What's the easiest way to tell if something is axial/equatorial when your cyclohexane is in the chair conformation from when you are looking at the molecule from the top when your substituents are either cis/trans? Thanks!
Axial and Equatorial for Cyclohexanes
- Thread starter jgalt42
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Learn to draw it like this, including the hydrogens. Notice that on each carbon, one of the hydrogens points either straight down or straight up, and the other is angled. Start by identifying those hydrogens that are clearly straight up or straight down. If the H is straight down, it is part of the "down" side of the molecule, and the other group attached to the same carbon must be "up". The opposite is true for the "up" groups -- if hydrogen points straight up, it is "up", and the other group is "down." Now all cis/trans means is that for cis, both groups are either "up" or "down." Trans means one is up and the other is down.
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Equitorial means it isn't pointing straight up or down. All the funny-angled ones are equitorial.
Pointing up or down = axial. Otherwise, equitorial. Notice that the "up" side of the molecule rotates between axial and equitorial as you move from carbon to carbon. For instance, pick any carbon with a hydrogen pointing up. This is axial. Now go to the next carbon, and here the carbon pointing "up" is equitorial. These two hydrogens are "cis," but notice one of them is axial and one equitorial. This will be the case any time the carbons are an odd number apart (1 to 2 or 1 to 4).
Move onto the next carbon, and you can see that the axial hydrogen is back to the "up" again. This means that the original hydrogen and this hydrogen are still cis, but notice they're both axial. This will be the case whenever you have an even number between carbons (1 to 3, or 1 to 5).
Pointing up or down = axial. Otherwise, equitorial. Notice that the "up" side of the molecule rotates between axial and equitorial as you move from carbon to carbon. For instance, pick any carbon with a hydrogen pointing up. This is axial. Now go to the next carbon, and here the carbon pointing "up" is equitorial. These two hydrogens are "cis," but notice one of them is axial and one equitorial. This will be the case any time the carbons are an odd number apart (1 to 2 or 1 to 4).
Move onto the next carbon, and you can see that the axial hydrogen is back to the "up" again. This means that the original hydrogen and this hydrogen are still cis, but notice they're both axial. This will be the case whenever you have an even number between carbons (1 to 3, or 1 to 5).
Ok, started exploring this a little further.
So let's start with a 1 to 4 molecule:
Cis 1,4-dimethyl-cyclohexane: the methyl groups would be axial and equatorial?
Trans 1,4-dimethyl-cyclohexane: the methyl groups would be both axial or both equatorial?
A 1 to 3 molecule
Cis 1,3-dimethyl-cyclohexane: the methyl groups would either be both axial or both equatorial?
Trans 1,3-dimethyl-cyclohexane: the methyl groups would be axial and equatorial?
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You got it.
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Yup. TBR hints that you should memorize the different orientations of cis/trans 1,2; 1,3; 1,4 substituted cyclohexanes, but that's pointless to me. It takes exactly no time to draw a quick chair (in your head or on paper) and see which way the bonds point.