Sciencemadness Discussion Board

Fries Rearrangement and Sulphonation

Claisen - 5-2-2011 at 06:34

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?

Nicodem - 5-2-2011 at 08:15

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?

madscientist - 5-2-2011 at 11:00

I think he just overestimated our knowledge! :D

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]

Claisen - 6-2-2011 at 06:52

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!

DJF90 - 6-2-2011 at 08:24

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.

Nicodem - 8-2-2011 at 15:12

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.

Claisen - 11-2-2011 at 07:08

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.


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.

Nicodem - 19-2-2011 at 02:54

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.

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