Nomenclature & Structure of the Carbonyl Group

The Carbonyl Group — Structure

Aldehydes, ketones and carboxylic acids all contain the carbonyl group, C=O. In an aldehyde the carbonyl carbon carries at least one hydrogen (R-CHO); in a ketone it is bonded to two carbon groups (R-CO-R').

Geometry and bonding: the carbonyl carbon is sp$^2$ hybridised and trigonal planar (bond angles ≈ 120°). The C=O bond is made of one σ bond and one π bond, like C=C, but it is strongly polar because oxygen is far more electronegative than carbon.

Polarity is everything: the bonding electrons are pulled toward oxygen, giving the carbon a partial positive charge (δ+) and the oxygen a partial negative charge (δ−). The carbon is therefore electrophilic (attacked by nucleophiles) and the oxygen is nucleophilic/basic (attacked by electrophiles or protons). This single fact controls the whole chapter.

Key Point: the carbonyl carbon is sp$^2$, planar and δ+ (electrophilic); the oxygen is δ−. Aldehyde = at least one H on C=O; ketone = two carbons on C=O.

Nomenclature of Aldehydes

Common names of aldehydes come from the common names of the corresponding carboxylic acids (replace -ic acid with -aldehyde): HCHO = formaldehyde, CH$_3$CHO = acetaldehyde, C$_6$H$_5$CHO = benzaldehyde.

IUPAC names (alkanal): replace the -e of the parent alkane with -al. The -CHO carbon is always C-1.

• HCHO → methanal
• CH$_3$CHO → ethanal
• CH$_3$CH$_2$CHO → propanal
• (CH$_3$)$_2$CHCHO → 2-methylpropanal

For a -CHO attached to a ring, add -carbaldehyde (e.g. cyclohexanecarbaldehyde, benzenecarbaldehyde = benzaldehyde).

Key Point: aldehyde IUPAC = alkane → alkanal; -CHO is C-1. Ring-CHO = carbaldehyde.

Nomenclature of Ketones

Common names of ketones name the two alkyl/aryl groups attached to the C=O, followed by ketone: CH$_3$COCH$_3$ = dimethyl ketone (acetone), CH$_3$COC$_2$H$_5$ = ethyl methyl ketone, C$_6$H$_5$COCH$_3$ = methyl phenyl ketone (acetophenone).

IUPAC names (alkanone): replace the -e of the alkane with -one, and give the carbonyl carbon the lowest possible number.

• CH$_3$COCH$_3$ → propan-2-one
• CH$_3$COCH$_2$CH$_3$ → butan-2-one
• CH$_3$CH$_2$COCH$_2$CH$_3$ → pentan-3-one

[JEE Tip] Number the chain from the end nearer the C=O. For aldehydes the -CHO is automatically C-1 (no number needed for the suffix); for ketones the position of the C=O must be shown (e.g. pentan-2-one vs pentan-3-one).

Key Point: ketone IUPAC = alkane → alkanone, with the lowest locant for C=O. Acetone = propan-2-one; acetophenone = 1-phenylethan-1-one.

Solved Examples

Example 1: IUPAC of an aldehyde

Give the IUPAC name of (CH$_3$)$_2$CHCH$_2$CHO.

Solution: The longest chain including -CHO is 4 carbons (butanal); the methyl branch is on C-3. Name: 3-methylbutanal.

Example 2: IUPAC of a ketone

Name CH$_3$CH$_2$COCH$_2$CH$_2$CH$_3$.

Solution: Six-carbon chain with C=O. Numbering from the nearer end puts the carbonyl at C-3: hexan-3-one.

Example 3: Common to IUPAC

Convert the common name "ethyl methyl ketone" to its IUPAC name.

Solution: CH$_3$-CO-CH$_2$CH$_3$ has a 4-carbon chain with C=O at C-2: butan-2-one.

Example 4: Aromatic carbonyl

Give the common and IUPAC names of C$_6$H$_5$COCH$_3$.

Solution: Common = acetophenone (methyl phenyl ketone). IUPAC = 1-phenylethan-1-one (or 1-phenylethanone).

Example 5: Why is C=O polar?

Explain why the carbonyl group is polar and what this means for its reactions.

Solution: Oxygen is more electronegative than carbon, so the C=O electrons (especially the π electron density) are pulled toward oxygen, giving C a δ+ and O a δ−. The δ+ carbon is electrophilic and is therefore attacked by nucleophiles — the basis of nucleophilic addition.

Example 6: Hybridisation and shape

What is the hybridisation and geometry of the carbonyl carbon in propanone?

Solution: The carbonyl carbon is sp$^2$ hybridised and the three groups around it lie in a plane at about 120° — it is trigonal planar.

Example 7: Ring aldehyde

Name the compound formed by attaching -CHO directly to a cyclohexane ring.

Solution: It is cyclohexanecarbaldehyde (the -CHO suffix on a ring uses -carbaldehyde).

Example 8: Lowest locant

Why is CH$_3$COCH$_2$CH$_2$CH$_3$ named pentan-2-one and not pentan-4-one?

Solution: The carbonyl must get the lowest possible locant. Numbering from the end nearer C=O gives position 2, so it is pentan-2-one.

Example 9: Distinguish aldehyde vs ketone by formula

Classify CH$_3$CH$_2$CHO and CH$_3$COCH$_3$.

Solution: CH$_3$CH$_2$CHO has a hydrogen on the carbonyl carbon → it is an aldehyde (propanal). CH$_3$COCH$_3$ has the carbonyl carbon between two carbons → it is a ketone (propanone).

Example 10: Naming a substituted aldehyde

Give the IUPAC name of OHC-CH$_2$-CH$_2$-CHO.

Solution: A four-carbon chain with -CHO at both ends is a dialdehyde: butanedial (the two -CHO carbons are C-1 and C-4).