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Topic: Base catalyzed hydrolysis  (Read 2255 times)

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Offline limonade

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Base catalyzed hydrolysis
« on: August 21, 2013, 06:21:30 PM »
I just want to make sure my reasoning is correct.

 In the picture I listed the following in order of most reactive to least reactive under conditions of base catalyzed hydrolysis. I just used the reasoning that the first step of a hydrolysis is a nucleophilic on a carbonyl carbon with delta positive charge character.


So I reasoned that order of reactivity might parallel electron withdrawing and donating effects.

The acyl chlorine being most reactive due to the inductive effect of chlorine increasing positive charge character on carbon.

Then the carboxylic ester with only two electron donating methyl groups, etc...


Offline Schrödinger

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Re: Base catalyzed hydrolysis
« Reply #1 on: August 31, 2013, 01:49:01 PM »
I have a feeling you also need to consider the effect that can be produced by an enolizable proton as compared to a structure that contains none. For instance, since 3 contains no such proton, this ester is probably more nucleophilic as compared to 2, which has one such proton. This is just a hunch. I might be wrong though.
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Offline spirochete

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Re: Base catalyzed hydrolysis
« Reply #2 on: September 02, 2013, 09:52:27 PM »
I doubt enolizable protons have anything to do with it. Most reagents used (IE hydroxide) are not strong enough bases to come anywhere close to quantitatively deprotonating that alpha carbon, so formation of enolate shouldn't factor into electrophilicity much.

I'd worry much more about steric effects with the alpha substitution, IE the trimethyl ester is less reactive than the dimethyl ester. That's because the electron donating ability of a Carbon-Carbon bond is about the same as the electron donating ability of a carbon-hydrogen bond, so they are roughly electronically equivalent.

In short, my best guess is rank them first using electronic effects (IE chloride most reactive, ester in middle, then amide), then use steric effects to rank within those categories.

I am less sure about the methylation at nitrogen, though.
« Last Edit: September 02, 2013, 10:16:42 PM by spirochete »

Offline opsomath

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Re: Base catalyzed hydrolysis
« Reply #3 on: September 03, 2013, 02:16:38 PM »
The #1 consideration is how good of a leaving group you have on the carbonyl. Do you know how to compare two groups in terms of leaving group stability? Hint, it is the same as base stability.

The #2 consideration is the effect of methylating the alpha carbon, which will only matter when the leaving group is the same. I would say that that will have the effect of slowing down the reaction, by steric hindrance of the nucleophile attacking the carbonyl.

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