Which is more reactive, Carboxylic acid or ester?

[eveangel6]

I'm really confused about this because the TBR ochem book and the EK ochem book say different things.

The EK book says carboxylic acids are more reactive than esters. But the tbr book says "carboxylic acids carry out similar organic reactions as esters, but are less reactive than esters"

Which one is right?

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[RickyJamesMD]

Hey Eveangel,


I think it's important to understand what they mean by "reactive". For example, in nucleophilic substitution, then the ester is more reactive than the carboxylic acid. The reason is that esters have better leavings groups than the hydroxyl group of the carboxylic acid. ANd since it's a carboxylic acid, there's a high chance it'll be a carboxylate (deprotonated) in its natural form. AS a result, the carboxylate is a terrible terrible electrophile and less prone to nucleophilic substitution.

I hope that helps!

 

[eveangel6]

that makes more sense. thanks so much!

 

[sigle]

Yay I dont have to create a new thread, I hope you dont mind @ the OP!
I have questions regarding the reactivity of COOH and its derivatives. I thought because COOH has a pka of ~5 that it would be more reactive than Ester which has a pka of 15-19. But I also know that reactivity for nucleophilic substitution decreases with increasing basicity; the leaving group -OH is more basic than O-R, right?
Can someone help me reconcile these two ideas please?

Thank you!

 

[Danlee07]

similar concept:
the stronger base isn't a better nucleophile; the stronger nucleophiles are stronger bases.

 

[loveoforganic]

Yay I dont have to create a new thread, I hope you dont mind @ the OP!
I have questions regarding the reactivity of COOH and its derivatives. I thought because COOH has a pka of ~5 that it would be more reactive than Ester which has a pka of 15-19. But I also know that reactivity for nucleophilic substitution decreases with increasing basicity; the leaving group -OH is more basic than O-R, right?
Can someone help me reconcile these two ideas please?

Thank you!

For carboxylic acids, you're going to have to deal with the proton somehow before you can look at substitution

 

[sigle]

For carboxylic acids, you're going to have to deal with the proton somehow before you can look at substitution

So I should just compare the derivatives to each other and not to COOH? Makes sense thanks.

 

[orgohacks]

So I should just compare the derivatives to each other and not to COOH? Makes sense thanks.

"Reactivity" also depends on what reaction you're talking about.

Nucleophilic substitution [addition/elimination, to be more specific] of carboxylic acids under basic conditions is a no-go because they just deprotonate the acid, giving the negatively charged carboxylate.

this might be helpful - a sheet summarizing the reactions of carboxylic acid derivatives: http://is.gd/coeOE

 

[BerkReviewTeach]

I have questions regarding the reactivity of COOH and its derivatives. I thought because COOH has a pka of ~5 that it would be more reactive than Ester which has a pka of 15-19.

What you've determined with those numbers is that the hydroxyl proton of a carboxylic acid group is more acid that the alpha proton of an alkyl ester. That tells you what would happen if you added a strong base to both compounds, but doesn't tell you about how good of an electrophile each compound may be.

 

[Phantastic]

What you've determined with those numbers is that the hydroxyl proton of a carboxylic acid group is more acid that the alpha proton of an alkyl ester. That tells you what would happen if you added a strong base to both compounds, but doesn't tell you about how good of an electrophile each compound may be.

Indeed! Just for clarification,

Consider a few reactions:

A carboxylic acid + -OEth = Acid base reaction, leaving most stable anion (conj base) of COO-
An ester + -OEth = Nucleophilic addition elimination before Acid base reaction (because attack of the carbonyl by -OEth is more likely to occur than the extraction of the alpha proton of the ester

Is that correct?

 

[orgohacks]

Indeed! Just for clarification,

Consider a few reactions:

A carboxylic acid + -OEth = Acid base reaction, leaving most stable anion (conj base) of COO-
An ester + -OEth = Nucleophilic addition elimination before Acid base reaction (because attack of the carbonyl by -OEth is more likely to occur than the extraction of the alpha proton of the ester

Is that correct?

Exactly - pKa of alcohol is ~17, pka of ester alpha-proton is ~25, so addition/elimination is favored over deprotonation of the alpha proton by the alkoxide.