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A hydrocarbon X reacts with excess HBr to yield only a single monobromo product. Complete combustion of 1 mole of X consumes exactly 6 moles of O2. How many carbon atoms are present in X?

1. 2
2. 3
3. 4
4. 5

Correct answer: 4

Solution

Cyclobutane (C4H8) satisfies 3n/2 = 6 (n=4) and has all eight hydrogen atoms equivalent due to its symmetric square ring, yielding only one monobromo product. Carbon count = 4.

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