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

Question

BlackTom AI · Accepted Answer

First, recall the key concept: for n-butane, the lowest-energy conformation about the C2–C3 bond is the staggered form in which the two methyl (CH3) groups are anti to each other, minimizing steric repulsion.
Option-by-option analysis:
- Option (i): In this projection, the two big methyl groups are not opposite each other; there is greater eclipsing or closer approach between substituents, which raises steric strain and energy relative to the anti staggered form. Hence not the lowest.
- Option (ii): Here, the arrangement places at least one methyl group in closer proximity to a hydrogen on the opposite side, and the relative orientation is not the ideal anti arrangement. This reduces the methyl–methyl clash but not to the minimum possible; energy remains higher than the anti form.
- Option (iii): This orientation again fails to achieve the perfect anti alignment of both methyl groups, leading to increased steric interactions compared with the true anti staggered conformation.
- Option (iv): In this depiction, the methyl groups are either eclipsed or partially eclipsed with neighboring substituents, which is energetically unfavorable due to direct steric crowding and torsional strain. Not the lowest energy.
- Option (v): This is the classic anti staggered arrangement where the two methyl groups lie 180° apart on the C2–C3 bond. This minimizes methyl–methyl steric interaction and avoids eclipsing, giving the lowest energy among the conformers.
- Option (vi): This orientation still involves some degree of steric interaction between substituents and does not place the methyl groups in the optimal anti position, resulting in higher energy than option (v).

Thus, among the shown conformers, the one that places the two CH3 groups anti to each other (option v) yields the least steric clash and the lowest torsional strain, making it the most stable conformation for n-butane about the C2–C3 bond.

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 LOWEST energy conformer of n-butane?

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