Para-hydroxyacetophenone

The mechanistic origin of the PFR products was a matter of discussion from the beginning. The archetypal rearrangement is that of phenyl acetate (10), which has been widely employed as a model to understand the photoprocess. Its irradiation in solution affords ort/io-hydroxyacetophenone (11), para-hydroxyacetophenone (12), and phenol (13) (Scheme 3). Two alternative mechanistic pathways were... 

Problem 19.16 Phenyl acetate undergoes the Fries rearrangement with AlCl, to form ortho- and para-hydroxyacetophenone. The ortho isomer is separated from the mixture by its volatility with steam. 

If we examine the formation of hydroxyacetophenones more closely, we can see (Fig. 4a) that on HY, at least part of ortho-hydroxyacetophenone could be formed directly from phenylacetate (i.e, intramolecularly) whereas the para-isomer is clearly a secondary product. On HZSM5 both compounds are secondary products (Fig. 4b). Moreover, one can see that the ortho-/para-hydroxyace-tophenone molar ratio changes with conversion especially over HZSM5 where it decreases from 6 to 1 as conversion decreases. Two explanations can be considered i) a consecutive transformation of para-hydroxyacetophenone which would not occur in the case of the ortho-isomer ii) a change in the ortho-/para-selectivity of the zeolite in the course of deactivation. The points at high conversion being obtained on the fresh catalyst, a preferential deactivation of the sites located outside of the particles will decrease the ortho-/para-hydroxyacetophenone molar ratio if one supposes that these sites which are easily accessible favour the formation of the ortho-... 

On HY, phenylacetate dissociates into phenol and ketene (reaction a). Ortho-hydroxyacetophenone is produced partly by the Fries rearrangement of phenylacetate (intramolecular reaction, reaction b) and by trans-acylation (reaction c) while para-hydroxyacetophenone is exclusively the result of trans-acylation (reaction d). Phenylacetate can also disproportionate into phenol and acetoxyacetophenones (reaction e). Para-acetoxyacetophenone can also be formed through transesterification between para-hydroxyacetophenone and phenylacetate (reaction f).The formation of secondary products like 2-methylchromone and 4-methylcoumarine is consecutive to the formation of... 

The mechanism of phenylacetate transformation can be described as shown in scheme 3. Phenylacetate adsorbs on a protonic center and then can either rearrange into ortho-hydroxyacetophenone (Fries rearrangement) or desorb phenol. The adsorbed acylium ion can then react with phenol (if the conversion is high enough) to give ortho- and para-hydroxyacetophenone which will then appear as secondary products. It can also react with phenylacetate to give ortho- and para-acetoxyacetophenones or react with ketene to give the condensation products. 

The Fries rearrangement of PA over H-BEA zeolites, which is a simple reaction, was chosen to introduce the competition for adsorption on the zeolite catalysts and its role on the reaction rate. Ortho- and para-hydroxyacetophenones (o- and p-HAP), para-acetoxyacetophenone (p-AXAP) and phenol (P) are the main products o-HAP, P and p-AXAP, which are directly formed (primary products),... 

Gas-phase acetylation of phenol using /3-zeolites gives phenyl acetate rapidly, which rearranges (see Fries rearrangement, vide infra) to orffto-hydroxyacetophenone and para-hydroxyacetophenone. The ojp ratio is high under these conditions. ... 

FIGURE 41.1 Stabilized derivatives of prostaglandin E2 ethylene ketal and crystalline ester with para-hydroxyacetophenone. - ... 

A continuous gas-phase catalytic acetylation of phenol with acetic anhydride to phenyl acetate followed by simultaneous Fries rearrangement to yield ortho- and para-hydroxyacetophenones over a silica-supported heteropoly acid has been described. A complete conversion of phenol to phenyl acetate is achieved at 140°C. Upon increasing the temperature to 200°C, the phenyl acetate formed rearranges into hydroxyacetophenones with 10% yield and 90% selectivity to the para isomer. 

Scheme 8.83. Representations of the pathway of the Fries rearrangement by which the acetyl ester of phenol is converted to the ortho- and para-hydroxyacetophenones in the presence of aluminum chloride (AICI3). It is important to note that the reactions are reversible (save for the final hydrolysis) so that both acetophenones can be reconverted to starting ester. It is argued that the orf/zo-hydroxyacetophenone is stabilized by the aluminum chloride complex between the two oxygen atoms.

Consider the structure of para-hydroxyacetophenone, which has a pKg value in the same range as a carboxylic acid, despite the fact that it lacks a COOH moiety. Offer an explanation for the acidity of para-hydroxyacetophenone. 

Based on your answer to the previous question, would you expect meta-hydroxyacetophenone to be more or less acidic than para-hydroxyacetophenone Explain your answer. 

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