecules are polar.

The presence of the carbonyl oxygen does allow to form attractions with water molecules. These molecules are more soluble in water than are alkanes, but less soluble than are similarly sized alcohol molecules.

Small aldehydes and ketones are completely soluble in water, but the solubility decreases as additional carbons are added to the chain.

Aldehydes and ketones are often used as solvents in industrial processes.

Reactions Involving Aldehydes and Ketones

Controlled oxidation of alcohol

Aldehydes and ketones are synthesized by the controlled oxidation of alcohol.

In a complete oxidation reaction (combustion reaction) of alcohol, the products are carbon dioxide and water. 

In the controlled oxidation of an alcohol, the reactant is not completely oxidized. In these types of reactions, oxygen atoms can be supplied by air or by compounds known as oxidizing agents

Oxidizing agents are oxygen-rich compounds such as potassium dichromate (K2Cr2O), hydrogen peroxide (H2O2), and potassium permanganate (KMnO4).

Alcohol controlled oxidation

When a primary alcohol is oxidized, the oxygen atom and one of the hydrogen atoms remain on the carbon atom, resulting in a carbonyl group on the terminal carbon atom, an aldehyde.

A secondary alcohol will also form a carbonyl group, producing a ketone rather than aldehyde.

Tertiary alcohols do not react in the same way because no hydrogen atom is available on an adjacent carbon atom.

To summarize the controlled oxidation reactions of alcohols:

  • a primary alcohol produces an aldehyde,
  • a secondary alcohol produces a ketone
  • a tertiary alcohol does not easily oxidize.

Hydrogenation of Aldehydes and Ketones

In the hydrogenation reaction of an aldehyde or a ketone, the net result is that a double bond is broken.

One hydrogen atom is added to the oxygen atom and another is added to the carbon atom to which the oxygen is bonded.

Hydrogenation of Aldehydes and Ketones

Hydrogenation of aldehydes or ketones occurs only under conditions of high temperature, pressure, and the presence of a catalyst.

The product of this reaction is an alcohol.