(MCQ from Karachi University Entry Test 2021)
Q: The IUPAC name for the alkane represented by the following structure
CH₃CH₂C(CH₃)₂CH₂CH₃ is:
A) 2,2-dimethyl-1-pentane
B) 1,1-dimethyl-2,2-methyl butane
C) 1-methyl-2,2-dimethyl butane
D) 2,2-dimethyl pentane
Correct Answer: D) 2,2-dimethylpentane
Explanation:
Given structure:
CH₃–CH₂–C(CH₃)₂–CH₂–CH₃
Breakdown:
- Longest continuous carbon chain = 5 carbon atoms → Pentane
- Two methyl groups (CH₃) are attached to carbon #2 of the main chain
Step-by-step IUPAC Naming:
- Longest chain: 5 carbons → “pentane”
- Substituents: Two methyl groups on carbon 2
- Final IUPAC name: 2,2-dimethylpentane
Other Options (Incorrect because):
- A) 2,2-dimethyl-1-pentane → No such position; “1-pentane” is not a proper base name
- B) & C) → Not standard IUPAC format; incorrect substituent placement
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