R and S Watch

cptbigt
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#1
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When looking at trans compounds, such as trans-1,2-diamino-cyclohexane, which amine group would take priority? I know that in cis-1,2-diaminocyclohexane its (S,R) configuration. I thought trans was (R,R) but I can't seem to work out how. Any help?
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EierVonSatan
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#2
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(Original post by cptbigt)
trans-1,2-diamino-cyclohexane
lol don't use the naming function either!

I get the trans to be (R,R), one of the amine groups is one carbon away so takes lower priority than the NH2 directly bonded to the chiral centre
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cptbigt
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(Original post by EierVonSatan)
lol don't use the naming function either!

I get the trans to be (R,R), one of the amine groups is one carbon away so takes lower priority than the NH2 directly bonded to the chiral centre
I seem to get (R,S) for the trans one.....or am I forgetting to rotate it so the lowest priority group is pointing away or something?
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Cyco
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Yep, point the lowest priority group away from you/into the plane of the board/screen/paper, and it's usually H.

You should then have your three groups coming off. Going from branch with lowest molecular mass, then middle, then highest, draw an arrow curving around the chiral centre. If it goes clockwise - it's R/Rectus, or counter-clockwise S/Sinister.
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cptbigt
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I'm going to look it up some more, still confusing me
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EierVonSatan
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(Original post by cptbigt)
I'm going to look it up some more, still confusing me
always helps to build a model then you will be able to see that both centres are the same
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cptbigt
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Where are you guys placing the third priority group? Are you going one carbon out from each direction from the chiral centre? I'm doing that, so the amine next to it becomes 2, and the C-C becomes the third...
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EierVonSatan
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(Original post by cptbigt)
Where are you guys placing the third priority group? Are you going one carbon out from each direction from the chiral centre? I'm doing that, so the amine next to it becomes 2, and the C-C becomes the third...
yeah: 1 = NH2, 2 = CHRNH2, 3 = CH2R, 4 = H
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cptbigt
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(Original post by EierVonSatan)
yeah: 1 = NH2, 2 = CHRNH2, 3 = CH2R, 4 = H
I must be rotating it wrong then... I tried using a model but that just confused me more Can't seem to get the rotation right so that the H is facing away.
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EierVonSatan
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(Original post by cptbigt)
I must be rotating it wrong then... I tried using a model but that just confused me more Can't seem to get the rotation right so that the H is facing away.
Maybe this will help...The R group represents the ring
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cptbigt
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(Original post by EierVonSatan)
Maybe this will help...The R group represents the ring
Ooo thanks so much! Its slowly starting to make sense....just takes a while with me
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EierVonSatan
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(Original post by cptbigt)
Ooo thanks so much! Its slowly starting to make sense....just takes a while with me
Welcome - you'll get the hang of it
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cptbigt
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#13
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I rechecked over the trans one and it comes out as (1S,2S) though It can be R,R if its drawn differently to what I have on the sheet. The one we have is on the attachment
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EierVonSatan
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#14
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(Original post by cptbigt)
I rechecked over the trans one and it comes out as (1S,2S) though It can be R,R if its drawn differently to what I have on the sheet. The one we have is on the attachment
yeah, it's because the ring can undergo a ''ring flip'' switches the equatorial with the axial positions
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