For chair conformers, substituents that are directed axial (above/below plane of ring) impart greater steric strain than substituents that are equatorial (directed horizontally away from the ring). Consider the following isomers of C6H12O6 (i – v) in the chair conformation.Which of i – v will be the lowest energy (most stable)?

Question

BlackTom AI · Accepted Answer

The question asks which chair conformer among i–v will have the lowest energy, i.e., be the most stable, given that axial substituents cause more steric strain than equatorial ones.
Option a: (i). In structure (i), several substituents appear axial (the red OH groups on both sides may be axial positions depending on the chair flip). If many substituents occupy axial sites, steric strain from 1,3-diaxial interactions would be high, making this conformer less stable. Since axial substituents raise energy, (i) would likely be less favorable.
Option b: (ii). Here, it seems that more substituents are in equatorial positions, which minimizes 1,3-diaxial interactions and lowers steric strain. If (ii) places bulky groups equatorially, this conformer should be more stable than those with axial crowding.
Option c: (iii). If (iii) has several bulky groups oriented axially, the steric clash would increase energy again, reducing stability relative to the equatorial-rich conformers. This would typically not be the lowest energy form.
Option d: (iv). Depending on the exact orientation, (iv) could present a mix of axial and equatorial substituents. If it introduces more axial bulk than (ii) but less than (i) or (iii), its stability would lie somewhere in between. Without more precise counting, it is uncertain but not obviously the lowest.
Option e: (v). If (v) reintroduces multiple axial substituents or places bulky groups in positions that cause significant 1,3-diaxial strain, the energy would be higher. Unless its arrangement somehow minimizes interactions more than (ii), it would not be the obvious lowest-energy option.
Overall, the core idea is that the lowest-energy chair is the one with the greatest number of bulky substituents in equatorial positions, thereby minimizing steric interactions. Among the given conformers, (ii) appears to maximize equatorial placement of substituents compared with the others, leading to reduced steric strain and thus lower energy.
Note: The assessment relies on recognizing 1,3-diaxial interactions and equatorial vs axial preferences for substituents on cyclohexane chairs. Visual inspection of axial crowding versus equatorial extension is the key heuristic for ranking these isomers’ stabilities.

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