Axial / equatorial?

[emitpeels]

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I'm having problems solving the types of problems that ask: "What is the most stable form of.... "? I don't understand the rules for this. All I know is groups that point up and down are axial. Groups that point left and right are equatorial. I know that in order to be the most stable, the largest bulkiest group needs to point equatorial. Does that mean the other group always has to go axial? Also, sometimes groups appear to be pointing diagonal... how do you tell if they are equi or ax? Ahhhh! Help!!!

 

[StumpMT]

In the chair conformation of cyclohexane the axial groups point either straight up or straight down. The equatorial groups point either diagonally upwards or diagonally downwards. Ideally any group(s) attached would be equatorial. However this isn't always the case and if there are more than one group attached and one has to be axial and the other has to be equatorial then you would have the largest group be equatorial. A couple examples are below...

(cis) 1-bromo-3-tert-butylcyclohexane: since this molecule is cis then both groups are either pointing up or down. The most stable would have them both be equatorial.

(trans) 1-bromo-3-ter-butylcyclohexane: since this molecule is trans then one has to point up and one has to point down. Since one points up and one points down then one is axial and one is equatorial. The most stable would have the tert-butyl group be equatorial.

If you have a subscription to Chad's videos he explains this really well. You could also try Khan for a video reference. Hope I helped 🙂

 

[Locdawgg]

StumpMT explained it in words very well, but i highly suggest Chads Videos @coursesaver.com he explains this topic and other topics very clearly and well.

 

[BrazilianRider]

So on this same note, what if it's a (trans) 1-bromo-3-methylcyclohexane? Is the methyl equatorial? Isn't it "bulkier" than the Bromine due to the hydrogens it has tacked on there?

 

[matty675]

Exactly. You would want the largest substituent to be in the equatorial position, and if possible you want both substituents to be in the equatorial position. However, in the case of trans-1-bromo-3-methylcyclohexane it is not possible to have both substituents in the equatorial position since one must be pointing up and the other must be pointing down. In this case you would chose the methyl group for the equatorial position and the smaller bromo group for the axial position- limiting steric interaction as much as possible. Conversely, in the case of trans-1-bromo-2-methylcyclohexane, both substituents would be assigned to the equatorial position in order to achieve the most stable chair conformation.

 

[BrazilianRider]

Awesome. I just wanted to make sure it was sterics > molecular weight.

 

[discensdentibus]

From largest "Axial (eq arrows) Equatorial" Keq to lowest : t-butyl, isopropyl, et, me, -OH, Cl/Br/I, F/CN, H

Where largest Keq would favor the product side of the rxn AKA equatorial.

Simplified: alkane>alc> halo/cyano> H

 

[CustomGrills51]

Trans/Cis doesn't matter ab eq or axial though right? Just matters if it's above or below the ring.

 

[discensdentibus]

yes. You can have a 1,2 cis which are ax/eq or eq/ax. You can have a 1,3 cis which are ax/ax or eq/eq.

 

[CustomGrills51]

Thank you and I bet knowing this will help us all be much better dentists!!! haha im jm but for real thanks for clearing that up

 

[BrazilianRider]

From largest "Axial (eq arrows) Equatorial" Keq to lowest : t-butyl, isopropyl, et, me, -OH, Cl/Br/I, F/CN, H

Where largest Keq would favor the product side of the rxn AKA equatorial.

Simplified: alkane>alc> halo/cyano> H

Sorry if this is stupid but I just gotta double check before I commit this to memory... So this is in order of which group gets equatorial priority? If you have an alkane and a halogen, you give it to the alkane first?

 

[discensdentibus]

yeah even methyl is more favored in eq. Think about it. Even a methyl will have a Carbon (bigger than an F b/c of trends) + 3 hydrogens poking out into space. 4 atoms beats 1 halogen. That halogen may be a big momma but 4 people on an elevator will take up more space. 2 atoms in hydroxyl groups beat out your 1 halogen. -CN is linear and there are some other reasons its not as stable as OH on equatorial but an ochem textbook will give you better approximated values.

 

[BrazilianRider]

yeah even methyl is more favored in eq. Think about it. Even a methyl will have a Carbon (bigger than an F b/c of trends) + 3 hydrogens poking out into space. 4 atoms beats 1 halogen. That halogen may be a big momma but 4 people on an elevator will take up more space. 2 atoms in hydroxyl groups beat out your 1 halogen. -CN is linear and there are some other reasons its not as stable as OH on equatorial but an ochem textbook will give you better approximated values.

Alright, awesome. Thanks dude!