Laser spectroscopic study of the nucleophilic photosubstitution of 4-chloroanisole and 4-fluoroanisole in aqueous solutions

3. HOMOGENEOUS SENSITIZED SYSTEMS170 3·1 General background 170 3·1·1 Singlet oxygen as oxidant 171 3·2 Sensitized oxidation of chlorophenols by singlet oxygen 172 3·3 Homogeneous reactions of chlorophenols with metal complexes 176 4. PHOTOINDUCED DEGRADATION OF CHLOROPHENOLS 180 4·1 Free radical oxidation of chlorophenols 180 4·2 Free radical processes in semiconductor-photocatalysed degradation of chlorophenols 182 4·2·1 TiO 2 -mediated degradation of chlorophenols 183 4·3 Sensitization of chlorophenol degradation by supported catalysts 188 4·4 Radiation induced degradation of 2,4,6-trichlorophenol 191 5. REACTIONS OF CHLOROPHENOLS WITH TRANSIENT INTERMEDIATES 193 6. CONCLUSIONS 197 7. ACKNOWLEDGEMENTS 198 8. REFERENCES 198 ABSTRACTThe photodegradation of mono-, di-, tri-and pentachlorophenols in aqueous solution is surveyed from several viewpoints, namely kinetic and mechanistic, the nature of the reactive intermediates and final products, and the potential of photochemical means of treating water contaminated by chlorophenols. In direct photolysis, the roles of heterolytic and homolytic processes are considered, and the appearance of carbene, in addition to ionic and radical, intermediates noted. Sensitized photolysis deals with the roles of singlet oxygen and of a variety of metal complexes. The induced degradation of chlorophenols refers to the oxidation of chlorophenols by free radicals generated from photo-and radiolytic systems, particularly illuminated semiconductors such as titanium dioxide. The article finishes with an overview of the reactions of various types of reactive intermediates with chlorophenols. 1. dIonization potentials of 2-chlorophenol (8·66 eV) and 4-chlorophenol (8·94 eV) in organic solvents have been determined from charge transfer spectra with TCNE [51] e Values obtained for 4-chlorophenol for OH bond dissociation energies by this method, photoacoustic calorimetry [52], pulse radiolysis [53] and theoretical calculations [54] vary over the range 84·4-90·3 kcal mol -1 f Calculated in [50] g From [55] h n-butanol solution [56]

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“…HCl elimination appears to require a polar, protic environment as it takes place neither in n-hexane nor in acetonitrile [78,81].…”

Section: ·2·2 4-chlorophenol (Pk a = 9·43)mentioning

confidence: 98%

Kinetics and Mechanism of Photodegradation of Chlorophenols

Burrows¹,

Ernestova²,

Kemp³

et al. 1998

prog. rct. kin. mech.

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“…= 420 nm for 4-chloroanisole and MeO-Py + is akin to the absorption spectrum of the previously reported cyclohexadienyl radical of the type shown below. 43 Moreover, the analogous 4-bromoanisole family of well-resolved transient spectra (T, donor yields the in Table 3) with A,,, = 410, 420 and 440 nm from the X-pyridinium acceptors with X = acetoxy, fluoro, and methoxy, respectively. Indeed, such a bathochromic shift of the absorption maxima with the variation in the X-substituent, accords with earlier observation of cyclohexadienyl radical^.^^.^^…”

Section: Fast Rate Processes Associated With the Charge-transfer Acti...mentioning

confidence: 99%

Time-resolved spectroscopy and charge-transfer photochemistry of aromatic EDA complexes with X-pyridinium cations

