Cyclohexanes and Chair Conformations Flashcards
Chair overview
- cyclohexane rings are free of torsional strain and bond angle strain
- they will adopt a chair conformation preferentially to minimize eclipsion between hydrogen atoms (same w boat conformation)
- chairs will flip back and forth to reduce bond angle strain and unfavorable steric interactions - the chair “flip” that is favored is the most stable chair conformation
what does a chair “flip” mean
a chair flip is when a chair tilts to alter the orientation of substituents from axial to equatorial and vice versa
equatorial vs axial
equatorial = off angle bonds or off axis
axial = straight up or down/ right angle, on axis
What happens in a chair flip conversion aka going from one chair conformation to another
- all axial substituents will adopt an equatorial position and vice versa
- the up positions or down positions will NOT change; they stay the same
- visually it looks like substituents rotate one position clockwise when converting from one chair conformation to its chair flip
Conformations of a cyclohexane
- cyclohexane can exist in a chair conformation (2 distinct chair flips), a half chair conformation, a boat conformation or a twist-boat conformation
Chair
- free of bond angle strain
- no eclipsed hydrogens or groups
- destabilized by 1,3 diaxial interactions and gauche interactions (depending on the substitution of a cyclohexane)
Boat
- free of bond angle strain
- destabilized by hydrogen atoms on opposite sides of the ring interacting
Twist boat
- slightly lower in energy due to the flagpole hydrogens no longer eclipsing with one another (less steric strain)
Half chair
- highest energy conformation
- they have bond angle strain and torsional strain from partial eclipsion of hydrogens
converting a perspective cyclohexane into a chair structure
- plotting a substituent on the chair structure in the top right corner - this position will have an up-axial and a down equatorial position
- plot the remaining substituents on the chair structure being sure to keep the following faithful to the perspective drawing:
- solid wedge substituents are UP in the chair
- dashed wedged substituents are DOWN on the chair
- the clockwise/counterclockwise relationship between substituents is preserved
A trans 1,2-diequatorial relationship between two non-hydrogen groups constitutes a destabilizing ____ interaction!
trans 1,2-diequatorial = GAUCHE interaction
Groups that are 1,3 diaxial will have penalizing ____ interactions
steric interactions on both sides of the “face” of the chair - top and bottom
- this is for DIAXIAL
What do 1,3 diaxial relationships occur between
two non-hydrogen groups or a hydrogen group and a non-hydrogen group
atoms/groups occupying an ___ position is more favorable energetically as there are no penalizing 1,3-diaxial interactions when a group is ___
equatorial! - more favorable energetically
Larger, more sterically penalizing substituents have a higher preference to occupy the ___ positions compared to ____ less sterically penalizing substituents
Larger, more sterically penalizing substituents prefer to occupy the equatorial position
Steric Penalty ranking
H< CN < F < I < Br < Cl < OH < CH3 < CH2CH3 < CH(CH3)2 < C(CH3)3
