idk - 19-7-2018 at 07:05
Is there any accurate model of predicting how fast decarboxylation rxn will occur including factors as boiling point of substrate which and needed RT?
If there isn't then could you tell me how to predict having a few compounds which one more likely undergos the reaction?
To simplify lets choose:
1. benzoic acid
2. 2-phenylacetic acid
3. 3-phenylpropanoic acid
4. heptanoic acid (I've chose it because hexane has the closest b.p. to benzene, and I analogously added COOH group to it, as to benzene)
I've seen some videos about making benzene from benzoic acid, altough IDK if mesomeric effect shouldn't cause it to be the most stable one.
I will be gratefull for help.
happyfooddance - 19-7-2018 at 08:03
I think electrophilicity of the carbonyl carbon is a big factor. I don't know that boiling point of your substrate is relevant to decarboxylation,
especially when considering that you will probably be decarboxylating in the presence of a strong nucleophile (i.e. base), which will render your
substrate non-volatile.
Heptane might have a close b.p. to benzene, but the alkyl group isn't nearly as electron withdrawing as the aryl group, so I wouldn't expect it to
decarboxylate so readily.
Sigmatropic - 19-7-2018 at 08:18
I think it depends on the mechanism at play. Sodium benzoate decarboxylation I belive goes through a phenyl anion, not though a carbonyl adduct.
Mesomeric effect has no influence on this mechanism. Assuming formation of the anion is rate limiting we get an sp2 carbanion from benzoic acid, a
benzylic anion from phenyl acetic acid and further homologues would go through a non conjugated sp3 carbanion. Considering the relative stabilities of
these anions I would therefore predict that Sp2 carbonions, having higher s-chatacter are more stable and thus form faster. Phenyl acetic acid with
its benzyl anion may just work too but I do not know of any precedent for the higher homologues or the aliphatic ones.
For an alternative mechanism consider the decarboxylation of salicylic acid under acidic conditions. In that mechanism I belive ipso protonation is
rate limiting and therefore the more electron rich the aryl group the more readily it occurs.
Does someone know through what mechanism hetereoaryl carboxylic acids such as niacin or pyrazole carboxylic acids decarboxylate?
Just realized that a phenyl carbanions can form from a carbonyl addict and likely does. See http://www.sciencemadness.org/talk/viewthread.php?tid=9717&a...
Now I wonder wether formation of the phenyl anion from the dianionic carbonyl adduct is the rate limiting step or the formation of that dianionic
carbonyl adduct...
[Edited on 19-7-2018 by Sigmatropic]
morganbw - 19-7-2018 at 11:39
A really nice question and I hope to learn a bit from this.
I would have a big smile on my face if I could honestly answer your question.
Fear not though. I may be the most knowledge deprived of this whole forum.