Carbonyl group

Ketones

Properties of the carbonyl group

Other molecules with carbonyl groups

Resources

A carbonyl group in chemistry is a group of atoms that consists of a carbon (C) atom covalently attached to an oxygen (O) atom by a double bond: C = O. The carbon atom, to satisfy its valence of 4, must also be attached by covalent bonds to two other atoms. The simplest type of molecule that contains a carbonyl group is a ketone. Other types of molecules that contain carbonyl groups are aldehydes, acids, esters, and amides.

A ketone is a compound whose molecules contain a carbonyl group and have two other groups attached to the carbon atom of the carbonyl group. There are many molecules that belong to this classification, but the simplest one is acetone. A condensed structural formula for acetone looks like the following formula.

In this formula, the C=O represents the carbonyl group, and the two CH3 groups satisfy the carbon atom’s valence of 4. In other molecules that contain the carbonyl group, the C=O is still present, but the two CH3 groups are traded for other atoms or groups of atoms.

Sometimes chemists need to talk about the entire class of possible ketone molecules. In this case, one uses a structural formula that looks like this formula.

In this picture, R and R’ can stand for any hydrocarbon-containing group and as CH3 -, C2 H5 -, etc.

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 excellent 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 will not 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.

KEY TERMS

Covalent bond— A chemical bond formed when two atoms share a pair of electrons with each other.

Double bond— A covalent bond consisting of two pairs of shared electrons that hold the two atoms together.

Hydrocarbon group— A portion of a molecule containing only carbon and hydrogen atoms.

Molecule— A single particle of a substance in which the atoms join together by covalent bonds.

Polarity— The distribution of electrical forces within a molecule. There are non-polar molecules, in which the electrical forces balance each other out, and polar ones, in which the complete molecule may still be neutral, but the electrical forces within it are not directed evenly throughout the molecule.

In many molecules that contain a carbonyl group, the other two groups of atoms are not hydrocarbon groups. Molecules like this are so different chemically that they belong to entirely different classifications. There are four major classes of molecules like this. Again, R stands for any hydrocarbon group. There is more information about these kinds of carbonyl-containing molecules in their entries in this encyclopedia.

See also Acids and bases; Amides; Ester.

BOOKS

Carey, Francis A. Organic Chemistry. Dubuque, IA: McGraw-Hill, 2006.

Hoffman, Robert V. Organic Chemistry: An Intermediate Text. Hoboken, NJ: Wiley-Interscience, 2004.

Stwertka, Albert. A Guide to the Elements. New York: Oxford University Press, 2002.

Tro, Nivaldo J. Introductory Chemistry. Upper Saddle River, NJ: Pearson Education, 2006.

G. Lynn Carlson