The carbonyl group

The essentials

The carbonyl group consists of a carbon atom joined to an oxygen atom by a double bond. The carbonyl carbon is sp² hybridised and forms three σ bonds lying in one plane at about 120° to each other; the π bond is formed by sideways overlap of a p orbital on carbon with a p orbital on oxygen. Because oxygen is more electronegative than carbon, the electrons of the C=O bond are pulled towards oxygen. The carbon therefore carries a partial positive charge and acts as an electrophilic (Lewis acid) centre, while oxygen carries a partial negative charge and acts as a nucleophilic (Lewis base) centre. This polarity makes carbonyl compounds polar and explains why nucleophiles attack the carbonyl carbon.

Key points

• Aldehydes: R–CHO; the carbonyl carbon is bonded to at least one H. Ketones: R–CO–R′; the carbonyl carbon is bonded to two carbon groups.
• IUPAC names: replace the final "-e" of the alkane by "-al" for aldehydes (ethanal, CH₃CHO) and "-one" for ketones (propanone, CH₃COCH₃). The aldehyde carbon is always C-1.
• Common names: formaldehyde (methanal), acetaldehyde (ethanal), acetone (propanone), benzaldehyde (C₆H₅CHO), acetophenone (C₆H₅COCH₃), benzophenone (C₆H₅COC₆H₅).
• When the –CHO group is attached to a ring, the suffix "carbaldehyde" is used, e.g. cyclohexanecarbaldehyde.
• In common names, positions of substituents are shown by Greek letters α, β, γ starting from the carbon next to the carbonyl group.
• The C=O bond is shorter and stronger than a C–O single bond and has a large dipole moment.

Exam tips

• Count the longest chain that contains the carbonyl carbon and number it so that this carbon gets the lowest possible number.
• Remember that the polarity of C=O is shown by a resonance form with C⁺ and O⁻.

Common mistakes

• Calling the carbon next to C=O the "α-carbon" in IUPAC numbering; in IUPAC names numbers are used, not Greek letters.
• Thinking the carbonyl carbon is sp³; it is sp² and trigonal planar.