The prototypical 13C NMR shifts of carbonyl carbons is in the range 190–200 ppm, however, carboxylic acid derivatives have a 13C NMR shift in the range 160–180 ppm instead. What structural factors cause this shift upfield? Shouldn't the carbon in the carboxylic acid be more deshielded than the analogous carbonyl, since there are two oxygens instead of one?
Shouldn't the carbon in the carboxylic acid be more deshielded than the analogous carbonyl, since there are two oxygens instead of one?
Well, no. I love this misconception and I'm always having fun when doing oral exams about reduction reactions and grignards, which is basically the same problem.
If you look at reactivity of acids/esters and ketones/aldehydes against nucleophilic attack (grignard reagents or hydride reagents) you will see that ketones and aldehydes are much more reactive towards nucleophiles than acids/esters are. So why are ketones and aldeyhdes more electron deficient on the carbonyl carbon?
Yes, in case of acid/ester you got two oxygens both with electron withdrawing -I properties, but the additional oxygen also got lone pairs so a +M effect is possible. This actually pushes more electron density towards the carbon than the -I is pulling away leaving the carbonyl carbon in acids and esters overall more electron rich than in aldehydes and ketones.