Question

Which of the following carbon atoms will display the signal most downfield in the 13C NMR spectrum? https://imgur.com/quYKwmQ

The answer is B but can somebody explain why? Would we have to remember the ppm numbers or is there another way to figure out this answer? Thank you.

EDIT: That is the whole question, there is no need for another structure to be added.

Answer 1

A signal is said to be shifted downfield when it occurs at a high δ (ppm) value relative to TMS (internal standard which appears at δ 0.0 ppm in the proton NMR spectrum and is not seen in the C-13 spectrum). A signal is said to be shifted downfield due to electronic and magnetic effects. Since the question compares different functional groups, we shall consider the electronic effect here.

The more electronegative a heteroatom bound to C, the more is the signal shifted downfield. This is due to the fact that the electronegative heteroatom reduces electron density of carbon by withdrawing the electron density toward itself. F and O are the foremost and second most electronegative elements in the periodic table. We shall expect compound D (having a C-F bond) to appear most downfield. Certainly, F is highly electronegative and withdraws the electron density of the C-F bond toward itself, rendering the carbon positively polarized. However, note that the said carbon is attached to three alkyl groups and alkyl groups are electron-donating. Hence, these alkyl groups push electron density toward the C-F bond, thereby, reducing the electron deficiency of carbon. Hence, compound D doesn’t show the most downfield shifted signal.

The most downfield shifted signal is shown by compound B. O is bound to carbon via a double bond and withdraws the electron density of the C-O bond toward itself. Infact, the carbonyl group resonates as a C⁺-O^- group with partially polarized carbon and oxygen atoms. It is due to this electron-withdrawing effect of the carbonyl compound that compound B has the signal that is shifted most downfield. There are two alkyl groups attached to the carbon, but the electron-withdrawing effect of the carbonyl group far outweighs the electron-pushing inductive effect of the alkyl groups. Infact, the C=O group appears around δ 200-220 ppm in the spectrum.

Note: You do not need to memorize exact values; but an idea about the possible ranges for the various functional groups is imperative.