Bockman¹,

Lee²,

Kochi³

1992

J. Chem. Soc., Perkin Trans. 2

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Direct photoexcitation of 1 : 1 aromatic EDA complexes with various N-substituted X-pyridinium cations (X = nitro, fluoro, methoxy and acetoxy) is achieved b y the specific irradiation of their charge-transfer (CT) absorption bands. Time-resolved picosecond spectroscopy refers t o chargetransfer activation by the identification of the aromatic cation radical as the initial transient (T,) formed in a photoinduced electron-transfer together w i t h the X-pyridinyl radical. The homolytic fragmentation of the latter varies with the X-substituent in the order X = NO, > F > AcO > CH30, and the addition of X' to the aromatic donors leads to a series of cyclohexadienyl adducts that are identified as longer -I ived transients (T,) by ti meresolved ( na n osecond/m i crosecond) spectroscopy.The phototransients T, and T, together account for the different types of aromatic product (resulting from ring substitution, side-chain substitution and dimerization) that are generated b y steady-state CT photochemistry of the aromatic EDA complexes w i t h X-pyridinium cations.Electrophilic aromatic substitution can be efficiently carried out with various X-pyridinium salts such as those from X = NO, (nitration),' F (fluorination),2 N O (nitr~sation),~ etc. In the ArH + X-Py' --+ ArX + H-Py' case of N-nitropyridinium cations, we recently showed that electrophilic aromatic nitration proceeds via a multistep pathway involving the pre-equilibrium formation of an electron donor-acceptor (EDA) ~o m p l e x , ~, ~ Scheme 1. ArH + O,NPy+ &% [ArH, O,NPy+] (2) [ArH, O,NPy+] a ArNO, + HPy+ (3) Scheme 1

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“…Several mechanisms for nucleophilic substitution of aryl halides have been proposed, namely S N 2(Ar*), S R+N 1(Ar*), and electron transfer . The results of our study are most readily accommodated by the S N 2(Ar*) reaction, in which a Meisenheimer complex is formed between the excited substrate and nucleophile, followed by departure of the substituent from the complex.…”

Section: Discussionmentioning

confidence: 55%

“…Photocyanations in the absence of added electron acceptors can take place through the intermediacy of an excimer, , such that attack on the excimer by nucleophile permits charge to be removed from the excimer in the form of the anion radical. To investigate whether an HCB excimer was involved in product formation, we measured φ diss as a function of HCB concentration.…”

Section: Resultsmentioning

confidence: 99%

“…As a nucleophile that is not easily oxidized, CN - encourages substitution by the S N 2Ar * mechanism, whereas oxidizable nucleophiles promote a formal nucleophilic substitution by an electron-transfer mechanism. , Cyanide ion is also notable for its ability to replace hydrogen in aromatic compounds, under conditions that favor one-electron oxidation of the photoexcited aryl substrate , prior to attack by CN - . Few examples have been disclosed in which cyanide replaces a halogen substituent, , and these examples are presently limited to monohalogenated aromatic compounds. In this paper we report a novel photosubstitution between hexachlorobenzene (HCB) and CN - in aqueous acetonitrile; successive photochemical replacement occurs with quantum yield > 0.1, affording highly cyanated phenolic products.…”

Section: Introductionmentioning

confidence: 99%

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Successive Photosubstitution of Hexachlorobenzene with Cyanide Ion

et al. 1998

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We report a novel nucleophilic polysubstitution reaction of hexachlorobenzene (HCB) with cyanide ion in acetonitrile/water. Successive photocyanations of HCB occur with high quantum yield (φ diss → 0.18) without the need for an electron acceptor, to give as products pentacyanophenol, 4-chloro-2,3,5,6-tetracyanophenol, and a dichlorotricyanophenol. The phenol functional group is introduced by competing hydrolysis of the polycyanochlorinated benzenes. Sensitization and quenching experiments indicate a triplet reactive excited state. Variation of [CN-] at constant [HCB] follows the expected relationship φ diss −1 ∝ [CN-]-1, but variation of [HCB] at constant [CN-] shows that the reaction becomes less efficient with increasing [HCB], consistent with the formation of an unproductive excimer.

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Laser spectroscopic study of the nucleophilic photosubstitution of 4-chloroanisole and 4-fluoroanisole in aqueous solutions

Cited by 21 publications

References 13 publications

Kinetics and Mechanism of Photodegradation of Chlorophenols

Kinetics and Mechanism of Photodegradation of Chlorophenols

Time-resolved spectroscopy and charge-transfer photochemistry of aromatic EDA complexes with X-pyridinium cations

Successive Photosubstitution of Hexachlorobenzene with Cyanide Ion

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