acidity of alpha hydrogens in a ketone

[thebillsfan]

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lets say you have acetone, except that one of the alpha carbons has a methyl group attached to it. would the hydrogens on that alpha carbon be less acidic than the hydrogen on the other alpha carbon?

also, if oxygen can bear a negative charge better than a carbon, why does an enolate carbon more likely to attack an electrophile than the enolate oxygen?

thanks for your help.

 

[maverick09]

i think the acetone with a methyl would yield less acidic hydrogens because a methyl group is electron donating (thus destabilizing the anion that would result should you remove a hydrogen). acidity is related to the stability of the anion

 

[Omni]

To you first question, my assumption would be that the methyl group on the alpha carbon will make the alpha carbon less acidic. If you had something electronegative, it would make it more acidic.

Your second question I'm not understanding. An enolate is an ion that is stabilized through resonance. The negative charge jumps between the carbonyl O and the alpha C, but stays mostly with the O. Hence, this thing would not be an electrophile.
http://en.wikipedia.org/wiki/Enolate_anion

 

[thebillsfan]

To you first question, my assumption would be that the methyl group on the alpha carbon will make the alpha carbon less acidic. If you had something electronegative, it would make it more acidic.

Your second question I'm not understanding. An enolate is an ion that is stabilized through resonance. The negative charge jumps between the carbonyl O and the alpha C, but stays mostly with the O. Hence, this thing would not be an electrophile.
http://en.wikipedia.org/wiki/Enolate_anion

omni,

my poorly phrased question basically was asking, in an enolate, is the oxygen atom or carbon atom more nucleophilic? I *know* the answer to be the carbon, because I always see the carbon attacking electrophiles. however, i know that oxygen is more electronegative, so am wondering why this would be more likely to attack an electrophile.

 

[maverick09]

enolate anions react at the carbon for two reasons. first, there is always a counterion such as Li+ or Na+ ion associated with the enolate anion and these are more tightly associated with the oxygen atom than the alpha carbon (resuliting in blockage of the electrophile and thus reducing the likelihood of a productive collision with the oxygen). the counterions are from the base used to pluck the hydrogen off. the second reason is a thermodynamic one and essentially boils down to the fact that stronger bonds form in the product when we react at the carbon as opposed to when we react at the oxgen. PM me if you want more explanation on this