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CHEM 263 (09/12/13) - NMR (con't)

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John Vederas

CHEM 263 (September 12, 2013)  H Bs also the most shielded not only because of its distance to the chlorine atom but also because it is “trans” to it as well. Diastereotopic Hydrogens Diastereotopic – (adj.) – resulting in the formation of diasteromers  Even though there are 2 methyl groups, one signal for this group shows up on the sprectrum because of the C-C bond (able to rotate freely) o The rotation around the C-C bond requires energy… 1-3kcal/mol o Total amount of energy available though, is 15-20kcal/mol (at room temperature)  Result = very fast rotation and because of this fast rotation, the hydrogen in the methyl groups are in the same environment (rotating to fast to be affect by the other intramolecular forces) o C i2 a stereogenic center (shown in above diagram) and there is no plane of symmetry. Because of this, the two hydrogen atoms attached to C (H 3ndHA) B are in different chemical environments (2 chemical shifts will be shown in the spectrum for these two atoms)  Being in a different chemical environment and having different chemical shifts result in them being diastereotopic hydrogen atoms.  Diastereotopic-ism can also be seen if you substitute one of the two hydrogen molecules attached to C wi3h deuterium (D):  Deuterium (D) - isotope of hydrogen with one proton AND one neutron  When you carry out this substitution, you can see the diastereomers created. You can even better see it if you draw a Newman projection.  As you can see, even though you replace the positions of H anA H , thB methyl groups will be in different positions relative to each other o These two hydrogen can NEVER be in the same environment Spin-spin Splitting (aka “Coupling”)  HA’s spi
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