The trap that costs marks in IUPAC nomenclature questions is numbering the parent chain from the wrong end. Under time pressure, students pick the first carbon they see and number forward — giving substituents locants like 4,5 when numbering from the other end would give 2,3. The exam does not reward partial naming; the entire name must be correct, and a wrong locant makes the entire answer wrong.
The IUPAC system in brief. NCERT Class 11 Chemistry Chapter 8, pages 263–266 (Organic Chemistry — Some Basic Principles and Techniques), lays out a deterministic algorithm: (1) identify the longest continuous carbon chain containing the principal characteristic group — this is the parent chain; (2) number the chain so the principal characteristic group gets the lowest possible locant; (3) when no principal characteristic group is present (hydrocarbons), give the lowest set of locants to substituents; (4) name substituents alphabetically with their locants; (5) assemble: locants + substituents + parent root + suffix.
The lowest-locant rule — where mistakes happen. "Lowest set of locants" means compare the full set, not just the first locant. For 2,3,5-trimethylheptane vs. 3,5,6-trimethylheptane, compare position by position: 2 < 3 at the first point of difference, so 2,3,5 wins. Students often compare sums (2+3+5 = 10 vs. 3+5+6 = 14); while the sum shortcut sometimes gives the same answer, the IUPAC rule is first-point-of-difference comparison, not sum comparison.
Functional group priority. When a functional group suffix is present (e.g., -ol, -al, -oic acid), that group must receive the lowest locant even if it means substituents get higher locants. Students confuse this with the hydrocarbon rule and minimise substituent locants instead.
Watch out: NEET questions on nomenclature are typically recall or single-step application — but the marks are lost to careless numbering, not to conceptual difficulty.
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
What is the IUPAC name of CH₃–CH(CH₃)–CH₂–CH₂–CH₃?
Answer: B. The longest chain is 5 carbons (pentane). A methyl group is on C-2 (numbering from the end nearest the branch). IUPAC name: 2-methylpentane. (NCERT Class 11 Chemistry, Chapter 8, page 264.)
Why A is wrong: A gives the methyl group locant 4, which means the chain was numbered from the wrong end — violating the lowest-locant rule (trap: wrong-end numbering).
Why C is wrong: C identifies only 4 carbons in the parent chain, missing the longer 5-carbon chain (trap: failing to find the longest chain).
Why D is wrong: D uses a common (trivial) name, not the IUPAC systematic name (trap: confusing trivial and IUPAC names).
In IUPAC nomenclature, when a compound has both a substituent and a principal characteristic group, which gets the lowest locant?
Answer: D. The IUPAC rule states the principal characteristic group (the suffix group) receives the lowest possible locant, even if this raises substituent locants. (NCERT Class 11 Chemistry, Chapter 8, page 266.)
Why A is wrong: A reverses the priority — substituents yield to the principal characteristic group for lowest locant (trap: confusing substituent-priority with functional-group-priority).
Why B is wrong: B describes a chain-tracing heuristic that is not part of the IUPAC rules (trap: inventing a rule from intuition).
Why C is wrong: C invokes molecular weight, which plays no role in IUPAC locant assignment (trap: importing an irrelevant criterion).
The IUPAC name of (CH₃)₃C–CH₂–CH(CH₃)₂ is:
Answer: A. The longest chain is 5 carbons (pentane). Numbering from the (CH₃)₃C– end gives methyl groups at 2,2,4. From the other end: 2,4,4. First-point-of-difference comparison: 2=2 (tie), then 2<4, so {2,2,4} wins. Name: 2,2,4-trimethylpentane. (NCERT Class 11 Chemistry, Chapter 8, page 264, which names this compound.)
Why B is wrong: B results from numbering the chain from the wrong end, giving locant set {2,4,4} instead of the lower set {2,2,4} (trap: lowest-locant rule — compare at the first point of difference, not by sum).
Why C is wrong: C incorrectly counts 6 carbons in the parent chain when the longest continuous chain has 5 (trap: including a branch carbon in the parent chain).
