O-Hydroxybenzophenones

The ultraviolet absorber could dispose of absorbed energy by radiation, by conversion into heat or by chemical changes leading to stable products. The most important commercial absorbers, such as the o-hydroxybenzophenones, o-hydroxyphenylbenzotriazoles and salicylates, appear to function by conversion of electronic energy into heat. The properties of the main types of ultraviolet absorbers are summarised in Table 7.7. 

The consecutive formation of o-hydroxybenzophenone (Figure 3) occurred by Fries transposition over phenylbenzoate. In the Fries reaction catalyzed by Lewis-type systems, aimed at the synthesis of hydroxyarylketones starting from aryl esters, the mechanism can be either (i) intermolecular, in which the benzoyl cation acylates phenylbenzoate with formation of benzoylphenylbenzoate, while the Ph-O-AfCL complex generates phenol (in this case, hydroxybenzophenone is a consecutive product of phenylbenzoate transformation), or (ii) intramolecular, in which phenylbenzoate directly transforms into hydroxybenzophenone, or (iii) again intermolecular, in which however the benzoyl cation acylates the Ph-O-AfCL complex, with formation of another complex which then decomposes to yield hydroxybenzophenone (mechanism of monomolecular deacylation-acylation). Mechanisms (i) and (iii) lead preferentially to the formation of p-hydroxybenzophenone (especially at low temperature), while mechanism (ii) to the ortho isomer. In the case of the Bronsted-type catalysis with zeolites, shape-selectivity effects may favor the formation of the para isomer with respect to the ortho one (11,12). 

In our case, all the compounds obtained by transformation of the intermediate, phenylbenzoate, were primary products. This indicates that the following parallel reactions occurred on phenylbenzoate (1) the intramolecular Fries transposition generated o-hydroxybenzophenone, (ii) phenylbenzoate acted as a benzoylating agent on phenol, to yield p-hydroxybenzophenone (with also possible formation of the ortho isomer) and phenol and (iii) phenylbenzoate acylated a second molecule of phenylbenzoate to generate benzoylphenylbenzoates, with the co-production of phenol. 

It is well known that o-hydroxyphenyl ketones phosphoresce very weakly and do not undergo photoreduction363,368 or photoelimination,366 presumably because of rapid enolization in the excited state. Lamola has performed quenching studies which indicate lifetimes of o-hydroxybenzophenone triplets of less than 10"8 sec. However, these lifetimes, as well as phosphorescence efficiencies, are markedly enhanced by the addition of alcoholic solvents,383 presumably because hydrogen bonding slows down photoenolization. 

Among the types of ultraviolet absorbers that may be used as stabilizers are salicylates, o-hydroxybenzophenones, o-hydroxyarylbenzotri-azoles, and certain acrylonitriles. The stabilization of polystyrene by other additives has also been reported. Matheson and Boyer (10) found that certain aliphatic amines and amino alcohols improved the light stability of the polymer. 

Thermal dehydration of o- (acylamino)phenols is the method of choice for the preparation of benzoxazoles (equation 96) and other annulated oxazoles. 0,iV-Diacyl derivatives of o-aminophenols cyclize at lower temperatures than do the monoacyl compounds. The synthesis is often carried out by heating the aminophenol with the carboxylic acid or a derivative, such as the acid chloride, anhydride, an ester, amide or nitrile. The Beckmann rearrangement of oximes of o-hydroxybenzophenones leads directly to benzoxazoles (equation 97). 

Some polymers show discoloration as well as reduction of the mechanical properties (e.g. aromatic polyesters, aromatic polyamides, polycarbonate, polyurethanes, poly (phenylene oxide, polysulphone), others show only a deterioration of the mechanical properties (polypropylene, cotton) or mainly yellowing (wool, poly(vinyl chloride)). This degradation may be less pronounced when an ultraviolet absorber is incorporated into the polymer. The role of the UV-absorbers (usually o-hydroxybenzophenones or o-hydroxyphenylbenzotriazoles) is to absorb the radiation in the 300-400 nm region and dissipate the energy in a manner harmless to the material to be protected. UV-protection of polymers can be well achieved by the use of additives (e.g. nickel chelates) that, by a transfer of excitation energy, are capable of quenching electronically excited states of impurities (e.g. carbonyl groups) present in the polymer (e.g. polypropylene). 

