nucleophilicity and basicity

[happyfellow]

I know this has been talked about in the past but I think it's an important concept, but an albeit confusing one.

I know that a nucleophile is a kinetic term while basicity is a thermodynamic one pertaining to proton transfer. I also know that there are strong bases that are nucleophiles and vice versa. However, how do we know if a base will act as a nucleophile or as a base in a reaction? For example, when we form enolates why does the -OH attack the alpha-hydrogens instead of the carbonyl carbon?

Thanks in advance

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[PiBond]

I know this has been talked about in the past but I think it's an important concept, but an albeit confusing one.

I know that a nucleophile is a kinetic term while basicity is a thermodynamic one pertaining to proton transfer. I also know that there are strong bases that are nucleophiles and vice versa. However, how do we know if a base will act as a nucleophile or as a base in a reaction? For example, when we form enolates why does the -OH attack the alpha-hydrogens instead of the carbonyl carbon?

Thanks in advance

tough question. i believe that the molecule that the nucleophile/base is acting on depends on whether or not we use the term nucleophile or base. for your enolate example, the alpha-hydrogens are sufficiently acidic for -OH to pluck off (their pka's are around 20 or so). however, were there no alpha-hydrogens then the -OH would probably have acted as a nucleophile at the carbonyl carbon.

just my .02

 

[boaz]

Have you learned about thermodynamic/kinetic control? In any case, it does attack the carbonyl carbon, but the reaction is reversible, leaving us again with the OH- and the carbonyl. It is then free to attack the hydrogen on the alpha-carbon. This reaction is also reversible, but since it is the more stable product, this is the one that we get.

 

[RogueUnicorn]

this is one thing i can confidently say i'll never understand.

 

[Isoprop]

Nucleophilicity is a kinetic concept but that doesn't mean we can't compare the thermodynamics of the nucleophilic attack of a carbonyl. Similarly, even though basicity is a thermodynamic concept, we can still talk about the kinetics of an acid-base reaction.

So let's compare the speed or kinetics. Generally speaking, there are usually more alpha hydrogens to react with a strong base than there are carbonyl carbons to react with a nucleophile. So the enolate reaction will happen faster because the base has a higher chance by reacting with the alpha hydrogens than it does with the carbonyl. So from a kinetics standpoint, the enolate formation is favored.

Now let's compare thermodynamics. The tetrahedral intermediate from a nucelophilic attack will produce a localized charge on the oxygen atom. The enolate formation will also produce a charge, but that charge is delocalized through resonance. So the enolate is more stable and more favored.

This is why a carbonyl + NaOH will produce an enolate.

 

[Isoprop]

Oh, and to put things in perspective, there is still relatively small amount of enolate created from NaOH. There's just less of the tetrahedral intermediate from the nucelophilic addition rxn.

Why is there a small amount of enolate? Because those "acidic" alpha hydrogens aren't that acidic at all. Remember, enolates are conjugate bases of the enols. And we know that enols are almost always tautormerized into the keto form. This is okay because as an electrophile reacts with the alpha carbon and shifts equilibrium to produce more enolate ions..