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I have been learning about nmr and I am confused about how to identify whether two hydrogen on different carbon atoms are homotopic, diastereotopic or emantiotopic. I can do identify this when two hydrogens are attached to same carbon atoms, but when it is different I am lost.I have searched lot on web, but they only include examples of two hydrogens on same atom, not on different ones. So can anyone help determine what the two hydrogen are in these compounds and why ? Thanks. enter image description here

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  • $\begingroup$ Honestly I think that it is hard to understand 3D symmetry by looking at 2D drawings on paper. A ball and stick model set really helps you understand better. Once you really have manipulated the 3D models then it is much easier to understand the 2D drawings. $\endgroup$ – MaxW Feb 19 '17 at 23:02
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    $\begingroup$ If two H atoms are related by a C2 axis they are homotopic, if they are related by an Sn improper axis (including plane and center of symmetry) they are enantiopotic. All the others are diastereotopic. Another approach is to change one hydrogen for a different group (say X) then you get compound A, then start with the original compound and change the other hydrogen for X and you get compound B. If A and B represent the same compound the hydrogens are homotopic, if A and B are enantiomers the H atoms are enantiotopic and if A and B are diastereomers the H atoms are diastereotopic. $\endgroup$ – Marko Feb 19 '17 at 23:05
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You determine by labeling. Take the two hydrogens in question and label them one at a time to get two molecules. What's the relationship between these two molecules? If the molecules are the same, then the hydrogens were homotopic. If they are enantiomers, the hydrogens are enantiotopic. Otherwise, if they are diastereomeric molecules, the hydrogens are diastereomeric.

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