Claisen
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Fries Rearrangement and Sulphonation
In Fries Rearrangement of phenyl ethanoate, low temperature favours the formation of p-isomer whereas high temperature favours the o-isomer.
In sulphonation of phenol, low temperature yields o-isomer whereas high temperature yields p-isomer in greater yield.
Why is there a difference?
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Nicodem
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Please open referenceless threads only in the Beginnings section. Surely you don't expect us to search for articles describing the temperature
dependence of the Fries rearrangement regioselectivity?
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Nicodem
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Thread Moved
5-2-2011 at 08:15 |
madscientist
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I think he just overestimated our knowledge! 
Low temperature products are kinetic products (lower energy transition states), and high temperature products are thermodynamic products (lower energy
products, more stable). So, it's probable that o-acylphenols are more stable than p-acylphenols, and p-sulfonated phenols are more stable than
o-sulfonated phenols. The inverse goes for the lowest energy transition states.
SO<sub>3</sub>H is a stronger electron withdrawing group than COR. Steric effects are also greater with SO<sub>3</sub>H than
COR, which would push SO<sub>3</sub>H away from OH. It could be that with the Fries Rearrangement, electronics outweigh sterics in
dictating product stability, the inverse being the case for sulfonation. Conversely, for transition states, it could be that with the Fries
Rearrangement steric effects outweigh electronic effects, the inverse being the case for sulfonation, due to the presence of a better electrophile.
Afraid that's all I can offer you - I'm already getting dangerously speculative! As Nicodem said, we'll need to read some papers to know the answer.
[Edited on 5-2-2011 by madscientist]
I weep at the sight of flaming acetic anhydride.
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Claisen
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Quote: Originally posted by Nicodem  | | Please open referenceless threads only in the Beginnings section. Surely you don't expect us to search for articles describing the temperature
dependence of the Fries rearrangement regioselectivity? |
My apologies. I was unaware of that.
Is there any page which mentions such do's and don't's of the forum?
@madscientist
That seems to be a possible explanation. Thanks!
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DJF90
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I propose:
The o-isomer of acylphenol is favoured under thermodynamic conditions (high temp) due to the increased stability of the product via
intramolecular H-bonding between the phenol and the carbonyl group. The p-isomer is the kinetic product due to steric effects (the aluminium
phenolate intermediate is going to be pretty big!).
The o-isomer of phenolsulfonic acid is likely due to H-bonding in the transition state, directing the sulfonating species towards the
o-position. This is sterically unfavourable and so at higher temperatures the p-product is favoured on the grouds of steric
interactions.
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Nicodem
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| Quote: Originally posted by Claisen | My apologies. I was unaware of that.
Is there any page which mentions such do's and don't's of the forum? |
There is no need to have threads specifically discuss the need of adhering to the basics of the scientific method when the title of the forum is
Sciencemadness. I would guess the forum tittle is enough.
Anyway, we are still waiting for you to present us your source of information about the regioselectivity of the Fries rearrangement. Don't you think
it is a bit unfair that you let members speculate about something that might not even be true as far as we know? It makes the whole discussion
needlessly tautologic while it could actually be interesting and constructive instead.
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
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Claisen
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| Quote: Originally posted by Nicodem |
There is no need to have threads specifically discuss the need of adhering to the basics of the scientific method when the title of the forum is
Sciencemadness. I would guess the forum tittle is enough. |
I don't have any idea about any such scientific method being a high school student. I post my doubts on other science forums too but they have never
asked for any references.
| Quote: Originally posted by Nicodem | Anyway, we are still waiting for you to present us your source of information about the regioselectivity of the Fries rearrangement. Don't you think
it is a bit unfair that you let members speculate about something that might not even be true as far as we know? It makes the whole discussion
needlessly tautologic while it could actually be interesting and constructive instead.
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I don't know about any online source that provide this information. It was written in my textbook and I thought experts here can clear such
introductory level doubts with ease.
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Nicodem
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| Quote: Originally posted by Claisen | | I don't know about any online source that provide this information. It was written in my textbook and I thought experts here can clear such
introductory level doubts with ease. |
I don't understand why do you find it so difficult to cite that textbook or verify its references (if it has any)? In any case, your textbook left out
the crucial information which is otherwise nicely pointed out in a review on the Fries rearrangement where for the acid catalysed version it says:
| Quote: | 8) the selectivity of the rearrangement to give ortho or para- substituted products largely depends on the reaction conditions (temperature, type, and
amount of catalyst, solvent polarity, etc.);
9) at high temperatures without any solvent the ortho-acylated product dominates while low temperatures favor the formation of the
para-acylated product;
10) with increasing solvent polarity the ratio of the para-acylated product increases
cited from Strategic Applications of Named Reactions in Organic Synthesis (Kürti, Czako, 2005) |
It seems obvious that under suppressed reversibility (solventless conditions) the rearrangement gives mostly the kinetic product (ortho selectivity),
while under high reversibility (polar solvents) the thermodynamic product is preferentially obtained (para selectivity). Hence, the reaction
regioselectivity does not differ that of sulfonation, except that here the formation of the more stable ortho-acylphenol complex with the Lewis acid
blocks further transacylation under solventless conditions where the dissociation from the metal is inhibited.
Attachment: Fries rearrangement from Strategic Applications of Named Reactions in Organic Synthesis.pdf (102kB)
This file has been downloaded 6461 times
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
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