Aldehydes and Ketones are molecules of carbon, oxygen and hydrogen that have, as part of their structure a reactive group called a carbonyl group, typically written C=O.

The carbon atom in a carbonyl group can still form two additional covalent bonds. If this carbon atoms is joined with two other carbon atoms then the compound formed is a ketone, but if this carbon is joined with a hydrogen, then the compound is an aldehyde.

The oxygen atom in the carbonyl group pulls the shared electrons towards it, causing an imbalance in the distribution of positive and negative charges. (The oxygen is said to be more electronegative). This polarity of charge has two effects on the physical properties of molecules containing this reactive group; aldehydes and ketones have higher boiling points, and they are more water soluble than similar hydrocarbons.

The unequal charges on the oxygen atom and the carbon atom of the carbonyl group also affects the kind and type of chemical reactions that these molecules undergo. For example, water will interact with the carbonyl group very, very rapidly forming a short lived intermediate with two -OH (hydroxyl groups) which then very rapidly breaks up into water and a carbonyl group once more.

The rapid addition of water (H-OH) to the carbonyl group produces a hydrate which is very unstable and cannot be isolated as a separate molecule or compound. However, if, instead of water, an alcohol (R-OH) is used a similar kind of chemical reaction takes place in which a hemiacetal is formed.

Most of these hemiacetals are not stable and quickly break down once more, however, there is at least one example where a molecule with five carbon atoms in a chain with an -OH group at one end, and an aldehyde group (-HCO) at the other end, forms a stable circular molecue; a cyclic hemiacetal.

This "internal" reaction between parts of a molecule has important and interesting consequences for organic, biological molecules such as saccharides and carbohydrates.