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I Have confusion, If we talk about Ethane Molecule, we can get different conformations. For ethane, the eclipsed and staggered are freely inter-convertible and in between these two extreme conformations lie infinite number of conformations where the dihedral angle may be 1degree or 2 degree or 10 degree etc. Thus, a slight change in the overall 3-D orientation in space results in a new conformer.

Now Coming to question, If rotation around single Bond is So free, Why Glucose Exist as (+)-Glucose, Why not C4 carbon get rotated simply by 180 and Get converted to glactose (as its rotation is free)?

What I am missing here?

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    $\begingroup$ You are totally missing the point. Re read the answer, made some draw, buid a little model of few molecule $\endgroup$ – Alchimista Nov 18 '17 at 9:08
  • $\begingroup$ Alright, I get that. I was thinking on the basis of Fisher projection. Thank you. $\endgroup$ – aman Nov 18 '17 at 13:18
  • $\begingroup$ chemistry.stackexchange.com/questions/59124/… $\endgroup$ – Mithoron Nov 18 '17 at 17:47
  • $\begingroup$ Geometric isomerism exists exactly because rotation can be blocked. $\endgroup$ – Mithoron Nov 18 '17 at 17:48
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I think you are confused between arrangement of different groups around carbon ($C$) and free rotation about $\ce{C-C}$.

In conformers, the orientation of groups in space does not change, but only rotation about $\ce{C-C}$ occurs. Note that this will never change the spatial configuration of groups around any carbon!

So + glucose will not get converted to any other optical isomer on changing conformations about any $\ce {C-C}$.

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  • $\begingroup$ isnt the rotation of C4 carbon by 180 degree will Give us Glactose ? $\endgroup$ – aman Nov 18 '17 at 7:58
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    $\begingroup$ @Aman No, It will not change configuration of groups around any carbon. On each chiral carbon, there is only two configuration possible. Rotation does not change them from one two another! $\endgroup$ – gilly Nov 18 '17 at 8:13
  • $\begingroup$ That is my question. If it is possible in ethane or butane, Why it is not rotating in case of chiral carbon?? $\endgroup$ – aman Nov 18 '17 at 8:41
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    $\begingroup$ I mean, rotation is of course possible, but it doesn't change a chiral center into another enantiomer. $\endgroup$ – Ivan Neretin Nov 18 '17 at 9:39
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    $\begingroup$ if you could rotate in 4 dimensions, then sure you could do it. can you rotate your left hand until it looks like your right hand? $\endgroup$ – gilleain Nov 18 '17 at 9:55

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