Bpentan-3-one-1-oic acid — both suffixes are listed simultaneously
C3-ketopentanoic acid — 'keto-' is the standard prefix for a ketone in a chain with a carboxylic acid
D5-oxopentanoic acid — the ketone is numbered from the opposite end
When a molecule contains both a ketone and a carboxylic acid, the carboxylic acid has higher naming priority and becomes the principal characteristic group, taking the -oic acid suffix. The carboxyl carbon is C1, so the chain is numbered from that end. The ketone, which cannot also take a suffix, is indicated with the prefix 'oxo-' at its position — here, C3. You cannot use two suffixes simultaneously (option B), 'keto-' is not standard IUPAC prefix notation (option C), and numbering from the wrong end gives 5-oxo incorrectly (option D).
Question 2 Multiple Choice
Which of the following is the correct IUPAC name for CH₃CH₂CHO?
Apropanal — a 3-carbon chain with an aldehyde terminal carbonyl
Bpropan-1-one — a ketone suffix at position 1
C1-propanaldehyde — a locant is needed to specify the aldehyde position
Dpropanone — the suffix for any 3-carbon carbonyl compound
CH₃CH₂CHO is an aldehyde: the C=O is at the end of a 3-carbon chain. Aldehydes use the suffix -al, replacing the terminal -e of the parent alkane (propane → propanal). Because the carbonyl carbon in an aldehyde is ALWAYS at position 1, no locant is needed — so '1-propanaldehyde' (option C) is both incorrect in format and redundant. 'Propan-1-one' (option B) incorrectly uses the ketone suffix; 'propanone' (option D) also incorrectly uses the ketone suffix and would refer to a different structure.
Question 3 True / False
In IUPAC nomenclature, the carbonyl carbon of an aldehyde is always carbon 1, so no position number is ever written as part of the -al suffix.
TTrue
FFalse
Answer: True
This is correct. Aldehydes have the C=O at the end of the carbon chain by definition, making that carbon C1 automatically. Because the position is fixed, including a locant would be redundant and is not used in IUPAC nomenclature — you simply write 'propanal,' 'butanal,' etc. Compare this to ketones, where the carbonyl can be at different interior positions and a locant (e.g., 'pentan-3-one') is required.
Question 4 True / False
In IUPAC nomenclature, the suffix '-oate' indicates an amide — a carbonyl group bonded to a nitrogen.
TTrue
FFalse
Answer: False
'-Oate' is the suffix for an ester, not an amide. Esters are formed when a carboxylic acid reacts with an alcohol; the suffix replaces '-oic acid' with '-oate,' and the alkyl group from the alcohol is named first (e.g., methyl ethanoate). Amides — carbonyl groups bonded to nitrogen — use the suffix '-amide' (e.g., ethanamide). Confusing these two is one of the most common errors in carbonyl nomenclature.
Question 5 Short Answer
How does IUPAC nomenclature handle a molecule that contains both a ketone and a carboxylic acid? Which group takes priority, and how is the lower-priority group indicated?
Think about your answer, then reveal below.
Model answer: The carboxylic acid takes priority over the ketone. The carboxyl carbon becomes C1 and the compound takes the '-oic acid' suffix. The ketone cannot also take a suffix, so it is instead indicated with the prefix 'oxo-' at its position number. For example, a 5-carbon chain with COOH at one end and a C=O at C3 is named 3-oxopentanoic acid.
IUPAC nomenclature establishes a strict hierarchy among functional groups to ensure each compound has exactly one unambiguous name. Carboxylic acids rank above aldehydes and ketones, so when both are present, the acid suffix (-oic acid) is used and the ketone's presence is indicated with the prefix 'oxo-'. This hierarchy — carboxylic acid > aldehyde > ketone — is consistently tested because students often try to use two suffixes simultaneously, which is not permitted.