optical diasteriomers

[ozzi22]

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Which of the following is identified by 1H NMR as having optical diastereomers?

A)

B)

C)

D) None of the above

Answer A

Can someone plz explain this to me.

 

[jHustle]

hate ochem

 

[nabilesmail]

I think its D) none of the above. I'm pretty sure HNMR cannot distinguish chirality

 

[ozzi22]

I think its D) none of the above. I'm pretty sure HNMR cannot distinguish chirality

Yeah that was my reasoning too and i choose D but that's not the answer. Here is the explanation Kaplan gave;
Rotation about the C-N bond in (A) is significantly hindered, so much so that it produces two peaks by PMR (see below). The C-C and C-O bonds rotate freely and would not exist as optical diastereomers.

 

[hiaips]

Their explanation isn't very helpful.

Since choice (A) is an amide, the C-N bond should have some double bond character (resonance). Therefore, we should expect restricted rotation about this bond. Since the resulting "cis" and "trans" forms are different, they may have different 1H NMR spectra.

(Note: We're talking about diastereomers here as opposed to enantiomers.)