Question

Stability of conformers. I understand that the most Stability of a conformer. I am having trouble identifying which group is larger.... a halogen or carbon molecules, like a methyl, ethyl, tert-butyl, isopropyl group. Are my answers correct?
@photon336

Answer 1

I would advise you read this fully and try to do this step by step before moving forward and asking any further questions because this is the foundation of this concept.

Check this out equatorial positions are like this. We first number the carbons. then we label them either up or down depending on their orientation.

This is an easy way because when we go from a cyclohexane flat ring to a chair form. the up down notation will be important. UP means it would be a wedge on our flat cyclohexane ring and down means it would be a dash.



notice our axial positions either go straight up or straight down.



Say if we had your first example.

notice how I labeled the carbons starting at carbon 1 right? and then I gave the designation up or down.

we have 1 up, 4 down.



so we know at carbon 1 the methyl CH3 group will be pointing UP a wedge
and carbon 4 will be a dash facing away from us.

Using this designation you'll be able to easily go from a flat ringed structure to a chair form easily,



Now when you look at it, 1,-3 di-axial interactions happen between I believe the interactions between axial groups that are 2 carbons away. this means that when we have large substituents they can actually cause repulsions, bigger electron clouds repelling. that's unstable for the molecule. If and when the molecule can, it will undergo a ring flip to reduce the number of axial positions.

In our figure above, both groups are in the axial positon.

when we do a ring flip, every carbon moves over 1 position and you keep the same relative positon. so watch

in our figure we had

1 UP
4 Down

so when we flip the ring we move everything over one carbon and keep the relative configuration.

so in the ring flip it will be

2 UP
5 Down

when we ring flip. and then we actually flip the molecule. so we must do that too. as in your drawing.



2 UP
5 Down

If you notice, both groups are in equatorial positions so this is going to be the more stable molecule.

Size of your substituent matters but it also matters more whether a ring flip will move that group into an equatorial positon.