Quantum yield of photohydrolysis

Halogenopyridines can be photohydrolized efficiently. The quantum yield of photohydrolysis is independent of the pH-value (5.2) 501 a>. 

Already, with one of the first nucleophilic aromatic photosubstitutions encountered, curious behaviour was found when studying the rate of reaction as a function of pH. m-Nitrophenyl sulphate shows no increase in the quantum yield of photohydrolysis with increase of hydroxide ion concentration up to values as high as 0-1 M. This behaviour, also found with one other compound (5-chloro-3-nitroanisole), is in clear contradistinction to what is... 

Quantum Yields of Photohydrolysis of some Halogenopyridines at Different pH Values 4>x, Disappearance of Halogenopyridine oH> Formation of Hydroxypyridine... 

It is seen from Table 1 that the series of halogenopyridines show quantum yields of photohydrolysis independent of pH in the neutral and alkaline region and decreasing only at pH-values where the pyridine nitrogen becomes protonated to a considerable percentage. 

The photohydrolysis of m-nitroanisole (mNA) has been thoroughly studied. The quantum yield of m-nitrophenol (mNP) formation reaches a limiting value of 0.23 at 0.07 M OH , and there is no wavelength dependency at pH 12 over the 254—334 nm range ). A linear plot of vs. [ OH]-i justifies the assump-... 

Table 3. Phosphorescence data and quantum yields ( ) of nitronaphthol formation in photohydrolysis of some methoxynitronaphthalenes...

Whichever this may be, the old anomalous photohydrolysis, where no increase of rate was found upon increase of hydroxide ion concentration, now seems consistently to fall into a wider category of reactions where the maximum quantum yield of conversion is reached merely by reaction with water. 

The photolysis of aqueous 3-chloroaniline was found to proceed via a clean photohydrolysis step to give 3-aminophenol with a quantum yield of

hydrolysis process or a fast reaction of a primarily formed aminophenyl cation with H20 (see Scheme 5). 

A number of other, but minor primary photoproducts was also found, among them the products expected from a radical (photo-Claisen) rearrangement and from photohydrolysis of the ortho chlorine 2- and 4-chlorophenol were detected too, but their formation remained unexplained. The photodegradation quantum yield of dichlorprop did not depend on pH and was 50 times smaller than that of the anionic form of the related monohalo-genated compound mecoprop (see above) [77]. This is another example of the marked influence of the pattern of ring halogen substitution on the course and on the efficiency of photodegradation. 

Chen, J., Peijnenburg, W.J.G.M., Quan, X., Zhao, Y, Xue, D. and Yang, F. (1998a) The application of quantum chemical and statistical technique in developing quantitative structure-property relationships for the photohydrolysis quantum yields of substituted aromatic halides. Chemosphere, 37, 1169-1186. 

Table I - Quantum yield of the photohydrolysis of chlorobenzene into phenol sensitized by acetone and hexadeuterated acetone, in degassed solution (18,19)...

Photohydrolysis of 3-chlorophenol, 3,5- and 3,4-dichlorophenol Our previous work (30) indicated that when 3-chlorophenol in anionic or molecular form was irradiated at 254 nm or 296 nm ( in aerated or degassed solution ( 5kI0"5—sxl0 3 mol.l l ) ), a specific conversion into resorcinol was observed. For a conversion rate of 5%, edsout 80% of the chlorophenol transformed were converted into resorcinol according to the HPLC analysis. Chlorine was quantitatively converted into Cl with an equal formation of H" ". The quantum yield of photoconversion of 3-chlorophenol was significantly higher with the anionic form ( 0.13 0.02 ) than with the molecular form ( 0.0910.01 ). The photochemical behavior of 3-chlorophenol is similar to the behavior of chlorobenzene. 

Only one difference appecurs with 3-chlorophenol the photohydrolysis could be sensitized by phenol (37), whereas this reaction did not occur with chlorobenzene. The quantum yield of the sensitized photohydrolysis depended on the concentration of the acceptor and increased from 0.24 to 0.45 when the concentration of 3-chlorophenol increased from 4.4x10 5 to 6kl0 mol.l. ... 

Earlier studies [11,12] have shown that photo excitation of aqueous 2-chloro-phenol or 2-bromophenol leads to contraction of the aromatic cycle to give cyclopentadienic acids (an example of a Wolff rearrangement [13]), and to substitution of the halogen by OH (photohydrolysis), with moderate quantum yields (< = 0.01-0.04). A carbene (2-oxocyclohexa-3,5-dienylidene) was suggested as a possible intermediate in the ring contraction pathway [11]. 

The quantum yields are the same, independent of whether the irradiation takes place with light absorbed in the first or the second ir- -it absorption band (exceptions were found later, e.g., in the case of the azulenes (Section 2), and in the photo-amination of nitrobenzene, (van Vliet et al., 1969). At long wavelength [X > 330 nm (for the photohydrolysis of m-nitroanisole)] a small but significant decrease in is observed, tentatively ascribed to absorption of part of the light by a hidden n - IT band. The obvious conclusion is that the reacting species is the aromatic molecule in its lowest excited... 

The photohydrolysis of 3,5-dinitroanisole (74) to 3,5-dinitrophenoxide ion (75) is catalyzed by hydroxide ion and studied in both aqueous and micellar systems (Scheme XXIX)74). The quantum yield for the reaction decreased in tetradecyltri-methylammonium chloride (TTAC1) micelles compared to aqueous solution. The results are explained by the decrease triplet lifetime of 3,5-dinitroanisole in the nonpolar micellar environment compared to the aqueous phase. 

Photonucleophilic substitution of fluoro- and chloro-anisoles has been the subject of three reports within the year. Cornelisse and co-workers have studied the photocyanation and photohydrolysis of 4-fluoro- and chloro-anisoles by laser spectroscopy and report that the initial step of the reaction involves formation of a triplet state transient complex composed of a ground state and an excited state aromatic molecule. Only in the presence of water does the complex yield radical ions and it is this process which determines the product quantum yield. The radical cation then reacts with the nucleophile to give a neutral radical which yields the substituted arene in a single step. Liu and Weiss report on anomalous effects during photonucleophilic aromatic substitution of 2- and 4-fluoroanisoles and also on the photo-... 

The photohydrolysis of monochlordbenzene in degassed solution can be sensitized by acetone and hexadeuterated acetone with similar quantum yields ( see table I ). 

The photocontraction of 2-halogenated phenolates could be extend ed to many derivatives studied (36,37). The quantum yield was evaluated to be 0.26 with 2-chloro-6-methylphenol in basic solution, and to be 0.10 with 2,4-dichloro-6-methylphenol ( pH= 8 to 13 ), without any influence of oxygen. The specificity decreased with increasing number of chlorine atoms on the ring with 2,5-dichlorophenolate, photocontraction competed with photohydrolysis. With 2,3- and 2,6-diohloro-phenolates, the accumulation of cyclqpentadienic acids was not quantitative. ... 

If the photohydrolysis does not compete with other routes of transformation, the quantum yield decreases with Increasing number of chlorine atoms on the ring. The same phenomenon was observed with chlorobenzenes. The quantum yield was generally higher with anionic forms than with molecular forms. This can be explained by the increase in the spin density on the ring which enhances the negative polarization of chlorine atom(s). ... 

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