Nomenclature of Aldehydes and Ketones

The naming of aldehydes and ketones follows the IUPAC system, based on the longest carbon chain containing the carbonyl group and the position of that group. For fundamentals of the carbonyl group, see Carbonyl Compounds.

Aldehydes

General formula: R–CHO

• Replace the -e of the parent alkane with -al (e.g., propane → propanal).
• The carbon of the aldehyde group is always carbon number 1.
• Common names often relate to the corresponding carboxylic acid (e.g., formaldehyde from formic acid).

Examples:

• HCHO → Methanal (Common: Formaldehyde)
• CH₃CHO → Ethanal (Common: Acetaldehyde)
• CH₃CH₂CH₂CHO → Butanal

Ketones

General formula: R–CO–R′

• Replace the alkane ending -e with -one.
• Number from the end nearer to the carbonyl to assign the lowest locant to the C=O carbon.
• Common names: list the two alkyl groups (alphabetical order) + “ketone”.

Examples:

• CH₃COCH₃ → Propan-2-one (Common: Acetone)
• CH₃COCH₂CH₃ → Butan-2-one
• CH₃CH₂COCH₂CH₃ → Pentan-3-one

Quick Tips

• Always include the position of the carbonyl group for ketones (e.g., butan-2-one).
• Cyclic ketones use the prefix cyclo- (e.g., cyclohexanone).
• Aromatic aldehydes have –CHO directly on the ring (e.g., benzaldehyde).

Related (Internal): Introduction to Aldehydes • Introduction to Ketones • Chemical Reactions of Aldehydes •

External References: Aldehyde – Wikipedia • Ketone – Wikipedia • NCERT: Aldehydes, Ketones & Carboxylic Acids (Ch-12)

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MCQ Quiz

1. The correct IUPAC name for CH₃CH₂CH₂CHO is:
   a) Propanal   b) Butanal ✅   c) Butanone   d) Ethanal
2. The carbonyl position in butan-2-one is on carbon:
   a) 1   b) 2 ✅   c) 3   d) 4
3. The common name of butan-2-one is:
   a) Diethyl ketone   b) Methyl propyl ketone   c) Methyl ethyl ketone ✅   d) Acetone
4. For aldehydes, the carbonyl carbon is numbered:
   a) Lowest possible   b) Highest possible   c) Always as C-1 ✅   d) Depends on substituent priority
5. Cyclohexanone is a(n):
   a) Aliphatic aldehyde   b) Aromatic aldehyde   c) Cyclic ketone ✅   d) Dicarboxylic acid

True or False

• Aldehyde group is always at the end of the chain. — True ✅
• Ketones do not require locants in IUPAC names. — False ❌ (locant is needed, e.g., butan-2-one)
• “Acetone” is the common name for propan-2-one. — True ✅
• Benzaldehyde is an aliphatic aldehyde. — False ❌ (it’s aromatic)
• In common names of ketones, alkyl groups are listed randomly. — False ❌ (alphabetical order)

Fill in the Blanks

• Aldehydes are named by replacing _____ with -al. → -e
• In ketones, numbering starts from the end nearer to the _____ group. → carbonyl
• CH₃COCH₃ in IUPAC is _____. → propan-2-one
• The aromatic aldehyde with –CHO on benzene is called _____. → benzaldehyde
• Common names of ketones use alkyl groups + the word _____. → ketone