Surviving Chair Structures

A How-to Review

Surviving Chair StructuresSurviving Chair StructuresCyclohexane and what you should be able to do:Draw in 2-D, showing stereochemTransfer to a chair conformationDraw the chair flipped conformationIdentify the high or low energy conformation (or calculate the values, if I give you the table of values)Step 1: Cyclohexane in 2-D2 groups on the ring can be Cis (on the same side) or Trans (on opposite sides)

Step 1: Cyclohexane in 2-D2 groups on the ring can be Cis or Trans:

Cis = Same Sides

Step 1: Cyclohexane in 2-D2 groups on the ring can be Cis or Trans:

Trans = Opposite Sides

Step 1: Cyclohexane in 2-DWhich is it? Cis or Trans?

Step 1: Cyclohexane in 2-DAnswer: One Up One Down Trans

Step 1: Cyclohexane in 2-DWhich are they? Cis or Trans?

Step 1: Cyclohexane in 2-DHow did you do?

Step 2: Draw cis-1, 3-dimethylcyclohexaneStep 2: Draw cis-1, 3-dimethylcyclohexaneStart with a hexagon

Step 2: Draw cis-1, 3-dimethylcyclohexane then pick a position to be #1 (and ANY position can be #1)

Step 2: Draw cis-1, 3-dimethylcyclohexane then number around the ring (CW or CCW) to find #3(since you are drawing a 1,3-disubstituted ring)

Step 2: Draw cis-1, 3-dimethylcyclohexane then attach the necessary groups (two methyl groups, in this case) to the #1 and #3 positions youve decided to use

Step 2: Draw cis-1, 3-dimethylcyclohexanethen 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

Step 2: Draw cis-1, 3-dimethylcyclohexane as would these, depending on where you placed your #1 and #3 positions

Step 3: Converting to a Chair structureStep 3: Converting to a Chair structureChair 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

Step 3: Converting to a Chair structureFirst lets remind ourselves about the different positions on a chair structure

Step 3: Converting to a Chair structureEvery cyclohexane chair has six carbons, three that zig-zag up and three that zig-zag down make sure you can identify them

Step 3: Converting to a Chair structureOn 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

Step 3: Converting to a Chair structureOn 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

Step 3: Converting to a Chair structureAll of the positions are shown below in one structure:

Step 3: Converting to a Chair structureAll 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!)

Step 3: Converting to a Chair structureTranslation 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

Step 3: Converting to a Chair structureRecall that any position on the cyclohexane can be #1

Step 3: Convert to a Chair structureand any position on the chair form can be #1

Step 3: Convert to a Chair structurefor practice and repetitions sake (so you are less likely to make errors), choose your #1 on both and always stay in the same positions

Step 3: Convert to a Chair structurefor the purpose of this example, we will use the #1 position on the hexagon shown below

Step 3: Convert to a Chair structureand this will correlate to the #1 position chosen here on this chair form Remember that YOU can chose whichever #1 YOU want to use

Step 3: Converting to a Chair structureIf this is #1 on the hexagon, number the ring accordingly CW or CCW

Step 3: Converting to a Chair structurethen do likewise on the chair structure again, CW versus CCW does not matter (its all RELATIVE to each other)

Step 3: Converting to a Chair structurenow you can see what carbons on the hexagon form of cyclohexane correlate to the chair form of cyclohexane

Step 3: Converting to a Chair structureNow lets consider stereochemistry on the 2-dimensional version of cyclohexane

Step 3: Converting to a Chair structureTry This: Draw trans-1,4-dichlorocyclohexane in 2-dimensions I numbered this one CW, just for fun

Step 3: Converting to a Chair structureHow 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

Step 3: Converting to a Chair structureNow you have to convert this into a chair structure Watch your UPs and DOWNs

Step 3: Converting to a Chair structurePosition #1 has in this 2-D drawing an UP chloro group and position #4 has a DOWN chloro group

Step 3: Converting to a Chair structureLabel 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

Step 3: Converting to a Chair structureYou 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.

