Carboxylic Acids, Ketones, Aldehydes

[MedPR]

For the life of me I can't keep this straight in my head. I'm hoping that typing it out here will make it stick and hopefully you guys will correct me where I'm wrong and offer some tips 🙂

Carboxylic acids DO NOT undergo nucleophilic addition reactions, but PREFER nucleophilic substitutions. This is because the carbonyl oxygen and the alcohol oxygen "share" the double bond, thus creating a large electron cloud very near the carbonyl carbon. Large cloud = nucleophile can't get close enough to add.

Ketones and Aldehydes PREFER nucleophlic addition because the carbonyl carbon carries a pretty large partial positive charge due to the EW nature of the carbonyl oxygen.

However, I'm not really clear on why ketones/aldehydes are less prone to nucleophilic substitution. Is it because the hydrogen (on aldehyde) and the alkyl groups (on ketones and aldehydes) are unreactive and won't accept an addition substituent?

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[chiddler]

Think of what happens to the carbon if there's nucleophilic substitution with a ketone. A carbanion has to form which is extremely unstable. Comparing this to ester Nu-substitution, an anion of something else forms, usually an oxy-anion of some sort forms.

here.

in the intermediate structure, it can swing both ways. which way would produce the more stable products? i should of added the by products, too. but keep those in your mind as you decide. oh and same applies to aldehydes. no compelling reason for hydrogen to become an anion in that case, either.

 

[MedPR]

Think of what happens to the carbon if there's nucleophilic substitution with a ketone. A carbanion has to form which is extremely unstable. Comparing this to ester Nu-substitution, an anion of something else forms, usually an oxy-anion of some sort forms.

here.

in the intermediate structure, it can swing both ways. which way would produce the more stable products? i should of added the by products, too. but keep those in your mind as you decide. oh and same applies to aldehydes. no compelling reason for hydrogen to become an anion in that case, either.

I understood what you were saying before I looked at that image. If you could somehow get a tetrahedral intermediate to form, why would the methyl product be preferred to the nucleophile product?

Nucleophile = has electrons to donate. Carbonyl carbon electrons = getting jacked by the oxygen, so wouldn't having a nucleophilic substituent help to reduce the partial positive on the carbonyl and stabilize it?

Edit: Oh wait, alkyl groups are EDG too...

I'm still not really sure why the tetrahedral intermediate would more likely lead to methyl substituent than Nu substituent. I thought the whole reason why ketones/aldehydes didn't undergo substitution had something to do with the fact that their existing substituents (alkyl and hydrogen) were very poor leaving groups.

I think my confusion might be from not having a good grasp on the mechanisms of both substitution and both elimination reactions. Going to make a thread about those now too 😀