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Cyclopropanone

Are the hydrogens that on the same carbon and those that are cis enantiotopic? And are they isogamous or anisogamous?

And will they anisochronous in a chiral environment NMR?

Isochronous - hydrogens that are in the same chemical environment leading to the same shift and splitting in NMR. If the hydrogens are enantiotopic they will be isochronous in an achiral nmr environment but anisochronous in a chiral environment.

I can see there is there is a C2 and two planes of symmetry but the TA said they will be isochronous in a chiral NMR resulting in a single singlet but as i did the replacement test I got two enantiomers which means that they will be anisochronous in chiral NMR, but I'm not sure.

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    $\begingroup$ They should be different in NMR in a chiral environment because they would appear to be diastereotopic in the presence of an external chiral reference. $\endgroup$
    – Zhe
    Jun 30, 2017 at 13:01

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Cyclopropanone is a reactive species, as such there aren't many experimental reports in which it is characterised, however some (possibly questionable data) does exist:

Cyclopropanone is a highly reactive compound. It is stable for a few days at liquid nitrogen temperature but rapid polymerisation takes place above o or even below if traces of water are present. The reaction is highly exothermic.

Ref: Recueil des Travaux Chimiques des Pays-Bas, 1966, 11, 1170. DOI:10.1002/recl.19660851113

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The NMR spectrum (neat) shows a singlet at 1.72 p.p.m. relative to tetramethylsilane.

Ref: Recueil des Travaux Chimiques des Pays-Bas, 1966, 11, 1170. DOI:10.1002/recl.196608511

It appears that in the absence of a chiral environment, all of the protons are in a single environment at 1.72 ppm (Hans Reich quotes a single peak at 1.65 ppm) which is what we'd expect (the molecule is essentially planar, and all of the protons are equivalent).

As you point out in your question, replacing any one of the protons (hypothetically) would yield two enantiomers, which, in the presence of a chiral environment, should be distinguishable. It can therefore be concluded that in a chiral environment, the protons should appear to be enantiotopic.*

* there are two slight caveats to this. Firstly, the compound would still be unstable and prone to polymerising, therefore there is no way of actually testing this. Secondly, the chiral environment must be able to interact with the molecule, for things like chiral alcohols, lanthanide shift reagents are (/used to be) commonly used, but I'm less convinced how you put a ketone into a chiral environment.

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  • $\begingroup$ A chiral solvent should perfectly present a chiral environment, should it not? $\endgroup$
    – Jan
    Jul 9, 2017 at 16:02
  • $\begingroup$ I can't think of any chiral solvents suitable for NMR (?). My point was that it should work, but I'm not sure if it does work. $\endgroup$
    – NotEvans.
    Jul 9, 2017 at 16:18
  • $\begingroup$ Every solvent is suitable for NMR afaik. You just want your hydrogens to be deuterium if you’re using proton NMR. $\endgroup$
    – Jan
    Jul 9, 2017 at 16:19
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    $\begingroup$ aha. One of us is being pedantic, I just can't quite tell whom. But yes, fine, I can't think of any chiral deuterated solvents. I mean actually, it doesn't even have to be deuterated (I often run in non-D THF/dioxan for reaction monitoring). I wonder if the viscosity becomes the issue, as a chiral alcohol is necessarily 'bigger' than methanol/chloroform etc. I think we need long (or Loong). $\endgroup$
    – NotEvans.
    Jul 9, 2017 at 16:21

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