3.11: Conformazione a sedia del cicloesano
The chair conformation is the most stable form of cyclohexane due to the absence of angle and torsional strain. The absence of angle strain is a result of cyclohexane’s bond angle being very close to the ideal tetrahedral bond angle of 109.5° in its chair conformer. Similarly, the torsional strain is also absent owing to the perfectly staggered arrangement of bonds.
The hydrogen atoms linked to carbons are arranged in two different axial and equatorial orientations to achieve this staggered form. The axial bonds are directed straight up or down, lying parallel to the ring axis, whereas the equatorial bonds are pointed sideways roughly along the equator of the ring. Out of the six axial bonds, three are pointed up, and the remaining three are pointed downward. Similarly, three bonds are slanted upwards among the six equatorial bonds, while the remaining three are slanted downwards. Thus, each carbon atom in the cyclohexane ring has an axial and an equatorial bond, pointing in opposite directions.
A chair conformation of cyclohexane can undergo a conformational change into another chair conformer by the partial rotation of C-C bonds. This chair-chair interconversion that leads to the generation of two equivalent energy forms is known as ring flipping. Upon ring flipping, the axial and equatorial bonds interchange their positions. The axial bonds in one chair conformation get converted to equatorial bonds in the other chair conformation, while equatorial bonds change their position to axial bonds.
