May 20, 2019, 04:55:19 PM
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Topic: Which carbonyl electrophile is more reactive?  (Read 305 times)

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Offline sharbeldam

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Which carbonyl electrophile is more reactive?
« on: May 12, 2019, 11:36:28 AM »
FFFCC(=O)OC(=O)CFFF

OR


FFFCC(=O)-Cl


My attempt: One side of the carbonyl is similar (CF3, yes its CF3, i didn't know how to do that in the pic) So I need to compare between the Cl group and  the OCOCF3 Group, chlorine is more electron withdrawing than oxygen so the the second option should be a better electrophile?

Thanks

Offline AWK

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Offline sharbeldam

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Re: Which carbonyl electrophile is more reactive?
« Reply #2 on: May 12, 2019, 12:32:53 PM »
Thank you haha, is my attempt correct though :P?

Offline AWK

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Re: Which carbonyl electrophile is more reactive?
« Reply #3 on: May 12, 2019, 12:47:02 PM »
What kind of reaction do you mean?
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Offline sharbeldam

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Re: Which carbonyl electrophile is more reactive?
« Reply #4 on: May 12, 2019, 12:48:29 PM »
Ester hydrolysis under basic condition, where the carbonyl of ester acts as an electrophile and an OH- attacks it and then a OR group leaves.

Offline AWK

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Re: Which carbonyl electrophile is more reactive?
« Reply #5 on: May 12, 2019, 12:55:52 PM »
These compounds are not basically esters. Their hydrolysis rate depends on the leaving group. You should check which one is better (Cl- or CF3COO-)
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Offline sharbeldam

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Re: Which carbonyl electrophile is more reactive?
« Reply #6 on: May 12, 2019, 01:12:46 PM »
is there a way to check that?
I can't rely on the electrophilicity of the carbonyl carbon? since if it has a larger positive charge it will be a faster reaction, chloride is more withdrawing group so logically it will make it a better electrophile, no?

Offline AWK

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Re: Which carbonyl electrophile is more reactive?
« Reply #7 on: May 12, 2019, 01:39:52 PM »
There is a general rule to approximately determine which leaving group has more favorable properties and indicates the chloride ion. But in the case of groups containing fluorine, there may be surprises.
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Offline hollytara

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Re: Which carbonyl electrophile is more reactive?
« Reply #8 on: May 14, 2019, 12:09:29 AM »
Usually leaving group ability correlates with the pKa of the conjugate acid of the leaving group: stronger acid = better leaving group.  (hence I>Br>Cl>>F).  Here you are comparing Cl- (conjugate acid HCl) and CF3CH2CO2- (Conjugate acid CF3CH2CO2H).  Which acid is stronger?  There can occasionally be other factors - solvation of the leaving group for example - that can work against this general rule.

This basic principle should be in your text or in the review /study guide by Fredlos.

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Re: Which carbonyl electrophile is more reactive?
« Reply #9 on: May 14, 2019, 07:03:09 AM »
You have an error in the formulas hollytara, it should be CF3COOH, not CF3CH2COOH.

Offline sharbeldam

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Re: Which carbonyl electrophile is more reactive?
« Reply #10 on: May 14, 2019, 06:36:16 PM »
But again since im being asked about which hydrolysis goes faster, shouldn't the electrophilicity of the carbonyl be considered?

chloride is more withdrawing group so logically it will make it a better electrophile ( more positive charge ) no?

But if i compare the basicity of the leaving groups

Cl- / conjugate acid is HCl with a pKa of -6.3 so it has a stronger conjugate acid than CF3COOH so Cl- is a weaker base and hence a better leaving group.
« Last Edit: May 14, 2019, 06:48:46 PM by sharbeldam »

Offline spirochete

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Re: Which carbonyl electrophile is more reactive?
« Reply #11 on: May 14, 2019, 07:27:27 PM »
But again since im being asked about which hydrolysis goes faster, shouldn't the electrophilicity of the carbonyl be considered?

chloride is more withdrawing group so logically it will make it a better electrophile ( more positive charge ) no?

But if i compare the basicity of the leaving groups

Cl- / conjugate acid is HCl with a pKa of -6.3 so it has a stronger conjugate acid than CF3COOH so Cl- is a weaker base and hence a better leaving group.

It sort of does depend on which step is rate limiting, I think. If attack at the carbonyl is rate limiting, then it matters more how electrophilic the carbonyl carbon is. If loss of leaving group is rate limiting, then it depends mainly on which leaving group is better. I recall reading that for esters, loss of leaving group is usually rate limiting but it could go either way. For acid chlorides I recall that attack at the carbonyl is rate limiting because chloride is such a great leaving group.

But in the end it doesn't really matter for predicting relative reactivity, because the pKa of the conjugate acid of the leaving group will still predict relative reactivity as an electrophile, regardless of what the rate limiting step is. The pKa is sort of a "nemonic" for predicting the correct answer. Thus you don't really need to know what the rate limiting step is. I guess it works because a more weakly basic leaving group also is more withdrawing toward the carbonyl carbon, prior to leaving.

If anybody has any examples where this turns out not to be true, I would be very curious to hear them!
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