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Hi guys, sorry to be really cheeky and sign up simply to ask for help, but here goes. I've done a simple Grignard addition of isopropyl magnesium bromide to 3-methoxy benzaldehyde and I'm just assigning the peaks, but I've come up stumped on one issue. Concerning the isopropyl -CH(CH3)2 proton (at approximately d=1.95p.p.m.), it's observed as a septet, but the integration is 2.14 and I'm stuck as to why. This Grignard addition should occur non-preferentially at either the Re-/Si- face of the aldehyde, so can it be explained by some form of entiomeric discussion? I also note that the vicinal proton appears as a doublet (coupling the the proton in question, 3JHH=6.9Hz), but that the isopropyl proton doesn't appear to couple in return (eg. the septet doesn't appear to be split), is this to be expected? I was wondering if this could be to do with similar J-coupling values.. It'd be great if anyone could shed some light on the situation, thanks |
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Hi, Brianstorm. Try to change temperature of sample. May be OH proton overlapped with septet CH(CH3)2 and give wrong integral. What about OMe and aromatic signals? 13C spectrum should be very simple. Check splitting in 1H NMR by COSY. |
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The integration could be off due to saturation of the resonances with longer T1 values. To obtain accurate integrals, I usually use a 30 degree pulse width to compensate for the longer T1 values. See Figure 4.2.5 in "Principles of Nuclear Magnetic Resonance in 1 & 2 Dimensions" by Ernst, Bodenhausen & Wokaun. I'm not sure why the additional coupling to the isopropyl proton does not appear, but I agree that you can check for coupling by running a COSY experiment. |
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