Step 3: Converting to a Chair structureTry 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

Step 3: Converting to a Chair structureCis-1-bromo-3-methylcyclohexane, in 2-dimensions

Step 3: Converting to a Chair structureDraw 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

Step 3: Converting to a Chair structureBased 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:

Step 3: Converting to a Chair structureBut if you picked a different #1 on the chair, it might look like:

Step 3: Converting to a Chair structureAnd 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

Step 3: Converting to a Chair structureTrans-1-chloro-2-ethylcyclohexane in 2-D.

Step 3: Converting to a Chair structureKeep 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

Step 3: Converting to a Chair structureHow 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

Step 4: Drawing a Chair Flip StructureStep 4: Drawing a Chair Flip StructureWhen 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.

Step 4: Drawing a Chair Flip StructureNotice how Axial substituents swapped places with Equatorial substituents, but those that were up stayed up and down stayed down

Step 4: Drawing a Chair Flip StructureNow 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

Step 4: Drawing a Chair Flip StructureDraw 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

Step 4: Drawing a Chair Flip StructureThen 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?

Step 4: Drawing a Chair Flip StructureThe diaxial UP positions become diequatorial UP positions:

Step 4: Drawing a Chair Flip StructureNow: Do it Again Draw the following chair in its flipped form:

Step 4: Drawing a Chair Flip StructureLike before lock onto the #1 position and flip the chair

Step 4: Drawing a Chair Flip StructureThere 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

Step 4: Drawing a Chair Flip StructureOne more time Practice makes PerfectDraw the chair flipped conformation for the compound shown below:

Step 4: Drawing a Chair Flip StructureFind the positions, as always note the up groups (#2, #3 and #4) none of the groups are downtwo are equatorial, one is axial.. Now flip the axials and equatorials

Step 4: Drawing a Chair Flip StructureAnd your answer should look like

Step 5: Evaluating Chair Conformations and their Relative EnergiesIf 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

Step 5: Evaluating Chair Conformations and their Relative EnergiesMake 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.

Step 5: Evaluating Chair Conformations and their Relative EnergiesIf 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.

Step 5: Evaluating Chair Conformations and their Relative EnergiesSo 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

Step 5: Evaluating Chair Conformations and their Relative EnergiesThe 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 Hs).

Step 5: Evaluating Chair Conformations and their Relative EnergiesThe larger the group, the larger the energy value for the A1,3 interaction.

Step 5: Evaluating Chair Conformations and their Relative EnergiesMolecules 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

Step 5: Evaluating Chair Conformations and their Relative EnergiesIf 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.

Step 5: Evaluating Chair Conformations and their Relative EnergiesIf 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.

Step 5: Evaluating Chair Conformations and their Relative EnergiesAs 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 Hs are EACH 4.8 kcal/mol. The total energy, for X = C(CH3)3 would be 9.6 kcal/mol.

Step 5: Evaluating Chair Conformations and their Relative EnergiesIf 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?

Step 5: Evaluating Chair Conformations and their Relative EnergiesWith two isopropyl-H interactions, the total strain energy would be twice the value of one, or 4.4 kcal/mol.

Step 5: Evaluating Chair Conformations and their Relative EnergiesWhat 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.

Step 5: Evaluating Chair Conformations and their Relative EnergiesWith 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.

Step 5: Evaluating Chair Conformations and their Relative EnergiesNow compare the two chair conformations and determine which one is more stable?

Step 5: Evaluating Chair Conformations and their Relative EnergiesThe ring with the most and largest groups in axial positions will be the most UNSTABLE and HIGHEST energy conformation. Which one is that, here?

Step 5: Evaluating Chair Conformations and their Relative EnergiesYes, the first conformation is definitely more unstable. The isopropyl group has two A1,3 interactions with Hs 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 ConclusionCyclohexane and what you should be able to do:Draw in 2-D, showing stereochemTransfer to a chair conformationDraw the chair flipped conformationIdentify the high or low energy conformation (or calculate the values, if I give you the table of values)Surviving Chair Structures In ConclusionIf you read through this IMMENSELY LONG slide show and found that it was helpful in learning about chair structures, drop me a note at jdiscord@towson.edu and let me know Thanks,Dr. Discordia