Consider the Newman projections (i - vi) looking down the C2 - C3 bond in n-butane. Select the highest energy conformer of n-butane?

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To determine the highest-energy conformer for n-butane when looking down the C2–C3 bond, we compare the interactions between substituents on the two adjacent carbons in each Newman projection. The key idea is that eclipsing interactions are higher in energy than staggered ones, and among eclipsed forms, the eclipse of two methyl groups (CH3–CH3) is the most energetic due to steric crowding.

Option a (i): Analyze whether in this projection the two front and back substituents are eclipsing or staggered. If the two methyl groups are not directly eclipsing each other and the conformer is not at the maximum CH3–CH3 overlap, then this form would be lower in energy than the CH3–CH3 eclipse case, likely a staggered or an eclipsed with CH3–H overlap. This would not be the highest-energy arrangement.

Option b (ii): This is the designated correct choice in the provided answer. Here, the methyl groups on C2 and C3 are eclipsed with each other (CH3–CH3 eclipse). Because CH3–CH3 eclipsing is the worst steric clash among the possible eclipsed interactions in n-butane, this conformer represents the highest-energy situation for the C2–C3 bond rotation. The energy penalty from two bulky methyl groups occupying the same ethane-like axis is greater than any eclipsing of CH3 with H, or any staggered arrangement.

Option c (iii): If this projection shows an eclipsed arrangement but with CH3 eclipsing H rather than CH3 eclipsing CH3, the steric strain is substantial but not as severe as CH3–CH3 eclipse. Therefore, this conformer lies lower in energy than the CH3–CH3-eclipsed form in option ii.

Option d (iv): If this projection is a staggered form (methyls separated by 60 degrees) or an eclipsed form with CH3–H overlaps, its energy is lower than the maximal CH3–CH3 eclipse. Staggered conformations are typically the lowest-energy states, and CH3–H eclipsing is also less energetic than CH3–CH3 eclipsing.

Option e (v): Similar reasoning applies to this projection; unless it again places the two methyl groups directly eclipsing each other, its energy will be lower than the CH3–CH3 eclipse. If it shows CH3–CH3 eclipse, that would be a candidate for high energy, but the given labeling suggests this one does not represent the maximum overlap.

Option f (vi): Depending on the exact arrangement, if this is a staggered conformation or an eclipsed form with only CH3–H overlaps, its energy would be lower than the CH3–CH3 eclipsed form. Without CH3–CH3 eclipse, it is not the highest energy.

In summary, the maximum steric strain in a butane Newman projection around the C2–C3 bond occurs when the two methyl groups are directly eclipsing each other (CH3–CH3 eclipse). Among the listed options, the one that shows this CH3–CH3 eclipse is the highest-energy conformer, which corresponds to option ii. The other options involve either CH3–H eclipses or staggered arrangements, which produce lower energies than the CH3–CH3 eclipsed form.

My LMS Subjects Practice LMS quiz, Week 1

Consider the Newman projections (i - vi) looking down the C2 - C3 bond in n-butane. Select the highest energy conformer of n-butane?

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