Carbonyls Nuc. add vs Acids Substition

atomos SG

Typo: title meant "substitution"

What would be a conise way to explain why carboxyllic acids undergo nucleophillic subsitution rather than nucleophillic addition?

My first thoughts would be the stability of the leaving group. Is there anything we can say based on the structure of the orginal molecules ?

UltimaOnline SG

Typo: title meant "substitution"

What would be a conise way to explain why carboxyllic acids undergo nucleophillic subsitution rather than nucleophillic addition?

My first thoughts would be the stability of the leaving group. Is there anything we can say based on the structure of the orginal molecules ?

No, you're totally correct that it's about the stability of the leaving group (which JCs don't even teach, and therefore almost all H2 Chem students (except for H3, Olympiad, and BedokFunland JC students) have no idea *why* in some cases nucleophilic addition occurs, *why* in other caes nucleophilic substitution occurs, they just blindly memorize their schools lecture notes without understanding).

For instance, for normal ketones and aldehydes, the only possible leaving group (other than the attacking nucleophile itself, which would be the backward reaction) are the H and R (ie. alkyl) groups. And since H- hydride ion and R-/C- carbanions are highly unstable, they just attack right back, and thus no group is willing to leave (ie. be eliminated).

Hence, the uninegative formal charge remains stuck on the most electronegative O atom for the time being, until it can resolve the negative charge by abstracting a proton (eg. from HCN), generating hydroxynitrile (aka cyanohydrin).

In contrast, for carboxylic acids electrophiles being attacked by alcohol nucleophiles, the H2O+ (if acid catalyzed) or even OH (if base promoted) leaving group has the electronegative O atom, which can stabilize the negative formal charge better, and hence makes for a more viable leaving group (since H2O and OH- are much more stable compared to H- or R-/C-).

As a tutor, your/our value is in helping your/our students understand H2 Chemistry by plugging in all the huge gaps that schools leave out (resulting in students blindly memorizing school notes rather than understanding chemistry). Not entirely the school teachers' fault, because (as ex-MOE teachers, you and I can empathize that) MOE teachers are overburdened with much too much non-teaching duties, that inevitably lower the quality of teaching possible, as well as time constraints on the poor JC students (also overburdened with much too much non-learning and time-wasting activities) whose perpetual lack of sleep causes cumulative brain damage and worsens their performance in the A level exams.

This is why there is, and will always be, a necessary, positive and (even) healthy role for good tuition teachers in Singapore, because the Education System in Singapore is much too flawed, and the MOE teachers and students suffer for it.

atomos SG

Thanks for the detailed writeup, helps to clarify quite a bit of missing gaps.

Many of your points resonate with me. Sometimes as school teachers, we do want to get into details on the concepts, but at times its hard to say anything without saying too much.

Then inevitably we'll get the pragmatic student asking "would this be tested in the exams?"

I believe students who view tutors as complements (rather than substitutes) to their school teachers will benefit much more.

Mugging overnight is a big no-no during exam periods for me, but its an uphill battle to convince students otherwise, need to enrol the help of parents for monitoring.