How to Name Aldehydes, Ketones and Carboxylic Acids (IUPAC and Common Names)
Chemistry · Aldehydes, Ketones And Carboxylic Acid · NEET
To name these compounds, take the longest carbon chain that has the functional group, then change the alkane ending: aldehyde becomes "-al", ketone becomes "-one", and carboxylic acid becomes "-oic acid". Always give the functional-group carbon the lowest possible number. Memory hook: "AL for aldehyde, ONE for ketone, OIC for acid" — and CHO/COOH is always carbon-1.
Quick naming map: change the alkane ending to -al, -one or -oic acid, keep the functional-group carbon as C1 (aldehyde and acid), number ketones from the nearer end, and remember COOH beats CHO beats C=O for suffix priority.
Your doubts, answered
From which end do I start numbering an aldehyde? Do I choose the lowest sum?
No. For aldehydes you do NOT pick the lowest-locant end freely. The -CHO carbon is ALWAYS carbon number 1, even if that gives higher numbers to substituents. So in OHC-CH2-CH3, the CHO carbon is C1. This is a fixed NCERT rule and a very common NEET trap.
For a ketone, where do I start numbering?
Start from the end NEARER to the carbonyl (C=O) carbon, so the ketone carbon gets the lowest possible number. Example: CH3-CO-CH2-CH2-CH3 is pentan-2-one, not pentan-4-one. The suffix is -one and you show the position, like 'pentan-2-one'.
When a molecule has BOTH an aldehyde and a ketone, which one wins?
The aldehyde has higher priority than the ketone. So the aldehyde carbon becomes C1 and takes the suffix -al, while the ketone is written as a prefix 'oxo-' (or 'keto-' in older naming). That is exactly why OHC-CH(CH3)-CO-CH=CH-CH3 is named as a ...hexenal (aldehyde suffix) with a keto/oxo prefix for the C=O.
How do I change the name from the alkane?
Take the parent alkane name and drop the final -e. Add -al for aldehyde, -one for ketone, -oic acid for carboxylic acid. Example: ethane to ethanal (CH3CHO), propane to propan-2-one (acetone), ethane to ethanoic acid (CH3COOH). The chain must include the functional-group carbon.
Why do I sometimes see 'oxo' or 'keto' inside an aldehyde name?
When a ketone C=O is present but the aldehyde has already claimed the -al suffix, the ketone group cannot also be a suffix. It is demoted to a prefix. In NEET-style answers this prefix appears as 'oxo-' or the older 'keto-'. Both mean a C=O sitting in the chain, named with its locant, e.g. 3-oxo... or 3-keto....
What are the common names I must still remember for NEET?
How do I name an aldehyde attached to a ring, like on benzene or cyclohexane?
When -CHO is attached to a ring, you cannot include the CHO carbon in the ring count. So you add the suffix 'carbaldehyde' after the full ring name: cyclohexanecarbaldehyde. For benzene the IUPAC name is benzenecarbaldehyde, but the common name benzaldehyde is also accepted.
⚠️ The NEET trap ✗ Students number the chain to give substituents the lowest locants and call OHC-CH(CH3)-CO-CH=CH-CH3 something like '5-methyl-4-oxohex-2-en-5-al'. ✓ The aldehyde -CHO carbon must be C1. Numbering from CHO gives: C1 = CHO (suffix -al), C2 = CH bearing CH3, C3 = C=O (ketone, so 'keto/oxo' prefix), C4=C5 double bond. Correct name: 3-keto-2-methylhex-4-enal. 🧠 CHO is the boss = carbon 1 ALWAYS. Aldehyde beats ketone, so ketone becomes an oxo/keto prefix, never the suffix.
Real NEET questions
NEET 2017
The IUPAC name of the compound OHC-CH(CH3)-CO-CH=CH-CH3 is:
A · 3-keto-2-methylhex-4-enal ✓
B · 5-formylhex-2-en-3-one
C · 5-methyl-4-oxohex-2-en-5-al
D · 3-keto-2-methylhex-5-enal
Solution: Count the longest chain containing the -CHO: it has 6 carbons (hex). The aldehyde group has the highest priority, so it takes the suffix -al and its carbon is numbered C1. Numbering from CHO: C1 = -CHO, C2 bears the -CH3 substituent, C3 is the ketone C=O (named as a keto/oxo prefix because the -al suffix is already used), and the C=C double bond lies between C4 and C5 (hex-4-ene). Combining: 3-keto-2-methylhex-4-enal. Option (A) is correct. (B) wrongly makes the ketone the suffix; (C) wrongly puts -al on C5; (D) puts the double bond at the wrong position.
NEET 2021
What is the IUPAC name of the organic compound formed when CH3COCH3 reacts with (i) C2H5MgBr, dry ether and (ii) H2O, H+?
A · pentan-3-ol
B · 2-methylbutan-2-ol ✓
C · 2-methylpropan-2-ol
D · pentan-2-ol
Solution: Ethyl magnesium bromide adds an ethyl group to the carbonyl carbon of acetone. After acidic work-up you get the tertiary alcohol (CH3)2C(OH)C2H5. To name it: longest chain through the -OH carbon is 4 carbons (butane), the OH carbon is C2 so it is butan-2-ol, and a methyl branch sits on C2. Final IUPAC name: 2-methylbutan-2-ol, option (B). This question tests both the Grignard product and correctly naming a branched alcohol chain.
Solved Aldehydes, Ketones And Carboxylic Acid NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Aldehyde uses -al (drop the -e of the alkane), ketone uses -one, and carboxylic acid uses -oic acid. For example ethane becomes ethanal, propan-2-one and ethanoic acid respectively.
Is the -CHO carbon always numbered 1?
Yes. In an aldehyde the carbon of the -CHO group is always carbon 1, so you do not usually write a locant for the -al. This is fixed by NCERT and is different from ketones, where you number from the end nearer the C=O.
Which has higher priority in naming, an aldehyde or a ketone?
The aldehyde. If both are in one molecule, the aldehyde takes the -al suffix and gets C1, and the ketone is written as an 'oxo' (or older 'keto') prefix with its position number.
Do I still need common names for NEET?
Yes. Common names like formaldehyde, acetaldehyde, acetone, benzaldehyde, formic acid, acetic acid and benzoic acid appear often in NEET reactions and reagent tests, so learn both the common name and the IUPAC name.
How is a carboxylic acid on a ring named?
When -COOH is on a ring, use the suffix 'carboxylic acid' after the ring name, for example cyclohexanecarboxylic acid, because the COOH carbon is not counted inside the ring. Benzoic acid (C6H5COOH) is the accepted name for the benzene case.
