Aldehydes and ketones are organic compounds which incorporate a carbonyl functional group, C=O.

Another class of organic molecules contains a carbon atom connected to an oxygen atom by a double bond, commonly called a carbonyl group. The trigonal planar carbon in the carbonyl group can attach to two other substituents leading to several subfamilies including aldehydes and ketones. Both contain a carbonyl group, a functional group with a carbon-oxygen double bond. The names for aldehyde and ketone compounds are derived using similar nomenclature rules as for alkanes and alcohols, and include the class-identifying suffixes –al and –one, respectively:

In an aldehyde, the carbonyl group is bonded to at least one hydrogen atom. In a ketone, the carbonyl group is bonded to two carbon atoms:

As text, an aldehyde group is represented as –CHO; a ketone is represented as –C(O)– or –CO–.

When naming compounds, aldehyde nomenclature follows -al and -aldehyde suffixes, and ketones follow and oxo- prefix, and the -one suffix:

• When naming compounds with an aldehyde group present, it is appropriate to replace the ending of a compound with -al (for example, methanal). In groups where there is a common name or parent chain, the suffix -aldehyde is also used (for example, formaldehyde).
• When naming compounds with a ketone group present, it is appropriate to replace the ending of a compound with -one (for example, propanone). In groups where there is a common name or parent chain, the suffix -aldehyde is also used (for example, diphenyl ketone).
• When naming compounds with both an aldehyde and ketone group present, it is appropriate to assign the ketone group with an oxo- prefix and to assign the aldehyde with the -al suffix.

In both aldehydes and ketones, the geometry around the carbon atom in the carbonyl group is trigonal planar; the carbon atom exhibits sp² hybridization. Two of the sp² orbitals on the carbon atom in the carbonyl group are used to form σ bonds to the other carbon or hydrogen atoms in a molecule. The remaining sp² hybrid orbital forms a σ bond to the oxygen atom. The unhybridized p orbital on the carbon atom in the carbonyl group overlaps a p orbital on the oxygen atom to form the π bond in the double bond. Like the C=O bond in carbon dioxide, the C=O bond of a carbonyl group is polar (recall that oxygen is significantly more electronegative than carbon, and the shared electrons are pulled toward the oxygen atom and away from the carbon atom). Many of the reactions of aldehydes and ketones start with the reaction between a Lewis base and the carbon atom at the positive end of the polar C=O bond to yield an unstable intermediate that subsequently undergoes one or more structural rearrangements to form the final product.

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Key Points

• Aldehydes and ketones are organic compounds which incorporate a carbonyl functional group, C=O.

• In an aldehyde, the carbonyl group is bonded to at least one hydrogen atom. In a ketone, the carbonyl group is bonded to two carbon atoms.

• As text, an aldehyde group is represented as –CHO; a ketone is represented as –C(O)– or –CO–.

• When naming compounds, aldehyde nomenclature follows -al and -aldehyde suffixes, and ketones follow and oxo- prefix, and the -one suffix.

• When naming compounds with both an aldehyde and ketone group present, it is appropriate to assign the ketone group with an oxo- prefix and to assign the aldehyde with the -al suffix.

• In both aldehydes and ketones, the geometry around the carbon atom in the carbonyl group is trigonal planar; the carbon atom exhibits sp² hybridization.

• Like the C=O bond in carbon dioxide, the

bond to yield an unstable intermediate that subsequently undergoes one or more structural rearrangements to form the final product.

Practice Questions

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Key Terms

Aldehyde: Organic compound containing a carbonyl group bonded to two hydrogen atoms or a hydrogen atom and a carbon substituent.

ketone: Organic compound containing a carbonyl group with two carbon substituents attached to it.

Polarized: Separation of positive and negative charges. In the case of carbonyl, the more negative charges go toward the oxygen and the more positive charges go toward the carbon.

Trigonal planar: In molecular geometry model with one atom at the center and three atoms at the corners of an equilateral triangle, called peripheral atoms, all in one plane.

Lewis Base: A compound or ionic species which can donate an electron pair to an acceptor compound.