What is a Carbonyl Group?

Carbonyl Group (>C=O)

A carbonyl group is a carbon doubly bonded to oxygen, written as >C=O. The carbon is sp2-hybridised and electrophilic. The oxygen holds two lone pairs and is nucleophilic. This unit features in aldehydes, ketones, carboxylic acids and many related families.

Geometry & Polarity of Carbonyl Group

Fig 8.1 Orbital diagram for carbonyl group

Fig 8.1  sp2 carbon showing p–π overlap with oxygen; electron cloud sits above & below the planar bonds.

What the diagram tells us

Carbonyl carbon is sp2-hybridised; three σ-bonds spread in one plane at about 120°.

A perpendicular p–p overlap forms the π bond, creating a flat yet rigid >C═O framework.

Resonance shifts electron density toward oxygen, polarising the bond and setting up a strong dipole.

Key Points:

  • Trigonal planar geometry; bond angles ≈ 120°.
  • π electron cloud lies above and below the plane.
  • Resonance form B: δ+ C, δ− O.
  • Dipole moment ≈ 2.3 D—more polar than ethers.

Aldehydes vs Ketones

Aldehyde

Terminal carbonyl; bonded to a carbon and ≥1 H atom.
General formula \( \mathrm{R{-}CHO} \).
Example: Ethanal \( \mathrm{CH_3CHO} \).

Ketone

Internal carbonyl; bonded to two carbon atoms.
General formula \( \mathrm{R{-}CO{-}R'} \).
Example: Propanone \( \mathrm{CH_3COCH_3} \).

Key Similarities

Both possess the polar carbonyl group \(>\!C{=}O\).
Undergo nucleophilic addition reactions.

Meet the Carboxyl Group

Carboxyl Group (–COOH)

A carbonyl carbon bonded to hydroxyl oxygen. This dual unit defines carboxylic acids and underpins their characteristic acidity.

Key Characteristics:

  • Formula –COOH: carbonyl \(C=O\) plus hydroxyl \(–OH\).
  • Shows both carbonyl (electrophilic) and hydroxyl (proton-donating) behaviour.
  • Resonance spreads negative charge over two oxygens, stabilising the conjugate base.
  • Resonance lowers carbonyl electrophilicity yet increases overall acidity.

Example:

In acetic acid, \(CH_{3}COOH\), deprotonation gives acetate \(CH_{3}COO^{-}\) stabilised by resonance over both oxygens.

IUPAC Nomenclature: Aldehydes & Ketones

Follow these five steps to name any simple carbonyl compound correctly.

1

Choose Parent Chain

Select the longest continuous chain containing the carbonyl carbon; this becomes the parent alkane.

2

Number the Chain

Give carbonyl carbon position 1 in aldehydes, or start from the nearer end in ketones.

3

Change the Suffix

Replace the terminal “-e” with “-al” for aldehydes or “-one” for ketones.

4

Add Substituents

List substituent names alphabetically with their locants; separate numbers by commas.

5

Name Ring Systems

On a ring, name the ring then add “carbaldehyde”; ring ketones are numbered with carbonyl as C-1.

Pro Tip:

Check locants—choose the set of numbers with the lowest overall value.

IUPAC Nomenclature: Carboxylic Acids

Follow these steps to derive systematic names for mono- and di-carboxylic acids.

1

Find Parent Chain

Choose the longest carbon sequence that contains every ‑COOH group.

2

Number the Chain

Assign carbon 1 to each carboxyl carbon; continue numbering for lowest possible locants.

3

Change the Suffix

Replace the terminal “-e” of the parent alkane with “-oic acid”.

4

Apply Multiplicative Prefix

For two or more carboxyl groups, add di-, tri-, etc., and state their positions: e.g., butane-1,4-dioic acid.

5

List Substituents

Prefix other groups alphabetically with their locant numbers before the base acid name.

Physical Property Highlights

Boiling Points • Solubility • Odour

Aldehydes & Ketones

Dipole-dipole forces lift b.p. above alkanes/ethers but, lacking H-bonding, stay below alcohols (see data p 235).

Carboxylic Acids

Strong hydrogen-bonded dimers double effective mass, giving the highest boiling points within the carbonyl family.

Water Solubility Trend

C₁–C₄ members mix completely; added CH₂ units weaken polarity, so solubility drops rapidly along the series.

Odour Evolution

High volatility C₁–C₃ acids and aldehydes smell sharp; larger or aromatic ones, less volatile, become mild and fragrant.

Identify the Family

Drag each structural formula to the correct functional-group family. Spot the carbonyl clues, then decide: aldehyde, ketone or carboxylic acid.

Draggable Items

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Drop Zones

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Tip:

Aldehyde = C=O linked to H; Ketone = C=O flanked by two carbons; Carboxylic acid = C=O bonded to –OH.

Key Takeaways

Carbonyl group \(>\!C{=}O\) is the common core of aldehydes, ketones and carboxylic acids.

Aldehyde has a terminal –CHO; ketone holds an internal C=O flanked by carbons.

Carboxyl group \(-\text{COOH}\) adds –OH to carbonyl, making acids stronger than alcohols or phenols.

IUPAC snapshots: aldehyde → –al, ketone → –one, carboxylic acid → –oic acid.

Boiling point trend: acids > ketones > aldehydes ≫ alkanes, driven by H-bonding and dipoles.

Collage of aldehyde, ketone & acid structures fading into a check-mark icon

Next Steps

Practise drawing each functional group, apply the IUPAC endings, and predict boiling points in the exercise set.

Thank You!

We hope you found this lesson informative and engaging.