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 C6H12O1Br5 (i – v) in the chair conformation.Which of i – v will be the highest energy (least stable)?

Question

BlackTom AI · Accepted Answer

To determine which is the highest energy (least stable) chair conformer, we need to evaluate the steric strain generated by axial bulky substituents, especially bromines, since axial substituents experience 1,3-diaxial interactions that increase energy.

Option (i): Assess how many Br substituents are axial in (i). If (i) has fewer axial Br or more equatorial Br, the steric strain is reduced compared to conformers with more axial substituents, making it comparatively more stable.

Option (ii): Examine (ii) for the distribution of Br substituents. If (ii) places a few Br in axial positions but not as many as in other options, its energy will be intermediate between highly axial cases and more equatorial cases.

Option (iii): This structure appears to place a larger number of Br substituents in the axial positions relative to the ring plane. Each axial Br contributes significant 1,3-diaxial repulsion with axial hydrogens on the neighboring carbons, elevating the overall energy substantially. When multiple bulky substituents are axial, the cumulative steric strain is the greatest, pushing this conformer to the highest energy among the set.

Option (iv): If (iv) has a mix of axial and equatorial Br substituents, with more Br in equatorial positions than (iii), the diaxial strain will be lower, resulting in a more stable conformer than (iii).

Option (v): Similarly, (v) might show a distribution that reduces axial interactions compared with (iii), perhaps by placing some Br in equatorial positions or by adopting a configuration that minimizes diaxial clashes, leading to lower energy than (iii).

In summary, the key factor is the number of bulky substituents (Br) in the axial positions. The conformer with the most axial bromine substituents will experience the greatest cumulative diaxial strain and thus the highest energy. Based on the provided diagram characteristics, conformer (iii) shows the greatest axial crowding among the options, making it the least stable candidate. Therefore, among i–v, (iii) is expected to have the highest energy due to maximal axial steric interactions.

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