CH 4-5: Cycloalkane Conformations I Cyclohexane Conformations: similar to alkanes, but: Very simple “zig-zag” 2-D structure. Note bonds are all vertical and in same plane. Cyclic structure limits conformations. Cyclohexane “chair” conformer is preferred:

3-D Cyclohexane Chair Conformations The ring is drawn as if it were viewed “on edge” (note “chair” shape): Bonds outside of the ring are called the Axial and Equatorial positions. Equatorial bonds (eq) slant “up” and “down” Axial bonds (ax) are all vertical, and alternate straight “up” and “down”:

Cyclohexane Chair Conformations: Geometry of Substituents 1-bromo-4-methyl cyclohexane What is the Relationship between these two structures? cis-1-bromo-4-methyl cyclohexane trans-1-bromo-4-methyl cyclohexane Cis/Trans isomers are also called Geometric Isomers.

Cyclohexane Chair Conformations: Cis-Trans Draw the two “chair” conformations for trans-1-bromo-4-methyl cyclohexane: Which is the correct “chair” conformation?

Conformational Stability is based on the position of the largest substituent The most stable “chair” conformer is with the largest substituent in the equatorial position.

Conformational Stability is based on the position of the largest substituent The least stable “chair” conformer is with the largest substituent in the axial position.

Cyclohexane Chair Conformations Axial conformers are less stable due to Steric Hinderance caused by the 1,3-diaxial groups “bumping into” each other.