July 08, 2020, 06:16:36 AM
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Topic: McMurray's book and one question from organic chemistry (isomers)  (Read 338 times)

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Offline windom626

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Hello everyone!

Is there any option to check the correct answers for questions at the end of mcmurray's organic chemistry? I mean i am  a self-learner and I don't have any option to ask anyone who is educated in organic chemistry etc.

Oh and by the way, I was doing this question, "Why does the 1,2-cis-dichlorocyclohexane has one axial and one equatorial group?" I think this is because steric strain would be too big, and the molecule would be unstable (if there were 2 axial or 2 equatorial groups). Am I correct? :p


Offline Babcock_Hall

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Re: McMurray's book and one question from organic chemistry (isomers)
« Reply #1 on: April 07, 2020, 05:06:41 PM »
You are going off on a wrong tangent.  Ask yourself what is true about trans-1,2-dimethylcyclohexane, and this might put you back on the right track.

Offline hollytara

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Re: McMurray's book and one question from organic chemistry (isomers)
« Reply #2 on: April 07, 2020, 10:59:13 PM »
McMurry may have a solutions manual you can buy. 

If you draw out the cis 1,2-dichlorocyclohexane you can see why.

I recommend the book by Jakob Fredlos - very inexpensive as an ebook - to help understand these things.

Offline Babcock_Hall

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Re: McMurray's book and one question from organic chemistry (isomers)
« Reply #3 on: April 08, 2020, 08:08:38 AM »
There is definitely a solutions manual for McMurry.  One thing to bear in mind is that ring flips change axial substituents into equatorial ones, and vice versa.  However, a ring flip cannot change a cis-cyclohexane derivative into a trans-isomer or vice versa.

Offline hollytara

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Re: McMurray's book and one question from organic chemistry (isomers)
« Reply #4 on: April 08, 2020, 11:15:48 AM »
And the terminology of "ring flip" has always been confusing to students. 

A lot of them think it means to "flip over to the other side" (turn upside down) when it means "flip the up end down and the down end up"

Offline windom626

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Re: McMurray's book and one question from organic chemistry (isomers)
« Reply #5 on: April 09, 2020, 08:03:57 AM »
I mean, I still don't see it, maybe I don't understand cis/trans isomers. In the picture I made, in the trans form (the one on the left side) we can see 2 axial groups that are in opposing positions, but If ring-flip occurs they are in equatorial positions, and this is still trans? Because for me it looks like cis conformetion...I'm confused...

Offline Babcock_Hall

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Re: McMurray's book and one question from organic chemistry (isomers)
« Reply #6 on: April 09, 2020, 08:35:05 AM »
As long as one group points even a little up (as it does when equatorial) and the other points a little down (as it also does when equatorial),  then they are trans to each other.  One cannot change a cis disubstituted cyclohexane into a trans isomer without breaking covalent bonds.  That will not happen at room temperature at an appreciable rate. 

Your drawings are a little misleading.  See this link for a good drawing of just cyclohexane:  https://en.m.wikipedia.org/wiki/File:Cyclohexane_with_H.svg

Hollytara makes a good point about terminology.  Maybe we should drop "ring-flip" in favor of "pseudorotation" or something less formidable.

Offline windom626

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Re: McMurray's book and one question from organic chemistry (isomers)
« Reply #7 on: April 09, 2020, 08:41:17 AM »
Oh that's perfect picture :D Thanks, now I get it! And with the drawings of my own i was just following step-by-step the method in McMurray but i was aware that in reality it is a little bit diffrent.

Offline hollytara

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Re: McMurray's book and one question from organic chemistry (isomers)
« Reply #8 on: April 09, 2020, 11:01:10 AM »
I have run into the "ring-flip" confusion many times. 

This is why they changed "chiral centers" to "stereocenters" to "stereogenic centers"..

Offline hollytara

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Re: McMurray's book and one question from organic chemistry (isomers)
« Reply #9 on: April 09, 2020, 11:04:33 AM »
PS for Windom626

When you did the "ring-flip" you did it a little incorrectly.  While both your structures are trans in one case and cis in the other, you moved things that shouldn't move. 

Here is the mistake:

In one structure, the one Cl is at the end, and the other is in the front corner.  Then in the other structure, the Cl's are at the end and in the back corner.  It can't move to the other corner - it has to stay in the front or back!

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