surviving chair structures. surviving chair structures a “how-to” review

Surviving Chair Structures

Surviving Chair Structures

A “How-to” Review

Step 1: Cyclohexane in 2-D

• 2 groups on the ring can be Cis (on the same side) or Trans (on opposite sides)


• 2 groups on the ring can be Cis or Trans:

– Cis = Same Sides

or


• 2 groups on the ring can be Cis or Trans:

– Trans = Opposite Sides

or


• Which is it? Cis or Trans?

Br

Br


• Answer: One Up… One Down… Trans…

Br

Br

TRANS


• Which are they? Cis or Trans?

Br

CH2CH3

HO CH3

H2N

OH


• How did you do? Br

CH2CH3

HO CH3

H2N

OH

CIS CISTRANS

Step 2: Draw cis-1, 3-dimethylcyclohexane


Start with a hexagon…


… then pick a position to be #1 (and ANY position can be #1)…

11

11

or or or etc...


… then number around the ring (CW or CCW) to find #3(since you are drawing a 1,3-disubstituted ring)

1

or1 1

3 3


… then attach the necessary groups (two methyl groups, in this case) to the #1 and #3 positions you’ve decided to use…

1

3

CH3

CH3


…then add stereochemistry to show the relative positions (cis = same side, trans = opposite sides). Since the compound is cis-1,3-dimethylcyclohexane, either of the answers below will be correct answers…

CH3

CH3

CH3

CH3

CH3

CH3


… as would these, depending on where you placed your #1 and #3 positions…

CH3

CH3

CH3H3C

CH3

H3C

CH3H3C

Step 3: Converting to a Chair structure


Chair structures are the 3-D representation of the cyclohexane ring… Remember that this view is from the side of the ring, not from above or below…

close side

far side


First – let’s remind ourselves about the different positions on a chair structure…


Every cyclohexane chair has six carbons, three that zig-zag up and three that zig-zag down… make sure you can identify them…

"up"

"down"


On the positions that zig-zag UP, you will find the vertical UP AXIAL positions… on the positions that zig-zag DOWN, you will find the vertical DOWN AXIAL positions…


On the positions that zig-zag UP, you will find the (semi-horizontal) DOWN EQUATORIAL positions… on the positions that zig-zag DOWN, you will find the (semi-horizontal) UP EQUATORIAL positions…


All of the positions are shown below in one structure:


All positions, together on the ring… make sure you can find them to draw them… Fill them in on the template shown… (answer on previous slide!)


Translation required: wedges (or bold lines) in 2-D are the “UP” positions and dashes in 2-D are the “DOWN” positions… Up is Up and Down is Down, regardless of axial or equatorial…

1

up, axial1

1

1up, equatorial


Recall that any position on the cyclohexane can be #1…

1

11

or etc...1

or or

Step 3: Convert to a Chair structure

…and any position on the chair form can be #1…

11

or etc...

1

or


…for practice and repetition’s sake (so you are less likely to make errors), choose your #1 on both and always stay in the same positions…


…for the purpose of this example, we will use the #1 position on the hexagon shown below…

1


…and this will correlate to the #1 position chosen here on this chair form… Remember that YOU can chose whichever #1 YOU want to use…

11


If this is #1 on the hexagon, number the ring accordingly… CW or CCW…

123

45

61


…then do likewise on the chair structure… again, CW versus CCW does not matter (its all RELATIVE to each other…)

1 1

23

56

4


…now you can see what carbons on the hexagon form of cyclohexane correlate to the chair form of cyclohexane…

123

45

61

23

4

56


Now let’s consider stereochemistry on the 2-dimensional version of cyclohexane…


Try This: Draw trans-1,4-dichlorocyclohexane in 2-dimensions… I numbered this one CW, just for fun…

12

34

5

6


How did you do? Your answer may not look exactly the same as the one below, but remember – its all RELATIVE… trans = one UP and one DOWN…

1

trans-1,4-dichlorocyclohexane

2

345

6

Cl

Cl


Now you have to convert this into a chair structure… Watch your UP’s and DOWN’s…

1


2

345

6

Cl

Cl


Position #1 has in this 2-D drawing an UP chloro group and position #4 has a DOWN chloro group…

1


2

345

6

Cl

Cl


Label position 1 and 4 on the chair structure you are using and then transfer the information. Position 1 has an UP chloro group and position 4 has a DOWN chloro group…

11

23

56

4

2

345

6

Cl

Cl


You need to fill in the groups on their positions (recall Axial and Equatorial positions). Position #1 has an UP chloro group and “up” on Position #1 will be AXIAL UP. Position #4 has a DOWN chloro group and “down” on Position #4 will be AXIAL DOWN.

