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How does the chair conformation reduce strain in cyclohexane?
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isomerism
Tetrahedral geometry of methane: (A) stick-and-ball model and (B) diagram showing bond angles and distances. (Plain bonds represent bonds in the plane of the image; wedge and dashed bonds represent those directed toward and away from the viewer, respectively.)
The planar form of cyclohexane is highly unstable due to significant torsional and angle strain. In a flat hexagon, the C–C–C bond angles are forced to 120°, which deviates from the ideal tetrahedral angle of approximately 109.5°–112°, and the molecule suffers from 12 pairs of eclipsed carbon-hydrogen bonds.12
The chair conformation reduces this strain through two primary mechanisms:
- Minimization of Angle Strain: By distorting from planarity, the chair form achieves a C–C–C bond angle of 111.5°, which is very close to the optimal tetrahedral angle.12
- Elimination of Torsional Strain: The chair conformation allows the carbon-hydrogen bonds to adopt a staggered arrangement, effectively eliminating the destabilizing eclipsing interactions present in the planar form.13
Because the chair conformation successfully minimizes both angle and torsional strain, it represents the energy minimum for cyclohexane, making it the most stable and predominant form at room temperature.123