[Question from NMRWiki Q&A forum] Simple 1H-NMR Assignment Query

[nmrlearner]

Simple 1H-NMR Assignment Query

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



Check if somebody has answered this question on NMRWiki QA forum