1 1

23

56

4

2

345

6

Cl

Cl

Cl

Cl


Try it again: Draw cis-1-bromo-3-methylcyclohexane as a chair structure… Start in 2-D… Go ahead – before you move to the next slide… Note that this time I numbered CW…

12

34

5

6


Cis-1-bromo-3-methylcyclohexane, in 2-dimensions…

1

Br

CH3

23

45

6


Draw cis-1-bromo-3-methylcyclohexane as a chair structure. Convert to 3-dimensions – find the positions you need… then determine axial versus equatorial… Down on #1… Down on #3…

12

34

5

61 2 3

456

Br

CH3


Based on MY numbering, you should have had “cis” drawn as two substituents in the DOWN positions, as shown below. The chair structure would look like:

12

34

5

6

12

3

456

Br

CH3

CH3Br


But if you picked a different #1 on the chair, it might look like:

1

Br

CH3

23

45

61 2 3

456

Br CH3


And again: Draw trans-1-chloro-2-ethylcyclohexane as a chair structure. Start in 2-D…. Pick a #1 and number CW or CCW… Add your groups… Give them stereochemistry (wedges/dashes) to show “trans”…

12

34

5

6


Trans-1-chloro-2-ethylcyclohexane in 2-D….

1

Cl

23

45

H3CH2C6


Keep Going: Draw trans-1-chloro-2-ethylcyclohexane as a chair structure. Remember, there are lots of alternatives, depending on where you placed YOUR #1 and if you went CW or CCW to find #2…

12

34

5

6

Cl

H3CH2C1

23

4

56


How did you do? Remember that when drawing trans-1-chloro-2-ethylcyclohexane as a chair structure, there are lots of alternatives, depending on where you placed YOUR #1 and if you went CW or CCW to find #2…

12

34

5

6

Cl

H3CH2C1

23

4

56Cl

H3CH2C

Step 4: Drawing a Chair Flip Structure


When the chair structure of a cyclohexane ring does a “flip”, all of the axial substituents become equatorial and vice versa. Lock onto an “up” carbon (see #1 labeled below) and find its corresponding “down” carbon in the second conformation.

1

14

4


Notice how Axial substituents swapped places with Equatorial substituents, but those that were “up” stayed “up” and “down” stayed “down”…

1

4 1

4


Now try to transfer the information from one chair to its “flipped” conformation… Start with cis-1,3-dimethylcyclohexane… In this form, you see the two methyl groups are both UP… axial on #1, axial on #3…

1

CH3CH3

4


Draw the “flipped” conformation of cis-1,3-dimethyl-cyclohexane… Remember the #1 carbon in the first form is an “up” carbon and must therefore be a “down” carbon in the flipped chair form… Number accordingly…

1

2 3 4

56

CH3CH3

1

4

23

56


Then add the groups to the positions they belong on… #1 and #3, in this case… Remember the methyl groups are both “up” and must stay “up”… the axial methyls become equatorial… What will that look like?

