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Priority Order of Functional Groups in IUPAC Naming

Chemistry · General Principles Of Organic Chemistry · NEET

When a molecule has more than one functional group, only ONE is chosen as the principal group (named as a suffix); the rest become prefixes. The seniority order is: carboxylic acid > ester > amide > nitrile > aldehyde > ketone > alcohol > amine. Memory hook: "COOH always wins, C=O of aldehyde beats C=O of ketone."
Seniority: only the top group becomes the suffixCOOH acidesteramidenitrilealdehyde -alketone -onealcohol -olamine -amineC=C / C=C: prefix onlyHIGH priorityLOW prioritySenior group = suffix + lowest locant; all others = prefixes (oxo, hydroxy, amino)
Functional group seniority ladder: the highest group present becomes the suffix and gets the lowest locant, while every lower group is named as a prefix.

Your doubts, answered

Which functional group becomes the suffix when a compound has two groups?

Only the SENIOR (highest priority) group is written as the suffix. Follow the order acid > ester > amide > nitrile > aldehyde > ketone > alcohol > amine > alkene/alkyne. The lower groups are written as prefixes. Example: a compound with both -OH and -CHO is named as an -al (aldehyde suffix) and the -OH becomes 'hydroxy' prefix, because aldehyde is higher than alcohol.

Is aldehyde or ketone higher priority in IUPAC naming?

Aldehyde (-CHO) is higher than ketone (>C=O). So if both are present, the chain is named with the -al suffix and the ketone becomes an 'oxo' or 'keto' prefix. This exact idea was tested in NEET 2017.

Why is carboxylic acid the highest priority group?

IUPAC fixes a fixed seniority list, and carboxylic acid (-COOH) sits at the top of the common groups you study for NEET. Whenever -COOH is present, it is always the principal group and gets the '-oic acid' suffix; every other group (OH, C=O, NH2, C=C) becomes a prefix.

Do C=C double bonds get priority over -OH?

No. Between an alcohol and a carbon-carbon double bond, the -OH is the principal group. The double bond is shown in the main chain name (e.g. -en-) but the compound ends in -ol. So numbering gives -OH the lower locant. Only the true 'top-list' groups (acid, aldehyde, ketone, alcohol, amine) become suffixes; C=C and C-C are structural features, not suffix groups on their own here.

How do I number the chain when there are multiple groups?

First pick the principal (senior) group. Then number the parent chain so that the principal group gets the LOWEST possible locant. The double bonds, prefixes and substituents accept whatever numbers follow. Priority for lowest locant: principal characteristic group first, then double/triple bonds, then prefix substituents.

⚠️ The NEET trap
✗ Giving the lowest number to a substituent or double bond instead of the senior functional group.
✓ The principal characteristic group (highest seniority) always gets the lowest locant first, even if that forces a substituent to take a higher number.
🧠 Senior group boards first. In NEET 2017, -CHO takes C-1, the methyl and ketone accept whatever numbers are left.

Real NEET questions

2017

The IUPAC name of the compound shown (a six-carbon chain bearing a terminal -CHO, an adjacent CH3 branch, a ketone C=O, and a C=C double bond) 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: The aldehyde (-CHO) is the principal characteristic group because aldehyde outranks ketone and alkene. So -CHO takes C-1 and the suffix -al. Numbering from the CHO end: C-1 aldehyde, C-2 methyl branch, C-3 ketone (keto/oxo prefix), and the double bond between C-4 and C-5. This gives 3-keto-2-methylhex-4-enal. Priority for numbering: aldehyde > ketone > alkene.

Solved General Principles Of Organic Chemistry NEET PYQs

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Frequently asked

What is the full priority order I must remember for NEET?

Carboxylic acid > sulphonic acid > ester > acid amide > nitrile > aldehyde > ketone > alcohol > amine > (then) alkene/alkyne as chain features. For most NEET questions the short chain acid > aldehyde > ketone > alcohol > amine is enough.

Can two functional groups both be suffixes?

No. Only one principal group is chosen as the suffix. All remaining groups are named as prefixes with words like hydroxy, oxo, amino, chloro, etc.

How are prefix groups ordered in the name?

Prefix substituents are written in alphabetical order before the parent name, each with its own locant. Only the principal group decides the suffix and the direction of numbering.

Where does a halogen or nitro group sit in this priority?

Halogens (Cl, Br, I, F) and -NO2 are ALWAYS prefixes; they never become suffixes. So they never decide the principal group, only add as prefix substituents.

Related concepts:
IUPAC Naming of Compounds with Functional GroupsLowest Locant Rule and Alphabetical NumberingWhat is a Functional Group in Organic ChemistryIUPAC Nomenclature of Branched Chain AlkanesNomenclature of Substituted Benzene Compounds
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