Possible orientation for disubstituted cyclohexanes

[cartman1980]

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From TBR Ochem page Chap2 114-

Trying to understand criteria using which a cis/tran is translated to axial/equatorial, but can't seem to make sense of all combinations. Guess i am missing the basic principle of such translation 🙁

1,2-trans --> c1,c2 axial OR c1,c2-equatorial
(why doesn't this lead to steric hinderance greater than c1-axial/c2-eq combination)

1,2-cis --> c1-axial, c2-equatorial OR c2-axial, c1-equatorial
(makes sense since both axial or both equatorial leads to larger steric hinderance)

1,3-cis --> c1,c3 axial OR c1,c3-equatorial
(why wouldn't axial-axial lead to lesser steric hinderance than axial-eq? i.e. when both subs are pointing up or pointing down, chances of hinderance appears more likely than one up/one side. Same question applies for eq-eq)

1,3-trans --> c1-axial, c3-equatorial OR c3-axial, c1-equatorial
(in trans, both subs are supposed to be on opposite sides. So shouldn't axial-axial or eq-eq cause lesser steric hinderance?)

1,4-trans -> c1,c4 axial OR c1,c4-equatorial
(makes sense)

1,4-cis-> c1-axial, c4-equatorial OR c4-axial, c1-equatorial
(axial-axial would no longer keep the compound cis. So, although this transformation makes sense, why doesn't the same logic apply for 1,3-cis)


As if this isn't enough, most stable orientation for 1,3,5- trimethycyclohexane is methyl groups in equatorial chair, and none in axial.

Utterly lost 🙁

 

[GreenRabbit]

Hindrance has nothing to do with these choices. cis-1,2 can ONLY be axial-equatorial or equatorial-axial. It cannot be axial-axial nor equatorial-equatorial due, because then they would be trans not cis. I hope you realize that equatorial groups do not point straight outward.

For the confusion over 1,3,5-trimethylcyclohexane, do you not understand why large groups prefer the equatorial position vs the axial?

 

[cartman1980]

cis-1,2 can ONLY be axial-equatorial or equatorial-axial. It cannot be axial-axial nor equatorial-equatorial due, because then they would be trans not cis. I hope you realize that equatorial groups do not point straight outward.

Doesn't axial means parallel to symmetery axis? i.e. IF cis-1,2 could assume axial-axial (although it doesn't) both subs could be pointing upwards or downwards thereby NOT causing it to take a trans configuration (which would be one up, one down i.e. on opposite sides). Unless i am misinterpreting the context, steric hinderance would be the cause of cis-1,2 being axial-eq oritentation.

For the confusion over 1,3,5-trimethylcyclohexane, do you not understand why large groups prefer the equatorial position vs the axial?

There should've been a 'cis' somewhere in the name of the compound. And even then, if we simply go by that logic, then the above mentioned examples should all have equatorial orientation i.e. it doesn't fit.

 

[GreenRabbit]

Doesn't axial means parallel to symmetery axis? i.e. IF cis-1,2 could assume axial-axial (although it doesn't) both subs could be pointing upwards or downwards thereby NOT causing it to take a trans configuration (which would be one up, one down i.e. on opposite sides). Unless i am misinterpreting the context, steric hinderance would be the cause of cis-1,2 being axial-eq oritentation.

No, cis-1,2 literally cannot physically be axial-axial. Nothing to do with stability or steric hindrance. If group A is axial "up" on carbon 1, group B on carbon 2 MUST be equatorial to be cis. The axial position is "down" and therefore not cis. That equatorial group points "up", which is cis.

cis- same side
trans- opposite sides

There should've been a 'cis' somewhere in the name of the compound. And even then, if we simply go by that logic, then the above mentioned examples should all have equatorial orientation i.e. it doesn't fit.

No, there shouldn't be a cis there. The book is saying that of all possible configurations, including cyclohexanes that have methyls cis and trans to each other, all-equatorial methyls is the most stable compared to the others.

I think you may need a model kit or more pictures to understand this.

 

[GreenRabbit]

Stability (i.e. steric hindrance) would influence something like trans-1,3. trans-1,3 has two choices, either both equatorial or both axial. Because axial groups would hinder each other (and the axial H on the 5th carbon), the both axial configuration is less stable than both equatorial position. Therefore, the equatorial-equatorial trans-1,3 configuration is more common than the axial-axial trans-1,3 configuration, though both exist.

trans-1,3 cannot however be axial-equatorial, or it would not be trans-1,3.

 

[cartman1980]

mucho thanks Greenrabbit..

after quite some deliberations on this topic, i finally saw the light.. had my aha moment a few minutes ago 🙂

its all clear now..