Chemistry · General Principles Of Organic Chemistry · NEET
IUPAC nomenclature is the official system for naming organic compounds. You build the name in 4 steps: (1) find the longest carbon chain (the "parent"), (2) number it so the most important group gets the lowest number, (3) name the main functional group as a suffix, and (4) add the branches as prefixes in alphabetical order. Memory hook: "Word + Root + Suffix", and always give the priority group the smallest number.
Naming CH3-CH(Cl)-CH(CH3)-CH(OH)-CH2-CH2-Br: the -OH (principal group) is given the lowest locant (C-3, suffix -3-ol); the bromo, chloro and methyl branches become prefixes listed in alphabetical order.
Your doubts, answered
What is a locant in an IUPAC name?
A locant is the number that tells you WHERE something sits on the carbon chain. In 2-methylbutane, the '2' is the locant. It says the methyl branch is on carbon number 2. Every branch and every functional group needs a locant so the name points to one exact structure.
What is the difference between a prefix and a suffix in IUPAC naming?
A suffix is the ending that names the MAIN (highest priority) functional group, like -ol for alcohol or -al for aldehyde. A prefix is written in FRONT and names lower-priority groups or plain branches, like chloro-, bromo-, methyl-. One compound has ONE suffix but can have MANY prefixes.
Why is the COOH or CHO group always given the lowest number?
Because IUPAC has a priority (seniority) list. The principal functional group must get the LOWEST possible locant, even if that makes a branch get a higher number. Order to remember for NEET: carboxylic acid > ester > amide > nitrile > aldehyde > ketone > alcohol > amine. So a -CHO on the end is carbon 1, always.
What does the -al ending mean, and how is it different from -one?
The suffix -al means an aldehyde (-CHO group), which sits at the END of a chain, so it is carbon 1. The suffix -one means a ketone (C=O inside the chain). Example: propanal = CH3-CH2-CHO (aldehyde), propanone = CH3-CO-CH3 (ketone). Aldehyde beats ketone in priority.
How do I find the longest carbon chain?
Trace every possible path of connected carbons and pick the one with the MOST carbons. It does not have to be a straight line on paper; it can turn a corner. If two chains have equal length, choose the one with more branches. This longest chain becomes the parent (root) name like pentane or hexane.
Is alphabetical order used for numbering the chain or for writing the name?
Alphabetical order is used for WRITING the prefixes, not for numbering. First you number the chain to give the priority group (or lowest locant set) the smallest numbers. THEN, when you write the name, you list the branch prefixes alphabetically. Example: 3-ethyl-5-methylheptane (ethyl before methyl).
What does 'oxo' or 'keto' mean inside a name?
When a ketone C=O is NOT the top-priority group (because a higher group like -CHO or -COOH is present), the C=O is named as a prefix: oxo- (modern IUPAC) or keto- (older style). Example: 3-oxobutanal has an aldehyde as the main group and a ketone C=O as the 'oxo' prefix on carbon 3.
⚠️ The NEET trap ✗ Numbering the chain from whichever end gives the branches the lowest numbers. ✓ Number so the PRINCIPAL functional group gets the lowest locant first; branch locants come second. 🧠 Priority group wins the number-race. Branches only decide ties.
Real NEET questions
NEET 2024
A compound with molecular formula C6H14 has two tertiary carbons. Its IUPAC name is:
A · 2-methylpentane
B · 2,3-dimethylbutane ✓
C · 2,2-dimethylbutane
D · n-hexane
Solution: A tertiary carbon is bonded to three other carbons. In 2,3-dimethylbutane, (CH3)2CH-CH(CH3)2, both central carbons (C-2 and C-3) are each bonded to three carbons, giving exactly two tertiary carbons. 2-methylpentane has only one, 2,2-dimethylbutane has a quaternary carbon, and n-hexane has none.
NEET 2022
The correct IUPAC name of CH3-CH(Cl)-CH(CH3)-CH(OH)-CH2-CH2-Br is:
A · 1-bromo-5-chloro-4-methylhexan-3-ol ✓
B · 6-bromo-2-chloro-4-methylhexan-4-ol
C · 1-bromo-4-chloro-5-chlorohexan-3-ol
D · 6-bromo-4-chloro-4-chlorohexan-4-ol
Solution: Parent chain = 6 carbons (hexane). The -OH is the principal group, so it must get the lowest locant. Numbering from the Br end: C1(CH2Br), C2, C3(CHOH), C4(CH-CH3), C5(CHCl), C6(CH3). Suffix -3-ol; prefixes cited alphabetically: 1-bromo, 5-chloro, 4-methyl. Name = 1-bromo-5-chloro-4-methylhexan-3-ol.
NEET 2022
The correct IUPAC name of the given compound (a 7-carbon chain with an ethyl and a methyl branch) is:
A · 3-ethyl-5-methylheptane ✓
B · 2,4-diethylhexane
C · 5-ethyl-3-methylheptane
D · 3,5-diethylhexane
Solution: The longest continuous chain has 7 carbons (heptane), not hexane. Numbering from the end giving the lowest locant set places ethyl on C-3 and methyl on C-5. Prefixes are written alphabetically (ethyl before methyl). Correct name = 3-ethyl-5-methylheptane.
Solved General Principles Of Organic Chemistry NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
(1) Choose the longest carbon chain as the parent. (2) Number it to give the principal functional group the lowest locant. (3) Name the main group as a suffix. (4) Add branches and minor groups as prefixes in alphabetical order.
Which functional group gets priority for the suffix in NEET?
Common order: carboxylic acid > ester > amide > nitrile > aldehyde > ketone > alcohol > amine > alkene/alkyne. The highest one becomes the suffix; the rest become prefixes.
Do double and triple bonds affect the ending of the name?
Yes. -ane means only single bonds, -ene means a C=C double bond, and -yne means a C triple bond, each with a locant, for example but-2-ene or pent-1-yne.
How do I name a compound that has both an aldehyde and a ketone?
Aldehyde is higher priority, so it becomes the suffix -al and is carbon 1. The ketone C=O is named as the prefix 'oxo'. Example: 3-oxobutanal.