Why D is wrong: D combines wrong parent-chain length (6 instead of 5) with wrong-end numbering (trap: compounded errors).
Which of the following represents the correct order of priority for selecting the parent chain in IUPAC nomenclature of hydrocarbons?
Answer: C. For hydrocarbons without a principal characteristic group: first select the longest continuous chain, then number to give the lowest set of locants to substituents, then name substituents alphabetically. (NCERT Class 11 Chemistry, Chapter 8, pages 264–265.)
Why A is wrong: A places 'maximum substituents' first — this is not an IUPAC criterion. The parent chain is selected by length, not by how many branches it carries (trap: inventing a maximise-substituents rule).
Why B is wrong: B reverses the order — you cannot assign locants until you have identified and numbered the parent chain (trap: applying rules out of sequence).
Why D is wrong: D places alphabetical naming before chain selection, confusing the naming step with the structural identification step (trap: mixing naming and numbering stages).
The IUPAC name of CH₃–CH₂–CH(C₂H₅)–CH(CH₃)–CH₃ is:
Answer: A. Longest chain: 5 carbons (pentane). Number from the end nearer to the first branch point. From the methyl end: ethyl at C-3, methyl at C-2 → locant set {2,3}. From the other end: ethyl at C-3, methyl at C-4 → locant set {3,4}. {2,3} < {3,4} at first point of difference, so: 3-ethyl-2-methylpentane. Substituents listed alphabetically: ethyl before methyl. (NCERT Class 11 Chemistry, Chapter 8, page 265.)
Why B is wrong: B numbers from the wrong end, placing the methyl at C-4 instead of C-2 — violates the lowest-locant rule (trap: wrong-end numbering).
Why C is wrong: C has the correct locants but lists methyl before ethyl — IUPAC requires alphabetical order: ethyl before methyl (trap: ignoring alphabetical-order rule for substituents).
Why D is wrong: D counts 6 carbons as the parent chain, likely including a branch carbon — the longest continuous chain is 5 (trap: miscounting the parent chain).
The IUPAC name of CH₃–CH(CH₃)–CH(CH₃)–CH₃ is:
Answer: A. The longest chain has four carbons (butane). Numbering from either end puts the two methyl groups on C-2 and C-3, and two identical substituents are written once with the multiplying prefix di-: 2,3-dimethylbutane. Longest chain and lowest numbers for the branch points are the rules in NCERT Class 11 Chemistry, Chapter 8, page 264.
Why B is wrong: B names the same substituent twice instead of combining identical groups with di- (trap: listing identical substituents separately).
Why C is wrong: C gives two locants but no multiplying prefix, so the name says one methyl group sits at two positions (trap: dropping di- while keeping both locants).
Why D is wrong: D counts a methyl carbon into the parent chain; no continuous chain here is longer than four carbons (trap: miscounting the parent chain).
A compound has the structure: CH₃–CH(OH)–CH₂–CH(CH₃)–CH₃. What locant does the –OH group receive in the IUPAC name?
Answer: C. The –OH (hydroxyl) is the principal characteristic group and must receive the lowest possible locant. Numbering from the –OH end: OH is at C-2, methyl substituent at C-4. Numbering from the other end: OH at C-4, methyl at C-2. Since the functional group has priority, number so OH gets 2 (the lower locant). (NCERT Class 11 Chemistry, Chapter 8, page 266.)
Why A is wrong: A gives –OH locant 4 by numbering from the wrong end — this minimises the substituent locant instead of the functional group locant (trap: applying the hydrocarbon lowest-locant rule instead of the functional-group-priority rule).
Why B is wrong: B places –OH at C-3, which doesn't match any consistent numbering of this 5-carbon chain — likely a counting error (trap: miscounting carbon positions).
Why D is wrong: D places –OH at C-1, but the hydroxyl is not on a terminal carbon in this structure (trap: misreading the structural formula).
