Nomenclature and Structure of Carbonyl Group

Full Notes

Nomenclature

In this unit, we explore organic compounds that contain the carbon–oxygen double bond (C=O), known as the carbonyl group. In aldehydes, the carbonyl carbon is bonded to one hydrogen and one carbon atom, while in ketones, it is bonded to two carbon atoms.

When the carbonyl carbon is also bonded to the oxygen of a hydroxyl group (–OH), the compound is classified as a carboxylic acid. If it is bonded to the nitrogen of an amine group (–NH₂) or to a halogen, the compounds are called amides and acyl halides, respectively. Esters and anhydrides are considered derivatives of carboxylic acids. The general formulas of these compound classes are shown below.

Common Names

Aldehydes:

Derived from corresponding carboxylic acid names by replacing “-ic acid” with “-aldehyde”.

The position of substituents is denoted using Greek letters: α (adjacent to –CHO), β, γ, etc.

Examples Common aldehyde names

• CH₃CHO = Acetaldehyde
• C₆H₅CHO = Benzaldehyde
• BrCH₂CH(CH₃)CH₂CHO = β-Bromobutyraldehyde

Ketones:

Named by combining names of alkyl/aryl groups bonded to the carbonyl group.

Substituent positions: α, α′, β, β′ (starting from carbon next to C=O).

Examples Common ketone names

• CH₃COCH₃ = Acetone
• C₆H₅COCH₃ = Acetophenone

IUPAC Names

Aldehydes:

Replace “-e” of the parent alkane with “-al”. Numbering starts from carbon of the aldehyde group.

Aldehydes on Rings: Add suffix carbaldehyde (e.g., cyclohexanecarbaldehyde).
Aromatic aldehyde: Benzene with –CHO is named benzenecarbaldehyde (IUPAC), but benzaldehyde is also accepted.

Ketones:

Replace “-e” with “-one”. Numbering starts from end closer to carbonyl group.

Cyclic ketones: Numbering starts at the carbon bearing the carbonyl group.

Structure of the Carbonyl Group

The carbonyl group (C=O) is a fundamental functional group in organic chemistry.

The carbonyl carbon is sp² hybridised, forming three sigma bonds:

• One with the oxygen atom (σ of C=O)
• Two with adjacent atoms (R groups or H)

The C=O bond consists of:

• One sigma bond (sp²–sp² overlap)
• One pi bond (sideways p–p overlap)

Geometry: Trigonal planar around the carbonyl carbon
➤ Bond angle ≈ 120°

Bond Polarity: Oxygen is more electronegative and pulls electron density toward itself.

The carbon gains partial positive charge (δ⁺), oxygen becomes partially negative (δ⁻). This makes the carbonyl carbon susceptible to nucleophilic attack.

Summary

• Aldehydes and ketones both contain the carbonyl (C=O) group.
• Aldehydes have –CHO, ketones have –CO– between carbons.
• Common names are derived from acids or alkyl groups; IUPAC uses “-al” and “-one”.
• The carbonyl carbon is sp² hybridised, giving trigonal planar geometry.
• C=O bond polarity makes the carbon electrophilic and prone to nucleophilic attack.