Wedge and dash to chair

[circulus vitios]

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Which is more stable: cis-1-methyl-3-isopropylcyclohexane or trans-1-methyl-3-isopropylcyclohexane. The answer is cis.

TPR solves it by saying the "rule of thumb" is to put the bulkier substituent on the equatorial position and decide if it's more stable, so their solution shows the isopropyl group as equatorial. They only do the first chair (boxed in blue) and ignore the ring flips. Trans shows interaction, cis shows no interaction. Cis is therefore more stable.

If I put the isopropyl group on the axial position, the trans chair shows interaction and the cis chair shows interaction. If I do a chair flip, the trans chair still shows interaction and the cis chair shows no interaction. Adding up the interactions from both chair flips, trans shows 2 interactions and cis shows 1 interaction. Cis is therefore more stable.

My question is: if I put the bulkier group on the equatorial position, will I only have to consider the first chair and never perform a chair flip? If I put the bulkier group on the axial position, will I always have to consider the chair and the chair flip?

 

[ChemTutor]

why do you want to put the bulkier group in axial position? this is not the conformation that is going to be more stable. why even consider this scenario?

 

[circulus vitios]

why do you want to put the bulkier group in axial position? this is not the conformation that is going to be more stable. why even consider this scenario?

For argument's sake.

(I chose it without thinking.)

 

[Jepstein30]

Which is more stable: cis-1-methyl-3-isopropylcyclohexane or trans-1-methyl-3-isopropylcyclohexane. The answer is cis.

TPR solves it by saying the "rule of thumb" is to put the bulkier substituent on the equatorial position and decide if it's more stable, so their solution shows the isopropyl group as equatorial. They only do the first chair (boxed in blue) and ignore the ring flips. Trans shows interaction, cis shows no interaction. Cis is therefore more stable.

If I put the isopropyl group on the axial position, the trans chair shows interaction and the cis chair shows interaction. If I do a chair flip, the trans chair still shows interaction and the cis chair shows no interaction. Adding up the interactions from both chair flips, trans shows 2 interactions and cis shows 1 interaction. Cis is therefore more stable.

My question is: if I put the bulkier group on the equatorial position, will I only have to consider the first chair and never perform a chair flip? If I put the bulkier group on the axial position, will I always have to consider the chair and the chair flip?

What interaction are you talking about? Gauche?

The bulkier group normally goes in the equatorial position. A common exception is a 1, 3 diaxial interaction but you don't have that here.

Cis is more stable because it can have BOTH the substituent groups in the equatorial position. Trans can only have one (and the isopropyl would be equatorial).

Ring flips don't come into play in this type of question. A ring flip switches equatorial and axial positions but in this case, since the bulkier group is favored to be in an equatorial position, there's no reason why the molecule would ever do a ring flip into a more unstable form.

If you put the bulkier group on the equatorial position, this is likely the most stable form and no ring flip needs to be considered. If you put the bulkier group in the axial position, you should do a ring flip because its most likely more stable in the equatorial position. A ring flip that is unfavored wouldn't play a role in the stability of the molecule.

No need to add up interactions like you did. The only thing to look out for are the 1-3 diaxial interactions. For instance if you add a bulky group in between the substituents on the cis molecule (with same orientation as the other groups), you wouldn't want the bulkiest group to be in the equatorial position because that would lead to the 1-3 diaxial interaction. In that scenario, the bulkiest group would be in the axial position (it would be a toss-up depending on how bulky the group is though, a t-butyl probably would still be more favorable in the equatorial position).

 

[Jepstein30]

For argument's sake.

(I chose it without thinking.)

There is an easy way to go from wedge and dash to the favored chair formation, eliminating the need for a chair flip (and saving time!).

Let me know if you want me to write up a post on it, it's fairly straightforward.

 

[circulus vitios]

A common exception is a 1, 3 diaxial interaction but you don't have that here.

Isn't my very first chair a 1,3-diaxial interaction?

I didn't think about the favorability of chair flips being a factor. That makes perfect sense. Thanks.

 

[Jepstein30]

Isn't my very first chair a 1,3-diaxial interaction?

I didn't think about the favorability of chair flips being a factor. That makes perfect sense. Thanks.

Well, there's technically always a 1-3 diaxial interaction but we only really care about it when two substituent groups are involved (i.e. not hydrogen-hydrogen or group-hydrogen because then the interaction isn't nearly as unfavorable).

Your first chair (top left) has the methyl group interacting with two hydrogens but again, that's of minimal concern (but if it does help, realize that this is part of the reason why groups favor the equatorial position).

The trans molecule can never have 1-3 diaxial interactions since the groups will always be opposite each other. The cis molecule technically could have 1-3 diaxial interactions (as you drew) but again, they'd both just be equatorial to avoid such unfavorable interaction.

Quick tip: As you go around your cyclohexane, the equatorial and axial positions flip. If you are INTO the page at position 1 and that is equatorial, then the group INTO the page at position 2 will be axial, the group INTO the page at position 3 will be equatorial, and so on. This helps in quickly deciding what chair formation would put as many groups in the equatorial position as possible as well as quickly being able to match the correct chair to the correct wedge and dash.