Use Newman projection formulas for ethylene glycol (1,2-ethanediol) and butane to explain why the gauche conformer of ethylene glycol is expected to contribute more to its ensemble of conformers than would the gauche conformer of butane to its respective set of conformers For ethylene glycol, provides stabilization of the gauche conformer. This cannot occur in the case of gauche butane
The correct answer and explanation is :
The gauche conformer of ethylene glycol (1,2-ethanediol) contributes more significantly to its conformational ensemble than the gauche conformer of butane due to intramolecular hydrogen bonding, a stabilizing interaction that is not possible in gauche butane.
Newman Projection and Conformational Analysis:
In butane, if we view the molecule along the central C–C bond (between C2 and C3), we observe different staggered conformations. The two main staggered forms are anti (with the two methyl groups 180° apart) and gauche (with the methyl groups 60° apart). The anti conformer is more stable due to minimal steric hindrance, while the gauche conformer is less stable due to steric repulsion between the two bulky methyl groups.
For ethylene glycol, we examine the molecule along the C–C bond (between the two carbon atoms bearing the hydroxyl groups). The molecule has two hydroxyl (-OH) groups, and the staggered conformations can also be described as anti or gauche, depending on the relative orientation of these groups. Unlike butane, the gauche conformer in ethylene glycol is stabilized by intramolecular hydrogen bonding—a hydrogen bond forms between the hydrogen of one hydroxyl group and the oxygen of the adjacent hydroxyl group. This favorable interaction lowers the energy of the gauche conformer significantly, making it more populated at equilibrium than the anti form.
Key Difference:
- Gauche butane: Destabilized due to steric hindrance between methyl groups.
- Gauche ethylene glycol: Stabilized due to intramolecular hydrogen bonding between hydroxyl groups.
Conclusion:
The Newman projections clearly show that in butane, the gauche form is energetically disfavored due to steric effects, while in ethylene glycol, the same gauche arrangement leads to a net stabilization. This hydrogen bonding does not occur in butane because it lacks polar functional groups capable of forming such bonds. Thus, the gauche conformer is more prominent in ethylene glycol’s conformational ensemble.