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I am forming N-hydroxysuccinimide ester (1) by DCC coupling with N-hydroxysuccinimide and 2,2-dimethoxyacetic acid using distilled THF as solvent.

2,5‐dioxopyrrolidin‐1‐yl 2,2‐dimethoxyacetate

What I see on $\ce{^1H}$-NMR is three major singlets: $\delta~\pu{4.840 ppm}~(1\ce{H}),$ $\delta~\pu{3.432 ppm}~(7.26\ce{H}),$ $\delta ~\pu{2.731 ppm}~(6.36\ce{H})$ and some impurities.

While the chemical shifts are well-aligned with theoretical predictions (in my opinion), integration ratios do not quite make sense. I wish I could purify it to have a clearer NMR spectrum and see whether I actually have the desired product, but I don't think my ester will stay stable in silica in column chromatography. Any suggestions?

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  • $\begingroup$ Maybe take or get an NMR of the reagents and theoretical side products and see if there can be any overlap with the signals you see. I'm not sure I understand what your doubt is, though. NMR integration is not always very precise, especially for acidic protons. $\endgroup$
    – user6376297
    Commented Jun 10, 2019 at 15:17
  • $\begingroup$ What is your next reaction with this product? $\endgroup$
    – Waylander
    Commented Jun 10, 2019 at 15:18
  • $\begingroup$ @user6376297 I have already taken the NMR of 2,2-Dimethoxy Acetic acid: $\delta \: \pu{4.867 ppm}$ (1H) and $\delta \: \pu{3.4615 ppm}$ (6.6H) (c.f. $\delta \: \pu{4.840 ppm}$ (1H) and $\delta \: \pu{3.432 ppm}$ (7.26H) in my product NMR) Not sure whether I can argue that peaks have 'shifted' in the product NMR $\endgroup$
    – chemrese
    Commented Jun 11, 2019 at 3:37

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If peaks for "some impurities" are appear around $\delta \: \pu{1.8 ppm}$, I assume you have residual THF in your crude product. The other multiplet of THF is probably burried under $\delta \: \pu{3.43 ppm}$ resonance. That would confirmed by your impurity integration, which should be equal to 1H unit equivalence. If I'm right, your 6.36H unit equivalence is actually overlap one of $\ce{CH3}$-resonance and one of $\ce{CH2}$-resonance.

On the other hand, the two resonances at $\delta \: \pu{3.43 ppm}$ and $\delta \: \pu{2.73 ppm}$ would very well be representing two $\ce{CH3}$-groups and (two $\ce{CH2}$-groups + part of THF impurity), respectively, as well.

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    $\begingroup$ Thanks. but I have thoroughly vacuum dried my product and there is no peak at δ1.8 ppm. $\endgroup$
    – chemrese
    Commented Jun 11, 2019 at 3:02

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