According to the general theory of electron attraction, I thought the central C in carbonates should have larger chemical shifts than in carboxylic acid esters because it is bonded to one =O and two -O, whereas the latter is bonded to one =O, one -O, and one -C. But the experimental result and software prediction both show that the chemical shift of carboxylic acid esters are higher than carbonates. Can you explain this phenomenon?
Electro attraction is not the only factor affecting chemical shifts, if you compare a ketone with a carboxylic acid ester, the 13C chemical shift of the carbonyl group is lower for the ester. This is due to conjugation between the carbonyl C=O double bond and the lone pairs of electron from the oxygene located between the 2 carbons. In a carbonate, the carbonyl is conjugated with the lone pairs of 2 oxygenes, thus the chemical shift is even lower.
Carboxylic acid esters have larger NMR chemical shifts than carbonates because the ester’s electron-withdrawing alkyl group increases the electron density around the carbonyl carbon, causing a more deshielded environment compared to carbonates, where the oxygen atoms are more evenly distributed. This leads to a higher chemical shift in esters.
Carbonates and esters have 2 oxygen atoms with single bonds and a main carbon bound to an oxygen atom with a double bond. The third atom which is attached to the core carbon is the crucial distinction. Due to the resonance effects, esters have a bit higher electron density and a lower electron-withdrawing activity, which causes the central carbon to undergo a higher chemical shift. Chemical shifts can also be influenced by other factors such as the molecular environment, solvent effects, and steric effects.
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