Which of the following compounds is not a structural isomer of 2,2,4-trimethylpentane?

Question

BlackTom AI · Accepted Answer

To tackle this question, I’ll list each option and evaluate whether it could be a structural isomer of 2,2,4-trimethylpentane (which has the formula C8H18).

Option a: 2,4-dimethylpentane. This structure would derive from a pentane backbone (C5) with two additional methyl groups, giving a total of C7. Since the carbon count is only seven, it cannot be an isomer of a C8H18 compound. Therefore, this option fails the fundamental criterion for being an isomer of 2,2,4-trimethylpentane.

Option b: 2,4-dimethylhexane. Here the base is hexane (C6) with two extra methyl groups, yielding C8, which matches the carbon count of 2,2,4-trimethylpentane. A quick structural check shows such a molecule can be drawn in a way that preserves the same molecular formula and can be rearranged to the same connectivity as an isomer of C8H18. This makes it a plausible structural isomer.

Option c: 3-ethylhexane. This compound has a hexane backbone (C6) with an ethyl substituent (C2), totaling C8. Like option b, the carbon count matches, and there exist connectivities that represent a different arrangement of the same eight carbons, which is characteristic of an isomer of 2,2,4-trimethylpentane.

Option d: octane. This is simply the straight-chain C8 alkane. It shares the same molecular formula with 2,2,4-trimethylpentane, and there are numerous structural representations that show it as a constitutional isomer of a branched C8H18. Therefore, it satisfies the requirement for being an isomer.

If we focus strictly on the question’s wording—identifying what is not a structural isomer of 2,2,4-trimethylpentane—each of options b, c, and d describes an eight-carbon alkane that can be a different connectivity isomer of the same formula. In contrast, option a describes a seven-carbon compound, which cannot be an isomer of an eight-carbon target. Hence, option a is fundamentally not a structural isomer due to carbon count, while the others are viable isomer candidates based on the same molecular formula.

In summary, the key distinction hinges on carbon count: seven carbons (option a) cannot be an isomer of a C8H18 compound, whereas eight carbons (options b, c, and d) can form isomeric structures with the same molecular formula. This explains why option a fails the isomer criterion, while the remaining options satisfy it. The analysis highlights common misconceptions, such as assuming any eight-carbon alkane must be an isomer without considering structural connectivity, or overlooking carbon count as a decisive factor.

MUF0042 Chemistry Unit 2 - Semester 2, 2025 1.8 Quiz: Hydrocarbons

Which of the following compounds is not a structural isomer of 2,2,4-trimethylpentane?

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