Writing Chairs and Chair Flips Methodology
Process:
1. If starting from a name draw planar molecule
a. Cis- and trans- show relative stereochemistry
i. cis- means same side – both wedge OR both dash
ii. trans- means opposite sides – one wedge and one dash
b. R and S show absolute stereochemistry and are exactly either a
wedge or dash. (8 Series Chap. 4, 118 Series Chap. 5)
2. Convert planar molecule to chair
a. Wedged groups are on the top face
b. Dashed groups are on the bottom face
c. Label atoms clockwise or counterclockwise to mark where groups
are. Keep all structures consistently clockwise or counterclockwise
to maintain the same stereoisomer. Labeling every structure the
same way will help prevent errors later due to habit formation.
3. Write the ring flip conformer
4. Compare axial/equatorial
a. Structure with fewer axial groups is lower in energy
b. If both structures have the same number of axial groups, those with
the larger group equatorial will be lower in energy
c. If both are identical = same energy
d. We will not deal with cases where one flip has one large group axial
and the other has two small groups axial – there is math for that but
we’re not going to worry about it.
e. H < X < OH < NH2 < CH3 < CH2R or 1o < CHR2 or 2o < CR3 or 3o
Example:
What is the low energy conformer of trans-1-ethyl-3-fluorocyclohexane?
F
F
Add
stereochemistry
F
1
2
34
5
6
1
2
3
65
4
Add substituents
wedge = up face
dash = down face
F1
2
3
65
4
Number
Carbons
Write chair
and number
F
1
2
3
6541
2
3
6
5
4
ring
flip
F
one axial (F)
one equatorial (ethyl)
one axial (ethyl)
one equatorial (F)
Both structures have one axial and one equatorial
Ethyl is larger so prefers equatorial
Left structure is lower in energy
© S. Lievens, November 2018
University of California, Davis For use in UCDavis Chemistry 8 and 118 Series
Writing Chairs and Chair Flips Practice
Write both chair flips of the given molecule and determine which is the lowest in
energy or if both ring flips are the same energy.
Br
OH
F
NH2
Br
OH
F
cis-1-bromo-2-methylcyclohexane
trans-1-ethyl-3-methylcyclohexane
cis-1-tert-butyl-4-isopropylcyclohexane
trans-1-sec-butyl-2-ethylcyclohexane
cis-1,3-dimethylcyclohexane
trans-1-bromo-4-isobutylcyclohexane
cis-1,2-diethylcyclohexane
A)
B)
C)
D)
E)
F)
G)
H)
I)
J)
K)
L)
M)
N)
O)
P)
Q)
R)
S)
T)
U)
V)
© S. Lievens, November 2018
University of California, Davis For use in UCDavis Chemistry 8 and 118 Series
Writing Chairs and Chair Flips Practice
NH2
trans-1-butyl-3-methylcyclohexane
cis-1-butyl-4-ethylcyclohexane
trans-1,3-dimethylcyclohexane
W)
X)
Y)
Z)
AA)
AB)
AC)
AD)
AE)
AF)
AG)
Cl
HO
cis-1-