[whyohme]

To solve this question, I came up with 6 drawings.




But there are only 2 drawings in the solution.  Can anyone please tell me why these two ?

[james_a]

For any given chair there are only two possibilities. Note the "both" in the question.

Some problems here are:
1) chair forms #2 and #3 on top are not completely correct (you have groups pointing up that should be pointing down). Axial groups should always alternate up-down-up-down-up-down around the ring. It's not always clear what's axial and equatorial.
2) in all structures except leftmost on bottom your substituents are going around the ring in the opposite direction from your flat drawing. In essence you've drawn the enantiomer. That's going to screw you up.
3) all your chairs are drawn in the same orientation. you need to learn how to draw the chair in the opposite form.
4) top row left is the same as bottom row middle. Top row middle is same as bottom row right.

In other words you probably didn't get full marks because you didn't show how to do a chair flip properly.

A walkthrough of how to do this.

1. Draw your "flat" cyclohexane and number it appropriately. Pay attention to whether you number clockwise or counterclockwise - you'll need to be consistent.

Draw your first chair. Your skeleton is OK. It would help if you drew in all the groups at first.  Put in the axial groups first - at the "headrest" put an axial pointing up, and at the footrest, put an axial pointing down. Then make sure you are alternating "up down up down up down" all along the ring. Then put in the equatorial groups, which will be pointing in the opposite directions from the axial substituents although not straight up or down. Next, decide which carbon is going to be #1, and *number the same direction as you did for flat cyclohexane*. Then, draw in your substituents. Methyl #1, methyl #2, and methyl #4 are all "up".
Then, draw the opposite chair conformation. Put all the axial and equatorial groups in. Then pick what will be carbon #1. Here's the key - the axial group MUST be pointing in the opposite direction from where it pointed in carbon #1 on your first chair. In a chair flip all axial groups become equatorial and all equatorial groups become axial. Then put in your methyl groups. You should have two nice looking chairs.

The final step is evaluation: which chair form has the fewest axial methyl groups? That will be your most stable chair.

Hope this helps

[whyohme]

For any given chair there are only two possibilities. Note the "both" in the question.

Some problems here are:
1) chair forms #2 and #3 on top are not completely correct (you have groups pointing up that should be pointing down). Axial groups should always alternate up-down-up-down-up-down around the ring. It's not always clear what's axial and equatorial.
2) in all structures except leftmost on bottom your substituents are going around the ring in the opposite direction from your flat drawing. In essence you've drawn the enantiomer. That's going to screw you up.
3) all your chairs are drawn in the same orientation. you need to learn how to draw the chair in the opposite form.
4) top row left is the same as bottom row middle. Top row middle is same as bottom row right.

In other words you probably didn't get full marks because you didn't show how to do a chair flip properly.

A walkthrough of how to do this.

1. Draw your "flat" cyclohexane and number it appropriately. Pay attention to whether you number clockwise or counterclockwise - you'll need to be consistent.

Draw your first chair. Your skeleton is OK. It would help if you drew in all the groups at first.  Put in the axial groups first - at the "headrest" put an axial pointing up, and at the footrest, put an axial pointing down. Then make sure you are alternating "up down up down up down" all along the ring. Then put in the equatorial groups, which will be pointing in the opposite directions from the axial substituents although not straight up or down. Next, decide which carbon is going to be #1, and *number the same direction as you did for flat cyclohexane*. Then, draw in your substituents. Methyl #1, methyl #2, and methyl #4 are all "up".
Then, draw the opposite chair conformation. Put all the axial and equatorial groups in. Then pick what will be carbon #1. Here's the key - the axial group MUST be pointing in the opposite direction from where it pointed in carbon #1 on your first chair. In a chair flip all axial groups become equatorial and all equatorial groups become axial. Then put in your methyl groups. You should have two nice looking chairs.

The final step is evaluation: which chair form has the fewest axial methyl groups? That will be your most stable chair.

Hope this helps

Hi I was wondering why  # 2 and #3 on top are not correct. As the drawing needs to be all-cis, I think all the substituents need to be in the same direction (all going up).

I am also confused at numbering the carbons. After drawing the chair, I can pick carbon#1 at anywhere ?

Thanks!

[james_a]

It's not clear that carbon #1 is equatorial in the drawing on the top middle. And in the top right drawing, you have carbon #1 *and* carbon #4 being axial up. This is not correct for a chair (it is more consistent with a boat).

By "picking" carbon #1 in the bottom section I am referring to the process drawing the chair skeleton first (without substitutents), and then selecting which position on the chair to put the first methyl group (i.e. "carbon #1). Note how in the answer key, the methyl group on carbon #1 is axial in one case  and equatorial in the other.

[whyohme]

I dont get the solution .

My textbook states that it is clockwise to do the ' ring flip '  , but the solution uses counterclockwise 'ring flip' .

I drew a ring that I thought what the solution should be.

anyone can please help me?

[whyohme]

It's not clear that carbon #1 is equatorial in the drawing on the top middle. And in the top right drawing, you have carbon #1 *and* carbon #4 being axial up. This is not correct for a chair (it is more consistent with a boat).

By "picking" carbon #1 in the bottom section I am referring to the process drawing the chair skeleton first (without substitutents), and then selecting which position on the chair to put the first methyl group (i.e. "carbon #1). Note how in the answer key, the methyl group on carbon #1 is axial in one case  and equatorial in the other.

Aha, I meant to draw to equatorial for the top right drawing. I see what you were saying now. No, my drawings are not that clear.

I dont get the solution given by the textbook. How come the ring flips in counterclockwise way ? I thought the #1 carbon (equatorial) would flip clockwise and become axial (going up).  #2 carbon would be equatorial (pointing up)  #4 carbon would equatorial as well and pointing up.

Thanks

[Dan]

The circled conformer and the pencil-drawn conformer are the same, just viewed from different angles (perhaps build a model).

[fledarmus]

Build models. Build a model of your structure 1, and rotate it. How many of your other structures are exactly the same? The ones that are the same are not ring-flipped structures, they are just the same molecule with pictures taken from different angles.

Now flip your structure to match one of the structures you were not able to match as described above. How many of your structures can you match by rotating this structure?