Quick/Easy Question on Drawing Chair Conformation of Cyclohexane

[MissionStanford]

If a cyclohexane is drawn with one substituent (ex: methyl group, t-butyl group, etc.) that is NOT on a wedge or dash, how do I know whether I draw that substituent in the equatorial or axial position in the chair conformation? In other words, if it's a wedge, we know it's up, and if it's a dash we know it's down, so we know whether to draw the substituent as axial or equatorial, but what if the substituent isn't on a wedge or dash?

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[Jepstein30]

If a cyclohexane is drawn with one substituent (ex: methyl group, t-butyl group, etc.) that is NOT on a wedge or dash, how do I know whether I draw that substituent in the equatorial or axial position in the chair conformation? In other words, if it's a wedge, we know it's up, and if it's a dash we know it's down, so we know whether to draw the substituent as axial or equatorial, but what if the substituent isn't on a wedge or dash?

Regardless whether it's "up" or "down," it can still be drawn as axial or equatorial. That doesn't have anything to do with it, it only tells you the relationship among multiple substituents.

If a substituent isn't on a wedge or dash, I'd assume its the only substituent on the cyclohexane (because otherwise, it has to be cis or trans), so it would be equatorial.

 

[MissionStanford]

Regardless whether it's "up" or "down," it can still be drawn as axial or equatorial. That doesn't have anything to do with it, it only tells you the relationship among multiple substituents.

If a substituent isn't on a wedge or dash, I'd assume its the only substituent on the cyclohexane (because otherwise, it has to be cis or trans), so it would be equatorial.

I'm not sure I understood the first part of what you said. For example, if there is a substituent on carbon number 2, and if that substituent is on a wedge, we know it's up, and on carbon 2, up can't be either axial or equatorial. It has to be one of the two.

The second part made sense though. Thanks.

 

[Jepstein30]

I'm not sure I understood the first part of what you said. For example, if there is a substituent on carbon number 2, and if that substituent is on a wedge, we know it's up, and on carbon 2, up can't be either axial or equatorial. It has to be one of the two.

The second part made sense though. Thanks.

I thought you decided whether a group is axial or equatorial based on whether it is "up" or "down." The group on carbon two that is on a wedge can be either axial OR equatorial. It would be equatorial with no other groups but if there are other groups, it would depend on their orientation/size.

You can't just make a decision on whether it is axial or equatorial based on whether it is on a wedge and dash alone.

 

[MissionStanford]

I thought you decided whether a group is axial or equatorial based on whether it is "up" or "down." The group on carbon two that is on a wedge can be either axial OR equatorial. It would be equatorial with no other groups but if there are other groups, it would depend on their orientation/size.

You can't just make a decision on whether it is axial or equatorial based on whether it is on a wedge and dash alone.

I went over Klein's Organic Chemistry and understand what you're saying now, so I guess my real question is, if you see a substituent in the hexagon-style drawing, and the substituent isn't on a wedge or dash, how do you know whether that substituent is "up" or "down" when drawing the chair conformation?

 

[sunflower18]

I went over Klein's Organic Chemistry and understand what you're saying now, so I guess my real question is, if you see a substituent in the hexagon-style drawing, and the substituent isn't on a wedge or dash, how do you know whether that substituent is "up" or "down" when drawing the chair conformation?

My understanding -- if there is only one substituent, draw it as equatorial. It's the most stable.

 

[Jepstein30]

I went over Klein's Organic Chemistry and understand what you're saying now, so I guess my real question is, if you see a substituent in the hexagon-style drawing, and the substituent isn't on a wedge or dash, how do you know whether that substituent is "up" or "down" when drawing the chair conformation?

Up or down doesn't matter on a chair conformation. Any axial is equivalent to any other axial position. Any equatorial equivalent to any other equatorial position.

All that matters is whether it is axial or equatorial. Even if it is "up" in the wedge and dash, it can still be axial pointing 'down' in the chair conformation. It's the same thing (with a ring flip, but it's the same thing for all intents and purposes).

The only thing you need to use the wedge and dashes for are to determine the relationship between different groups. If, for instance, there is a tert-butyl group pointed "up" at carbon 1 and a methyl group pointed "up" at carbon 2...

1) Determine which group is bulkiest (or if 3+ groups, which combination of groups are bulkiest) and put that group equatorial.
2) Equatorial and Axial switch off as you go around the cyclohexane. If "up" at carbon 1 is equatorial than "up" at carbons 2 and 6 are axial. If "down" is axial at carbon 2, "down" at carbon 4 and 6 are axial.
3) Using that rule, place the other groups.

So for that mini-example up there, we would put tert-butyl equatorial which would make methyl axial. Doesn't matter which way they are pointing on the chair itself.

 

[MissionStanford]

Up or down doesn't matter on a chair conformation. Any axial is equivalent to any other axial position. Any equatorial equivalent to any other equatorial position.

All that matters is whether it is axial or equatorial. Even if it is "up" in the wedge and dash, it can still be axial pointing 'down' in the chair conformation. It's the same thing (with a ring flip, but it's the same thing for all intents and purposes).

The only thing you need to use the wedge and dashes for are to determine the relationship between different groups. If, for instance, there is a tert-butyl group pointed "up" at carbon 1 and a methyl group pointed "up" at carbon 2...

1) Determine which group is bulkiest (or if 3+ groups, which combination of groups are bulkiest) and put that group equatorial.
2) Equatorial and Axial switch off as you go around the cyclohexane. If "up" at carbon 1 is equatorial than "up" at carbons 2 and 6 are axial. If "down" is axial at carbon 2, "down" at carbon 4 and 6 are axial.
3) Using that rule, place the other groups.

