quick aldehyde vs ketone question

[rmm30]

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i have it written in the margin of my notes that aldehydes are generally better nucleophiles than ketones. Im not sure if this is accurate. I know aldheydes are more acidic making their conjugate bases more stable and this seems to suggest that they are weaker nuclephiles. anybody know how this works?

 

[rmm30]

so ive given it a little more thought and am even more confused. ketones and aldohydes are only nucleophilic in enol (not very) and enolate form (ie after deprotination at alpha carbon) since aldehyde enolates form more readily would they then be better nucleophiles?

 

[JamesBourne]

I am not 100% sure but, I think it might have to do with steric hinderance even though aldehyde is a weaker base comparing to a ketone. Since ketone has 2 carbon chains next to the carbonyl group, it would be less favorable in a nucleophilic addition wherease aldehyde only has one carbon chain next to the carbonyl group, thus has less hinderance for the reaction. Hope it helps =)

 

[rmm30]

good point and thanks for response. i would still like to know how acidity and formation of enolate fits in to its nucleophilicity.

 

[sbdento]

Aldehydes are more reactive than ketones because ketones are stabilized by the inductive effect. Remember R groups are considered electron donating groups. The extra R group on a ketone donates electrons and therefore stabilizes the carbonyl C. I would say that this stabilization by induction is more important than the said steric hindrance.

 

[rmm30]

Aldehydes are more reactive than ketones because ketones are stabilized by the inductive effect. Remember R groups are considered electron donating groups. The extra R group on a ketone donates electrons and therefore stabilizes the carbonyl C. I would say that this stabilization by induction is more important than the said steric hindrance.

I appreciate your help but you are talking about about ketones and aldehydes in a different context. what you are talking about is carbonyl compounds as electrophiles. Everything you said is true but it applies to the carbonyl carbon when ketone/aldehyde is acting as an electrophile.
As nucleophiles, ketones and aldehydes are nucleophilic at the alpha carbon. in this case ketones as conjugate bases are less stable due to induction than are aldehydes, making aldehydes more acidic. as for steric hinderance jamesbourne is right but thats only part of the story. My question was since ketones are the less stable/stronger conjugate base than are they better nucleophiles than aldehydes. I have it in my notes that they are not because the aldehyde forms a conjugate base more readily. I just wanted to double check.

 

[sbdento]

OoPs I read over that part 😛
Now I feel dumb for explaining something you already know.
Anyway, good luck with the test !!