Cyclohexanes and Chair Conformations

Question 1

Chair overview

Answer 1

• 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

Question 2

what does a chair “flip” mean

Answer 2

a chair flip is when a chair tilts to alter the orientation of substituents from axial to equatorial and vice versa

Question 3

equatorial vs axial

Answer 3

equatorial = off angle bonds or off axis

axial = straight up or down/ right angle, on axis

Question 4

What happens in a chair flip conversion aka going from one chair conformation to another

Answer 4

• 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

Question 5

Conformations of a cyclohexane

Answer 5

• cyclohexane can exist in a chair conformation (2 distinct chair flips), a half chair conformation, a boat conformation or a twist-boat conformation

Question 6

Chair

Answer 6

• 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)

Question 7

Boat

Answer 7

• free of bond angle strain

- destabilized by hydrogen atoms on opposite sides of the ring interacting

Question 8

Twist boat

Answer 8

• slightly lower in energy due to the flagpole hydrogens no longer eclipsing with one another (less steric strain)

Question 9

Half chair

Answer 9

• highest energy conformation

- they have bond angle strain and torsional strain from partial eclipsion of hydrogens

Question 10

converting a perspective cyclohexane into a chair structure

Answer 10

• 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

Question 11

A trans 1,2-diequatorial relationship between two non-hydrogen groups constitutes a destabilizing ____ interaction!

Answer 11

trans 1,2-diequatorial = GAUCHE interaction

Question 12

Groups that are 1,3 diaxial will have penalizing ____ interactions

Answer 12

steric interactions on both sides of the “face” of the chair - top and bottom

• this is for DIAXIAL

Question 13

What do 1,3 diaxial relationships occur between

Answer 13

two non-hydrogen groups or a hydrogen group and a non-hydrogen group

Question 14

atoms/groups occupying an ___ position is more favorable energetically as there are no penalizing 1,3-diaxial interactions when a group is ___

Answer 14

equatorial! - more favorable energetically

Question 15

Larger, more sterically penalizing substituents have a higher preference to occupy the ___ positions compared to ____ less sterically penalizing substituents

Answer 15

Larger, more sterically penalizing substituents prefer to occupy the equatorial position

Question 16

Steric Penalty ranking

Answer 16

H< CN < F < I < Br < Cl < OH < CH3 < CH2CH3 < CH(CH3)2 < C(CH3)3