So for that mini-example up there, we would put tert-butyl equatorial which would make methyl axial. Doesn't matter which way they are pointing on the chair itself.

I think I get what you're saying, but according to the Klein's Organic Chemistry book, a wedge means "up," and a dash means "down." You, however, say that "up" with a wedge can still be "down" in the chair conformation. This seems to contrast with the book.

In the book, a wedge is always drawn as "up" in the chair conformation. For example, in the book there is a Br on a wedge on carbon 1 and a Cl on a dash on carbon 3. The author draws the chair conformation with Br in the "up" axial position and Cl in the "down" equatorial position.

However, I think what you are suggesting is that the wedge and dash simply imply that the Br and Cl have to be opposite in terms of "up" and "down" (One is "up" and one is "down") since we have a wedge and a dash rather than two wedges or two dashes. But, I think what you're saying also implies that Br should be in the "down" equatorial position, and Cl should be in the "up" axial position since Br is bigger than Cl.

 

[Jepstein30]

I think I get what you're saying, but according to the Klein's Organic Chemistry book, a wedge means "up," and a dash means "down." You, however, say that "up" with a wedge can still be "down" in the chair conformation. This seems to contrast with the book.

In the book, a wedge is always drawn as "up" in the chair conformation. For example, in the book there is a Br on a wedge on carbon 1 and a Cl on a dash on carbon 3. The author draws the chair conformation with Br in the "up" axial position and Cl in the "down" equatorial position.

However, I think what you are suggesting is that the wedge and dash simply imply that the Br and Cl have to be opposite in terms of "up" and "down" (One is "up" and one is "down") since we have a wedge and a dash rather than two wedges or two dashes. But, I think what you're saying also implies that Br should be in the "down" equatorial position, and Cl should be in the "up" axial position since Br is bigger than Cl.

Chair conformation is 3D as drawn on paper. So while a wedge is always drawn as "up" in the chair conformation in the book, you can just turn the page upside down and it is now "down" (but obviously still the same molecule). Wedge and dash is 2D representation of a 3D object so you can't just turn things upside down, obviously.

So while that may be the book's convention and can simplify things a bit, that is not absolutely correct. A wedge can be "up" axial or "down" axial, or "up" equatorial or "down" equatorial.

If you have Br and Cl on NEIGHBORING carbons with opposite orientations of "up" and "down":
1) Br is largest, should be equatorial
2) Since Cl is on a neighboring carbon with opposite orientation, it too can be equatorial
They both would be equatorial. Whether they are "up" or "down" equatorial doesn't matter as again, these positions don't actually exist.

If you have Br and Cl on carbons 1 and 3 for instance with opposite orientations of "up" and "down":
1) Br is largest, should be equatorial
2) Since Cl is on carbon 3 with opposite orientation, it must be axial.
Again "up" and "down" axial/equatorial don't exist. You can prove this to yourself by turning the page upside down and noticing the same chair conformation (i.e. same molecule) can have "up" or "down" . It's all about axial or equatorial when it comes to chair conformations.

 

[MissionStanford]

Chair conformation is 3D as drawn on paper. So while a wedge is always drawn as "up" in the chair conformation in the book, you can just turn the page upside down and it is now "down" (but obviously still the same molecule). Wedge and dash is 2D representation of a 3D object so you can't just turn things upside down, obviously.

So while that may be the book's convention and can simplify things a bit, that is not absolutely correct. A wedge can be "up" axial or "down" axial, or "up" equatorial or "down" equatorial.

If you have Br and Cl on NEIGHBORING carbons with opposite orientations of "up" and "down":
1) Br is largest, should be equatorial
2) Since Cl is on a neighboring carbon with opposite orientation, it too can be equatorial
They both would be equatorial. Whether they are "up" or "down" equatorial doesn't matter as again, these positions don't actually exist.

If you have Br and Cl on carbons 1 and 3 for instance with opposite orientations of "up" and "down":
1) Br is largest, should be equatorial
2) Since Cl is on carbon 3 with opposite orientation, it must be axial.
Again "up" and "down" axial/equatorial don't exist. You can prove this to yourself by turning the page upside down and noticing the same chair conformation (i.e. same molecule) can have "up" or "down" . It's all about axial or equatorial when it comes to chair conformations.

Ok, I think I basically get what you're saying, but based on that last part, does that mean the book's drawing is incorrect?

 

[Jepstein30]

Ok, I think I basically get what you're saying, but based on that last part, does that mean the book's drawing is incorrect?

What molecule is the book trying to draw?

EDIT: "In the book, a wedge is always drawn as "up" in the chair conformation. For example, in the book there is a Br on a wedge on carbon 1 and a Cl on a dash on carbon 3. The author draws the chair conformation with Br in the "up" axial position and Cl in the "down" equatorial position. "

so this is trans 1-3 substituted.

Br would be equatorial meaning the atom that is cis to Br on carbon 3 would also be equatorial. Since Cl is trans at carbon 3, it would be axial.

Not sure why the author would put Br in axial when it is bigger than Cl..

 

[PlumbLine]

Not sure why the author would put Br in axial when it is bigger than Cl..

I believe the fact that it is a bigger atom is the very reason it's axial. It has a longer bond length, therefore fewer i/a with the other axial atoms.

 

[MissionStanford]

I believe the fact that it is a bigger atom is the very reason it's axial. It has a longer bond length, therefore fewer i/a with the other axial atoms.

You're right. I was looking through my college organic chemistry textbook (not Klein's), and it said, "The C-Br bond is longer than the C-Cl bond, which causes bromine to be farther away than chlorine from the other axial substituents. Apparently, the longer bond more than offsets the larger diameter."