Tautomerization vs Enolate formation using a base

[wahcha]

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First post-- go easy on me.

How do you know whether a base + carbonyl will result in tautomerization ... or stop half way as an enolate? Do all reactions using a carbonyl + base just make use of the enolate intermediate whereas the same reaction w/ acid (different mechanism) makes use of the enol form? (IE base catalyzed halogenation vs acid catalyzed halogenation) I know this is nit-picky stuff but I'd rather get a full understanding of the material than go for raw memorization ... it tends to make the mechanism easier to recall.

And while we're on the subject -- when adding H2O to a carbonyl (reagents = carbonyl + h2o + -OHfinal product = diol), why does the -OH act as a nucleophile and attack the carbonyl carbon instead of acting as a base and removing a C-H bond from the alpha carbon?

Thanks in advance.

 

[Elmnt]

Organic isn't my strongest subject but i'll give it a try. For your first question, I would think that in basic conditions the enolate would predominate because it is unlikely for the alkoxide to be protonated in basic conditions. Remember the pKa of the alpha proton is ~20 so the base will be decently strong, stronger than the alkoxide.

for the second question. im not certain what conditions you are specifying. You said adding H2O then switched to -OH. The best explanation I can think of is that neither the -OH or the H2O are strong bases and plus the carbonyl carbon is slightly electrophilic due to resonance with the oxygen, thus the carbonyl carbon is attacked instead of the alpha-proton.

 

[plsfoldthx]

First post-- go easy on me.

How do you know whether a base + carbonyl will result in tautomerization ... or stop half way as an enolate? Do all reactions using a carbonyl + base just make use of the enolate intermediate whereas the same reaction w/ acid (different mechanism) makes use of the enol form? (IE base catalyzed halogenation vs acid catalyzed halogenation) I know this is nit-picky stuff but I'd rather get a full understanding of the material than go for raw memorization ... it tends to make the mechanism easier to recall.

I think elmnt is correct on this one... depends on pH

And while we're on the subject -- when adding H2O to a carbonyl (reagents = carbonyl + h2o + -OHfinal product = diol), why does the -OH act as a nucleophile and attack the carbonyl carbon instead of acting as a base and removing a C-H bond from the alpha carbon?

Thanks in advance.

Because H2O is not a strong enough base to deprotonate the alpha carbon. H2O will act as a nucleophile on the carbonyl in the presence of a good leaving group. That is, the resulting carboxylic acid is more stable than, for example, a carbonyl halide. In terms of nucelophilic substitution, the product with the worse leaving group will be favored.