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Topic: Decreasing reactivity of various carbonyl coumpounds  (Read 10995 times)

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Memy

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Decreasing reactivity of various carbonyl coumpounds
« on: September 15, 2005, 11:22:51 AM »
Hello,

I have tried to put an order to the reactivity of these different carbonyl compounds to a given nucleophile Nu- . Can you please check that the order is right and if not why? thank you

-Reactivity of different carbonyl compounds:

the reactivity of carbonyl compounds is affected by :
-how well is the atom attached to the carbonyl carbon at giving/pulling the electrons of the bond (inductive and conjugation effect) and therefore increase or decrease the positve character of the carbonyl carbon
-how well will the atom attached to the carbonyl carbon depart (leaving group ability)

(the following order goes from the most reactive to the least reactive)

1) ACID CHLORIDE ( Cl is a good leaving group and strong -I inductive effect)
2)ANHIDRIDE (-OCOR is a good leaving group and not effective delocalization)
3)THIOL ESTER(no effective delocalization as no effective orbital overlap between C and S)
4)ESTER(orbital overlap between C and O but less than in an amide)
5)AMIDE
6)N,N DIALKYLAMIDE (-NR2 is a worse leaving group than -NH2 in an amide)
7)CARBOXYLIC ACID ( it usually get deprotonated and not nucleophilic attack)
8 )FORMALDEHYDE ( -H is a bad leaving group)
9)ALDEHYDE (-H and -R are bad leaving groups)
10)KETONE ( -R is a bad leaving group and electron donating)
11)CARBOXYLATE ANION ( complete effective delocalization and the negative charge will repel the lone pair/ the negative charge of the nucleophile)

where can I put the ACID NITRILE ( CN is a bad leaving group) and HYDRAZIDE in the previous series?

thank you very much
« Last Edit: September 15, 2005, 11:24:03 AM by Memy »

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Re:Decreasing reactivity of various carbonyl coumpounds
« Reply #1 on: September 15, 2005, 01:05:58 PM »
Looks okay for the most part, however, you should consider that the leaving group ability might not come into play.  For example, if I reacted a Grignard reagent (Nu-) with formaldehyde I would expect a very rapid reaction since there is essentially no steric hindrance and the carbonyl carbon is quite electron defficient.  Even though there is not a good leaving group, formaldehyde is still very, very reactive towards nucleophiles, you just tend to get the alcohol as the product.  So, given that, where would you place formaldehyde relative to the other carbonyl compounds?  Think more in terms the electron density at the carbonyl carbon.  Which other class of compounds would this argument apply to?

Two other points:
- While thio-esters don't exhibit the same level of electron donation that regular esters do, it's not right to say that they don't have any electron donation, because they do have some.
- For acid nitriles, actually CN- is quite a good leaving group.  There is a rather famous reagent called Mander's Reagent which has the form (NC)-CO-OMe and is often reacted with enolate anions to make beta-keto esters.  What is the leaving group in this case?  What does that tell you about the relative leaving group ability of CN- and MeO-?

Memy

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Re:Decreasing reactivity of various carbonyl coumpounds
« Reply #2 on: September 15, 2005, 03:25:04 PM »
Hello, thank you for the answer...
so then formaldehyde and aldehydes in general can be put under acid chlorides , in order of reactivity, like this ?:
1) acid chlorides
2)acid nitriles
3)formaldehyde
4)aldehydes
5)anhydride

etc...

Ketones can be left in the same place in the order of reactivity that I have written due to the fact that there is an electron donating effect and steric hindrance?

Ok, i have found that CN is a good leaving group and an electron withdrawing group, so it's pretty reactive as well, so maybe it can go under the acyl chlorides, before the formaldeyde?

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Re:Decreasing reactivity of various carbonyl coumpounds
« Reply #3 on: September 15, 2005, 04:36:13 PM »
I think that the acid nitrile will be less reactive an anhydride, but I'm not positive of that.  They will probably be pretty close.  Anhydrides are probably more reactive than formaldehyde and other aldehydes too, but then again it kind of depends on the specific reaction.  For example, an anhydride and an aldehyde will react rapidly with water to for the acid or the gem-diol.  The difference is that the acid won't spontaneously go back to the anhydride, but the gem-diol can eliminate water to reform the aldehyde (the two would likely be in equilibrium).

As for ketones, again consider the electron donating ability of the alpha carbons relative to the donating ability of an oxygen atom.  Would a ketone or an ester be more reactive?

Memy

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Re:Decreasing reactivity of various carbonyl coumpounds
« Reply #4 on: September 15, 2005, 06:24:06 PM »
hello,

mm i think ketones will be more reactive than esters, as the electron donating effect of the oxygen is more pronounced than the electron donating effect of alkyl groups..is that right?
So, the reactivity list should look like something like this:

1)acyl chlorides
2)Anhydrides/acyl nitriles
3)Formaldehyde
4)Aldehydes
5)Ketones
6)Thiol ester ( the electron donating effect of S is bigger than the electron donating effect of the alkyl groups of ketones)
7)ester
8 )amides
9)N,N di-alkylamides
10)carboxylic acids
11)carboxylate anion

Hydrazides are before the amides and under esters in the reactivity list, as they have another nitrogen atom that pulls the electrons toward it and increase the positive character of the carbonyl carbon?

thank you very much
« Last Edit: September 15, 2005, 06:27:49 PM by Memy »

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Re:Decreasing reactivity of various carbonyl coumpounds
« Reply #5 on: September 16, 2005, 12:16:12 PM »
That looks good to me.

I'm not sure about the acid hydrazides, but your explanation seems to be good.  They will definitely appear somewhere more reactive than amides, but less reactive than ketones, but I'm not sure how they would compare with esters.

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