Q.12 IUPAC name of the following molecule is
(A) 3–Bromo–3,5–dimethyl hexane
(B) 4–Bromo–2,4–dimethyl hexane
(C) 3–Bromo–2–isobutyl butane
(D) 4–Bromo–2–methyl–4–ethyl pentane
The correct IUPAC name of the given molecule is 4‑Bromo‑2,4‑dimethylhexane, which corresponds to option (B).
Introduction: IUPAC name of the following molecule
For competitive exams, questions like “IUPAC name of the following molecule” test conceptual clarity in organic nomenclature, including selection of the parent chain, correct numbering, and prioritization of substituents. The given structure is a haloalkane with branching, so careful application of IUPAC rules is essential to avoid common traps in the options.
Step‑by‑step solution to find the correct IUPAC name
1. Identify the longest carbon chain
From the drawn structure (rewritten linearly):
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Start at the leftmost carbon and trace the longest continuous chain:
C1−C2−C3−C4−C5−C6. -
This gives a 6‑carbon parent chain, so the parent hydrocarbon is hexane.
Any option whose parent name is not hexane is automatically incorrect, because the longest chain clearly has six carbons.
2. Locate substituents and functional group
On the main 6‑carbon chain:
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A bromo substituent is attached to carbon 4.
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A methyl group is attached to carbon 2.
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Another methyl group is attached to carbon 4 (same carbon that carries bromo).
Thus, relative to the six‑carbon chain, there are:
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Two methyl substituents (at positions 2 and 4).
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One bromo substituent (at position 4).
3. Number the chain correctly
The chain must be numbered so that the set of locants is lowest according to IUPAC rules:
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Numbering from left to right:
Substituents at C‑2 (methyl), C‑4 (bromo + methyl) → locants: 2,4,4. -
If numbered from right to left:
Substituents would be at C‑3 (methyl), C‑5 (bromo + methyl) → locants: 3,5,5.
Since 2,4,4 is lower than 3,5,5 in the first point of difference, numbering from left to right is correct.
4. Write the complete IUPAC name
Combine substituents in alphabetical order:
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Substituents: bromo, methyl, methyl.
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Alphabetical order (ignoring numerical prefixes di, tri, etc.): bromo (B) comes before methyl (M).
So the full name:
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Bromo at C‑4 → 4‑Bromo
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Two methyls at C‑2 and C‑4 → 2,4‑dimethyl
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Parent chain → hexane
Final IUPAC name: 4‑Bromo‑2,4‑dimethylhexane.
This matches option (B).
Detailed analysis of each option
Option (A): 3‑Bromo‑3,5‑dimethylhexane
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This option assumes numbering of the same 6‑carbon chain from the opposite end, giving substituents at 3 and 5 rather than 2 and 4.
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As discussed, the correct lowest set of locants is 2,4,4, not 3,5,5, so this violates the IUPAC low‑locant rule and is incorrect.
Option (B): 4‑Bromo‑2,4‑dimethylhexane (correct)
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Uses the correct 6‑carbon parent chain (hexane) and correct numbering (2,4,4).
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Includes both methyl groups via the prefix dimethyl and places bromo and methyl correctly at carbon 4, hence this is the correct IUPAC name.
Option (C): 3‑Bromo‑2‑isobutylbutane
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Takes a 4‑carbon chain (butane) as parent and treats a larger branch as an isobutyl substituent.
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This violates the basic rule that the longest continuous chain must be selected as the parent; a 6‑carbon chain exists, so butane cannot be the parent, making this option incorrect.
Option (D): 4‑Bromo‑2‑methyl‑4‑ethylpentane
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Takes a 5‑carbon chain (pentane) as the parent and treats one end carbon as an ethyl substituent.
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Again, ignoring the available 6‑carbon chain breaks the longest‑chain rule, so pentane cannot be the correct parent and this option is incorrect.
Key exam takeaways
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Always choose the longest continuous carbon chain as the parent, even if it appears less “straight”.
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Among possible numbering directions, select the one that gives the lowest set of locants to all substituents collectively.
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When writing the final name, list substituents in alphabetical order, ignoring multiplicative prefixes like di‑, tri‑, etc.
These principles ensure you obtain 4‑Bromo‑2,4‑dimethylhexane as the correct IUPAC name for the given molecule in such MCQ problems.
