Why do carboxylic acid derivatives show a more upfield shift than carbonyls in 13C-NMR?
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.