Carbonyl Group

The carbonyl group is somewhat polar. That means that one end of it (the carbon atom) has a slight positive electric charge, and one end of it (the oxygen atom) has a slight negative charge. This makes the entire molecular a polar molecule.

The polar nature of the carbonyl part of the molecule affects the physical properties of the entire molecule. For instance, small ketone molecules, with fewer than six carbon atoms in all, are soluble in water, a very polar solvent. At the same time, small ketone molecules are themselves often good solvents for other compounds with polar groups. This is in contrast to small hydrocarbon molecules with no carbonyl group—they are insoluble in water, and they won't dissolve other polar molecules.

A carbonyl group in a molecule is often the most chemically reactive portion. When a molecule containing a carbonyl group undergoes a chemical reaction, it is often this polarity that controls which reaction will take place. Usually a chemical reaction in a molecule containing only a carbonyl group and hydrocarbon groups will take place at the carbonyl group.