Acrylic polymers are fairly insensitive to normal uv degradation since the primary uv absorption of acrylics occurs below the solar spectrum (59). The incorporation of absorbers, such as o-hydroxybenzophenone [117-99-7] further improves the uv stability (59). Under normal use conditions acrylic polymers have superior resistance to degradation and show remarkable retention of their original properties. 

Arenes of the type ArX-Y, where X may be O, S, or NR, frequently undergo facile photoinduced homolytic cleavage of the X-Y bond with the Y radical subsequently attacking the aromatic ring. The photo-Fries reaction is the most common process of this type and has been reported within the year for aromatic esters which form part of a ptolymer chain or are pendant groups on a polymer chain. In the former case, the rearrangement of fluorene-based polyacrylates [for example, (292)] was studied. Formation of the o-hydroxybenzophenone moiety in the product (293) was monitored by u.v. and... 

Kulka " developed a synthesis of o-hydroxybenzophenones in which a t-butyl group is used to block the para position of a phenol methyl ether to permit o-benzoylation and is then removed, along with the ether group, with aluminum chloride. 

In contrast, the o-hydroxybenzophenone had no noticeable effect on the decomposition chonistry. Though it has been conmonly referred to as an ultraviolet absorber, it actually behaves more like a deactivator since the screen film containing it had a lower oxidation rate in Fig. 11 than the underlying clear film devoid of additives. Absorption can hardly account for the protective action of the stabilizer in thin films which are almost completely transparent to 300nm radiation. Neither can the hydroxybenzo-phenone be an important terminator of radical chain reactions because it is much less effective than other hindered enols in inhibiting oxidation in the dark. 

Figure 11. Photooxidation rate of branched polyethylene films containing 0.1% Neotex ISO carbon black (9) and 0.1% substituted o-hydroxybenzophenone (H). Also shown are the rates of unprotected films exposed to direct radiation (A), to radiation transmitted by the black screen (O), and by the film cotaaining the...

The reaction rate was shown to be related to the accessibility of the acid sites-H-MFI zeolite was practically inactive and Nafion was the most active catalyst. With all the catalysts, not only the desired p-hydroxybenzophenone (/ -HBP) product but also j3-benzoxybenzophenone (p-BXBP) and o-hydroxybenzophenone (o-HBP) were formed. The para products result from benzoylation of phenol (formed by hydrolysis of PB) or of PB and o-HBP from the intramolecular rearrangement of PB. The selectivity to p-HBP can, therefore, be significantly improved by adding phenol to the reactant mixture [4]. Unfortunately, thermodynamic limitations arising from the low polarity of the medium employed (even with very polar solvents such as sulfolane) seems to preclude the use of a solid acid for the production of diphenol monomers from diphenyl benzoate [4]. 

Arylation of salicylaldehydes. o-Hydroxybenzophenones are obtained when salicylaldehydes and diaryliodonium salts are brought together with PdCl2-LiCl. This same reaction had been realized before using Arl instead of the iodonium salts. 

MCl2(OMe)2L] HL = salicylaldehyde, acetoacetanilide, benzoylacetanilide, 2-hydroxyacetophenone, O-hydroxybenzophenone, benzoylacetophenone 15... 

Here, a is the slope of a plot of the wave amplitudes versus the optical densities of the solutions at different concentrations of di-tert-butyl peroxide the calibration constant a is the slope of a similar plot involving solutions containing variable amounts of o-hydroxybenzophenone in place of di-tert-butyl peroxide. That substance converts all of the absorved radiation into heat in a few nanoseconds (3). 

Ultraviolet absorbers absorb primarily in the UV range while avoiding absorption in the visible hght range so that the appearance of the plastic material s colour is not changed. Once absorbed, UV radiation may be converted to heat or to new chemical products. The most important commercial absorbers include o-hydroxybenzophenones, o- hydroxyphenylbenzotriazoles and sahcylates. These convert absorbed radiation to heat. 

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