How do you determine axial vs. equatorial?

[Sammy1024]

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I'm a little confused by something I believe I saw in chad's videos.

It said something along the lines that the groups alternate, eq, ax, eq, etc. but then I get confused when a question asks me what is the position of the molecules in the picture.

Ex: Imagine a cyclohexane with a CH3 and OH para to each other, would that mean they're both equitorial?

 

[t5Nitro]

Hey Sammy, there are axial and equitorial positions at all carbons. See this pic:

To answer your question: it might be. If CH3 and OH are para to each other, then the preferred position would be to have both equitorial for greater stability (assuming the other bond to those carbons is to hydrogens). Axial is all your vertical bonds.

 

[puppylatte]

You can tell in a chair conformation. I would define equatorial and axial in words but it sounds unnecessarily confusing. Look up chair conformation determine the axial/equatorial positions. If something is "para" in a chair conformation than you will have, say, a CH3 on carbon 1 and the OH on carbon 4.

 

[Sammy1024]

So would it be best to have them both be in eq position? If it was something incredibly bulky, such as t-butyl then would it be t-butyl as eq and then OH as ax?

 

[t5Nitro]

Yep! Bulky substituents have the equitorial position priority. Note that, when looking at an image with wedges and dashes, to identify them as "up" and "down".

Here's an example. If both substituents are on a wedge, then you can call them "up", and on those carbons you would draw them both "up". Up/down does not correlate to equitorial/axial. Check it out here. Both are on wedges, both are "up" then, and when drawing the chair conformation, one is axial and another equitorial. In the second set, one substituent is "down" and the other is "up". When labeling the chair, it turns these two specifically to be both equitorial. Note that the "up" position at carbon 1 is axial while the "up" position at carbon 2 is equitorial. You can go around the ring and keep adding "up" and "down" labels to each carbon this way. Does this help?

 

[TXhopeful]

From Berkeley Review Organic Chemistry, Structure Elucidation, p. 105: "[In chair conformation] Equatorial is more stable than axial, so the most stable conformation of a cyclohexane compound has the largest constituents in the equatorial position".

 

[Cawolf]

So would it be best to have them both be in eq position? If it was something incredibly bulky, such as t-butyl then would it be t-butyl as eq and then OH as ax?

If by best you mean the lowest energy conformation, then yes.

T-butyl is so large that for steric reasons many conformations would be so high energy if the group was axial that they are not reasonably attainable or seen at room temperature.

 

[Sammy1024]

Yep! Bulky substituents have the equitorial position priority. Note that, when looking at an image with wedges and dashes, to identify them as "up" and "down".

Here's an example. If both substituents are on a wedge, then you can call them "up", and on those carbons you would draw them both "up". Up/down does not correlate to equitorial/axial. Check it out here. Both are on wedges, both are "up" then, and when drawing the chair conformation, one is axial and another equitorial. In the second set, one substituent is "down" and the other is "up". When labeling the chair, it turns these two specifically to be both equitorial. Note that the "up" position at carbon 1 is axial while the "up" position at carbon 2 is equitorial. Does this help?

Omg! Yes thanks so much! I think for the longest time I was getting what the wedge meant during translation from ring to chair! Thank you so much! :]

 

[t5Nitro]

Omg! Yes thanks so much! I think for the longest time I was getting what the wedge meant during translation from ring to chair! Thank you so much! :]

Glad it helped. Keep the "up" and "down" stuff in mind then as you continue. If you go a step further and look at the 3rd image in those figures, then you'll have to keep in mind "up" and "down" when labeling a ring flip. Up and down never changes, but equitorial and axial may change. Notice how the CH3 is "up" in the first set. Then look at the ring flip. The CH3 is still "up", but the orientation changed from axial to equitorial (edit: I'm referring to the blue one). This is a good example of why you don't want to link up and down to equitorial and axial.