In $\ce{CDCl3}$ solution, the different diastereoisomers of 3-amino-2-butanol have different 1H spectra: for one diastereomer, the coupling constant between H atoms on C-2 and C-3 is 6 Hz, for the other it is 3 Hz. In the solvent DMSO which can bond to the $\ce{-OH}$ and $\ce{-NH2}$ groups, the coupling constants are the same. Why is this observed?

My working is such: enter image description here

I suppose since in $\ce{CDCl3}$ no H-bonding is possible, so the molecule H-bonds to itself. Whereas in DMSO hydrogen bonding occurs with the $\ce{-OH}$ and $\ce{-NH2}$ groups and this causes rotation and an overall average value of NMR is seen. Could someone please explain this 1H NMR properly to me? (Diagrams would really help!)

  • 2
    $\begingroup$ Can you draw out what the molecules look like when they have intramolecular H-bonding? Look closely at the relative positions of the H atoms and relate this to the Karplus equation. $\endgroup$ – long Dec 17 '15 at 19:08
  • $\begingroup$ @long do you mean INTERmolecular bonding? $\endgroup$ – justbehappy Dec 17 '15 at 19:31
  • $\begingroup$ @long i know the shape of the molecule will be different in DMSO but im having trouble visualizing what you mean and that is my main problem... $\endgroup$ – justbehappy Dec 17 '15 at 19:32
  • $\begingroup$ That's a detail but chloroform can make weak h-bonds with other molecules $\endgroup$ – Mithoron Dec 17 '15 at 19:42
  • 2
    $\begingroup$ Long meant intramolecular. You have an OH and a NH2 group next to each other - it's likely that in the absence of solvent (or in the presence of a solvent that cannot form hydrogen bonds), the most stable conformation will be dictated by intramolecular hydrogen bonding. Use Newman projections to help. $\endgroup$ – orthocresol Dec 17 '15 at 22:55

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.