Finding the chemical shift of a proton in between two different functional groups.
The molecule I'm looking at has, starting at the left, a CH2 leading to an Ether Oxygen, leading to another CH2, which goes into the carbonyl carbon of a ketone. How would you determine the shift of that carbon in the middle?
I know the ether one would be around 3.7ppm, and the ketone around 2.1ppm, but I've never seen one with a carbon in between before so I'm not sure how to take both those into account.
Thanks a lot!
Posted Sep 26, 2017, 1:23 AM UTC
11
If I am understanding your description correctly, you have something like RCH2OCH2C(O)R', with the bolded hydrogens being what you're trying to predict the chemical shift. The deshielding effects of structures are, in general, additive, so you can expect that methylene to be shifted another ppm or so downfield from the diethyl ether methylene, so around 4.7 ppm.
Alkyl hydrogens are usually around 1, so that is your base number. So an ether shifts ~2.7 ppm downfield and the ketone would shift ~1.1 ppm downfield. The deshielding and shielding effects of functional groups are additive, but it isn't strictly addition. There is some variation and the farther away functional groups are the less effect they have on chemical shift.