When comparing two possible numbering schemes for substituent locants, the IUPAC rule uses:
Answer: B. The IUPAC lowest-locant rule compares locant sets term by term at the first point of difference — not by sum. (NCERT Class 11 Chemistry, Chapter 8, page 265 compares 2,5,6 with 3,5,7 this way.)
Why A is wrong: A describes the 'lowest sum' shortcut, which sometimes gives the same result but is NOT the IUPAC rule — the official criterion is first-point-of-difference comparison (trap: using the sum shortcut as if it were the rule).
Why C is wrong: C confuses alphabetical naming order with numbering priority — alphabetical order determines how substituents are listed in the name, not how the chain is numbered (trap: conflating naming order with numbering order).
Why D is wrong: D introduces a 'larger substituent' criterion that does not exist in IUPAC nomenclature — substituent size has no bearing on locant assignment (trap: inventing a rule based on intuition).
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Given
A branched-chain hydrocarbon with the condensed formula above. No functional group suffix is present — this is a substituted alkane.
Required
The correct IUPAC name with proper locants.
Concept
IUPAC naming for saturated hydrocarbons: (1) find the longest continuous carbon chain, (2) number from the end giving the lowest set of locants to substituents (first-point-of-difference comparison), (3) name substituents alphabetically with locants.
Identify the parent chain
Trace all possible paths. The longest continuous chain runs through all 6 backbone carbons: C1–C2–C3–C4–C5–C6 → hexane.
Number and locate substituents
• From the left end: methyl at C-2, methyl at C-4, methyl at C-4 → locant set {2, 4, 4}• From the right end: methyl at C-3, methyl at C-3, methyl at C-5 → locant set {3, 3, 5}
First-point-of-difference: 2 < 3. So numbering from the left gives the lower set: {2, 4, 4}.
Assemble the name
Three methyl substituents → trimethyl. Locants: 2,4,4. Parent chain: hexane.
Name: 2,4,4-Trimethylhexane
Final answer
2,4,4-Trimethylhexane.
Common trap
Numbering from the right end gives {3,3,5}. A student using the sum shortcut: 2+4+4 = 10 vs. 3+3+5 = 11 — the sum gives the same answer here. But consider a case like {2,3,5} vs. {1,4,5}: sums are both 10, yet first-point-of-difference picks {1,4,5}. Always use the official IUPAC method (first-point-of-difference), not the sum shortcut.
Similar NEET-style question
Write the IUPAC name of: CH₃–C(CH₃)₂–CH₂–CH(C₂H₅)–CH₃. (Answer: 2,2,4-trimethylhexane. The longest chain runs THROUGH the ethyl group — CH₃–C–CH₂–CH–CH₂–CH₃ is six carbons, not the five you see written left to right — so the parent is hexane and there is no ethyl substituent at all. Numbering from the left gives {2,2,4}, from the right {3,5,5}, so {2,2,4} wins. Treating ethyl as a substituent on pentane is the lowest-chain error this lesson is about.)
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Rules: (1) longest carbon chain = parent. (2) Number to give lowest locants to substituents. (3) Functional groups have priority order; principal group as suffix. (4) Substituents as prefix. (5) Multiple substituents alphabetised.
-- NCERT Class 11 Chemistry, Ch. 8, p. 262These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.
Category: Similar Terms
Student numbers carbon chain from wrong end, giving higher locants to substituents than necessary.
IUPAC naming question with multiple substituents.
Choose end that gives the LOWEST set of locants for all substituents (compare set, not first-encountered). Functional group has priority for lowest locant.
Root cause: rushed under time pressure
Try numbering from BOTH ends; pick the one giving the lowest SET of locants. Functional group has priority for lowest locant.
More in Organic Chemistry — Some Basic Principles: 7 exam traps and mistakes · 1 formula · 1 question pattern from its other lessons.
2 questions from NEET 2022, 2024. Answers verified against NTA official keys.
A compound with a molecular formula of C₆H₁₄ has two tertiary carbons. Its IUPAC name is :
The correct IUPAC name of the following compound is
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