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Topic: Help with conformations of cis-decalin  (Read 10089 times)

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Offline taurean

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Help with conformations of cis-decalin
« on: October 18, 2008, 03:11:19 PM »
In the ring flip show in the picture, why the red bonds (A & B) are not going to diaxial?  Please someone respond.

Offline azmanam

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Re: Help with conformations of cis-decalin
« Reply #1 on: October 18, 2008, 03:34:17 PM »
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why the red bonds (A & B) are not going to diaxial?

They do.  the A you drew on the right is not the same proton as the A on the left.  Same with B.  Build yourself a model to convince yourself.
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Offline taurean

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Re: Help with conformations of cis-decalin
« Reply #2 on: October 18, 2008, 03:42:40 PM »
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why the red bonds (A & B) are not going to diaxial?

They do.  the A you drew on the right is not the same proton as the A on the left.  Same with B.  Build yourself a model to convince yourself.

My question is why A and B are still equatorial, even after the ring flip.

A and B (down and up respectively in the left figure) changed to A and B (up and down respectively in the right figure).  Why they did not become diaxial after the ring flip.  In a ring flip all the equatorials should become axial right?

Offline macman104

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Re: Help with conformations of cis-decalin
« Reply #3 on: October 18, 2008, 03:57:15 PM »
Azmanam is trying to tell you that the hydrogen you labeled with an A on the right, is not the same hydrogen that is labeled A on the left.

Offline taurean

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Re: Help with conformations of cis-decalin
« Reply #4 on: October 18, 2008, 04:01:16 PM »
Azmanam is trying to tell you that the hydrogen you labeled with an A on the right, is not the same hydrogen that is labeled A on the left.

Sorry, but I am unable to see.  Can you explain clearly please?

Offline azmanam

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Re: Help with conformations of cis-decalin
« Reply #5 on: October 18, 2008, 04:21:55 PM »
You asked why A (currently equatorial) and B (also currently equatorial) in the decalin on the left do not become axial after a chair flip.

The answer is because the atoms you are indicating are not the same atom on the right and left.  See below.  You asked why A1 does not become A2.  The answer is because A1 is not A2.  A1 remains A1 on both the right and left.  A1 is equatorial on the left.  The same A1 becomes axial on the right.  Same for B. 

Build yourself a model to convince yourself
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Offline azmanam

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Re: Help with conformations of cis-decalin
« Reply #6 on: October 18, 2008, 04:24:19 PM »
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A and B (down and up respectively in the left figure) changed to A and B (up and down respectively in the right figure)

THIS is your key something is amiss.  When comparing protons between ring flips, you need to compare up protons with up protons.  You compared an up proton on the left with a down proton on the right.  Can't do that.
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Offline taurean

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Re: Help with conformations of cis-decalin
« Reply #7 on: October 18, 2008, 04:37:33 PM »
You asked why A (currently equatorial) and B (also currently equatorial) in the decalin on the left do not become axial after a chair flip.

The answer is because the atoms you are indicating are not the same atom on the right and left.  See below.  You asked why A1 does not become A2.  The answer is because A1 is not A2.  A1 remains A1 on both the right and left.  A1 is equatorial on the left.  The same A1 becomes axial on the right.  Same for B. 

Build yourself a model to convince yourself

Thanks, Azmanam.  Thank you very much for taking time to draw the structures.  I got the answer.

Offline taurean

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Re: Help with conformations of cis-decalin
« Reply #8 on: October 18, 2008, 04:42:42 PM »
You asked why A (currently equatorial) and B (also currently equatorial) in the decalin on the left do not become axial after a chair flip.

The answer is because the atoms you are indicating are not the same atom on the right and left.  See below.  You asked why A1 does not become A2.  The answer is because A1 is not A2.  A1 remains A1 on both the right and left.  A1 is equatorial on the left.  The same A1 becomes axial on the right.  Same for B. 

Build yourself a model to convince yourself

Thanks, Azmanam.  Thank you very much for taking time to draw the structures.  I got the answer.

I have one more question, what happens to A1 and B1 if we change the configuration of the carbon to which D is attached, I mean if we make it trans-decalin.

Offline azmanam

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Re: Help with conformations of cis-decalin
« Reply #9 on: October 18, 2008, 05:00:16 PM »
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Build yourself a model
Knowing why you got a question wrong is better than knowing that you got a question right.

Offline taurean

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Re: Help with conformations of cis-decalin
« Reply #10 on: October 18, 2008, 06:30:18 PM »
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Build yourself a model

I built the model, it is the structure on right expect for D is axial.  Am I right?  Here is the picture.

Offline azmanam

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Re: Help with conformations of cis-decalin
« Reply #11 on: October 20, 2008, 09:21:02 AM »
I agree with you that that is the lowest energy confirmation.  Are you asking what will happen to A and B if trans decalin ring flips?  Or were you just trying to find the lowest energy confirmation?
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