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Which of the following compounds contains exactly one isopropyl group?

1. 2,2,3,3-Tetramethyl pentane
2. 2,2-Dimethyl pentane
3. 2,2,3-Trimethyl pentane
4. 2-Methyl pentane

Correct answer: 2,2,3-Trimethyl pentane

Solution

2,2,3-Trimethylpentane (CH3-C(CH3)2-CH(CH3)-CH2-CH3) has C3 bearing one H and one methyl, flanked by the quaternary C2 (bearing two methyls). The -CH(CH3)- at C3 along with the two methyls on the adjacent C2 means the C2-C3 unit forms one isopropyl group as viewed from the chain. This is the only compound among the four with exactly one isopropyl group.

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