1

2 3 4

56

CH3CH3

1

4

23

56


The diaxial UP positions become diequatorial UP positions:

1

2 3 4

56

CH3CH3

1

4

23

56

H3C CH3


Now: Do it Again… Draw the following chair in its flipped form:

Cl

HO


Like before – lock onto the #1 position and flip the chair…

1

2 3 4

56

1

4

23

56

Cl

HO


There is a “down” chloro on MY #2 and an “up” alcohol group on MY #6… they must stay DOWN and UP, respectively… Axial becomes Equatorial and vice versa…

1

2 3 4

56

1

4

2

3

56

Cl

HOOH

Cl


One more time… Practice makes Perfect…Draw the chair flipped conformation for the compound

shown below:

CH3

CH3H3C


Find the positions, as always… note the “up” groups (#2, #3 and #4)… none of the groups are “down”…two are equatorial, one is axial.. Now flip the axials and equatorials…

1

23 4

56

1

4

2

3

56CH3

CH3H3C


And your answer should look like…

1

23 4

56

1

4

23

56CH3

CH3H3C

CH3

CH3

CH3

Step 5: Evaluating Chair Conformations and their Relative Energies

If the two chair conformations are exactly the same, they will be the same energy. See the two examples below. Both sets are equal in energy…

Br

Br Br

Br


Make sure you can see that in this example you have two of the same groups, both up, with one axial while the other one is equatorial. They will have the same energy value.

Br

Br Br

Br


If the two chairs are not the same, they will have different energies, caused by different interactions (e.g. sterics). Note how in the first chair form, the bromo group is axial. In the second it is equatorial. They are different, and therefore different in energy values.

Br

Br


So – why are they different in energy? The equatorial groups are a lower energy state because they point out and away from the molecule. The axial groups, however, have a higher energy steric interaction…

Br

Br


The steric interaction that needs to be considered is called the A1,3 interaction. It occurs for any axial group (top or bottom of ring) with the other axial groups on the same side (even H’s).

XH

H

1

2

2'3

3'


The larger the group, the larger the energy value for the A1,3 interaction.

XH

H


Molecules have been studied for their energy values. The A1,3 interactions are quantified on tables that you can look up. As long as you know what X is, you can determine the energy of the system. For example, if X is a CH3, the A1,3 interaction is 0.9 kcal/mol for EACH CH3-H interaction…

X H

H


If the X group is a CH3, the A1,3 interactions occur with EACH H on the same side of the ring – thus the TOTAL energy is 0.9 kcal x 2… The total energy, for X = CH3 would be 1.8 kcal/mol.

X H

H


If the X group is a CH2CH3, the A1,3 interactions occur with EACH H on the same side of the ring – thus the TOTAL energy is 0.95 kcal x 2… The total energy, for X = CH2CH3 would be 1.9 kcal/mol.

X H

H


As the groups get larger, so does the energy of the conformation. If X group is a t-butyl group (-C(CH3)3], the A1,3 interactions with the H’s are EACH 4.8 kcal/mol. The total energy, for X = C(CH3)3 would be 9.6 kcal/mol.

X H

H


If X is an isopropyl group, the A1,3 interaction with one H would be 2.2 kcal/mol. What would be the total energy of the system shown below?


With two isopropyl-H interactions, the total strain energy would be twice the value of one, or 4.4 kcal/mol.

H

H


What about this one? Watch both the top AND the bottom of this ring. Remember the A1,3 value for methyl-H is 0.9 kcal and for ethyl-H is 0.95 kcal/mol.


With two methyl-H interactions on top and two ethyl-H interactions on the bottom, the total would be (2 x 0.9) + (2 x 0.95) = 3.7 kcal/mol total energy.

CH3H

H

HH


Now compare the two chair conformations and determine which one is more stable?


The ring with the most and largest groups in axial positions will be the most UNSTABLE and HIGHEST energy conformation. Which one is that, here?


Yes, the first conformation is definitely more unstable. The isopropyl group has two A1,3 interactions with H’s adding to 4.4 kcal/mol. The other has two methyl-H A1,3 interactions that add to 1.8 kcal/mol. The first is higher energy! By 2.6 kcal/mol…

Surviving Chair Structures… In Conclusion…

Cyclohexane and what you should be able to do:• Draw in 2-D, showing stereochem• Transfer to a chair conformation• Draw the chair flipped conformation• Identify the high or low energy conformation (or

calculate the values, if I give you the table of values)

Surviving Chair Structures… In Conclusion…

If you read through this IMMENSELY LONG slide show and found that it was helpful in learning about chair structures, drop me a note at [email protected] and let me know…

Thanks,Dr. Discordia

mailto:[email protected]

surviving chair structures. surviving chair structures a “how-to” review

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