See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

Concept: The heat of combustion of alkane isomers depends on their degree of branching. For a given molecular formula (same number of carbons and hydrogens), more branched isomers are more stable (lower in potential energy) due to hyperconjugation and other stabilizing interactions. Because more branched isomers have lower potential energy, they release less energy upon combustion. Conversely, straight-chain (unbranched) alkanes have the highest heat of combustion among isomers, and the most branched isomers have the lowest heat of combustion. Step 1: Identify the structures. - Structure A is a straight-chain alkane (n-alkane), with no branching. It is the least stable (highest potential energy) among isomers. - Structure B is a moderately branched alkane (2,4-dimethylhexane or similar), with two methyl branches. - Structure C is the most highly branched alkane (e.g., 2,2,3-trimethylpentane), with the most branching and thus the most stable (lowest potential energy). Step 2: Rank by heat of combustion (most exothermic to least exothermic). - Since A (straight chain) has the highest potential energy, it releases the most energy upon combustion: highest heat of combustion. - B (moderate branching) is more stable than A but less stable than C, so it releases intermediate energy: intermediate heat of combustion. - C (most branched) has the lowest potential energy, so it releases the least energy upon combustion: lowest heat of combustion. Step 3: Therefore the ranking most exothermic to least exothermic is A > B > C. Why other options fail: - (a) B > A > C: Incorrect, because the straight-chain A is less stable than branched B, so A should release more energy than B, not less. - (c) C > A > B: Incorrect, because C is the most stable (most branched) and should release the least energy, not the most. - (d) C > B > A: Incorrect for the same reason as (c); more branching means less heat of combustion. Therefore, the correct answer is A.