Although heterolytic cleavage of the ozonide is by far the most important process, there seem to be cases, where homolytic cleavage can occur. Strong experimental evidence for this has been obtained from the ozonolysis of dichloromaleic acid at pH 3 (largely the monoanion). The chloride yield is 3.9 mol per mol ozone and reduces to 1.7 in the presence of tertiary butanol (see paragraph 6.6.6) pointing to the formation of †Cl, which subsequently induces a short chain reaction. Quantum-chemical calculations (Naumov & von Sonntag, 2009, unpublished) indicate that, for example, reaction (26)/(27) is slightly exergonic (ΔG°=–2 kJ mol−1) as are subsequent decarboxylation plus O†−2 elimination [reaction (28), ΔG°=–108 kJ mol−1] and the hydrolysis of the ketene [reaction (29), ΔG°=–49 kJ mol−1]. There are other potential exergonic routes to the chain carrier†Cl, but at this stage it is premature to discuss other alternatives, as detailed product studies are missing.
C
6.5 DETECTION OFα-HYDROXYALKYLHYDROPEROXIDES
α-Hydroxyalkylhydroperoxides are often quite stable, as equilibrium (8) is usually only slowly attained at pH 7 (Bothe & Schulte-Frohlinde, 1980).α-Hydroxyalkylhydroperoxides react, similarly to H2O2, with molybdate-activated iodide by forming I3−. The kinetics of such reactions are markedly different and can be used to characterise and quantify such intermediates (Dowideit & von Sonntag, 1998; Flyuntet al., 2003b; Stemmler & von Gunten, 2000). As theα-hydroxyalkylhydroperoxides react much more slowly than H2O2(for a compilation of rate constants see Table 6.2), this approach can even be used to quantify H2O2andα-hydroxyalkylhydroperoxides when formed side by side. The very reactive hydroperoxides, formic and acetic peracids, react even without molybdate catalysis. Formic peracid is reduced by S(CH2CH2OH)2to formic acid [reaction (30),k=220 M−1s−1(Flyuntet al., 2003a)], and this reaction
can be followed by changes in the conductance, as formic peracid (pKa=7.1) is barely dissociated at pH 6 while formic acid (pKa=3.75) is fully dissociated under such conditions.
S(CH2CH2OH)2+HC(O)OOHO=S(CH2CH2OH)2+H++HC(O)O− (30)
6.6 OZONE REACTIONS OF OLEFINS–PRODUCTS AND REACTIONS OF REACTIVE INTERMEDIATES
In this paragraph, the ozonolysis products of a large number of olefins will be presented. This information will allow predictions on the ozonolyses of olefins for which no data are available. The high regioselectivity of ozone reactions with unequally substituted olefins assists these predictions.
6.6.1 Methyl- and halogen-substituted olefins
Final products and some of the longer-lived intermediates of some methyl- and chlorine-substituted olefins are compiled in Table 6.3.
The reactions of propene have been discussed above [reactions (21) and (23)]. Ethene is symmetrical, and from the above one expects equal amounts of formaldehyde and hydroxymethylhydroperoxide. This is indeed observed (Table 6.3). A more convenient source to produce hydroxymethylhydroperoxide in ozone reactions is buten-3-ol [reaction (31)].
C C C CH3
OH H
H H
O3
C H
H OH
O OH + H C O
C CH3 H H H
(31)
Table 6.2 Compilation of half-lives of the reactions of molybdate-activated iodide with some hydroperoxides
Hydroperoxide t½/s Reference
HC(O)OOHa 0.0032 Dowideit & von Sonntag, 1998
CH3C(O)OOHb 0.022 Flyuntet al., 2003b
CH3C(O)C(O)NHC(O)N=C(OOH)C(O)Hb 0.12, 0.16c Flyuntet al., 2002 5-Hydroperoxy-5-methylhydantoinb 0.71, 0.9c Flyuntet al., 2002 H2O2d
2.5 Dowideit & von Sonntag, 1998
H2O2b 1.8×10−3 Flyuntet al., 2003a
HOCH(CH3)OOHd 13.7 Dowideit & von Sonntag, 1998
HOCH2OOHd 203 Dowideit & von Sonntag, 1998
HOOCH2C(CH3)2OH ∼260c Schuchmann & von Sonntag, 1979
CH3OOH 790 Flyuntet al., 2003b
HOOCH2CH(CH3)OH 420c Flyuntet al., 2003b
HOC(CH3)2OOHd ∼35 000 Dowideit & von Sonntag, 1998
aHydroperoxide reacts equally rapidly without molybdate catalysis.bReaction without molybdate catalysis.
cReagent A [0.4 M KI, 3.6×10−2M KOH, 1.6×10−4M (NH4)6Mo7O24], reagent B (0.1 M potassium hydrogen phthalate) and probe at 1 : 1 : 2.dMolybdate catalysis, [I−]=0.17 M.
Its good solubility in water and its high rate constant (Table 6.1) makes buten-3-ol a very convenient competitor for the determination of ozone rate constants (Chapter 2).
With halogenated olefins, reactive halogen-containing intermediates are formed which show an interesting chemistry. With vinyl chloride the main reaction path leads to hydroxymethylhydroperoxide and formyl chloride [reaction (15)]. Formyl chloride rapidly decomposes into CO and HCl [reaction (19), k=600 s−1; 94%] (Dowideit et al., 1996). Its hydrolysis into formic acid plus HCl [reaction (18), 6%] is slow in comparison but gains in importance at high pH [reaction (17),k(OH−)=2.5×104 M−1 s−1]. A minor route leads to chlorohydroxymethylhydroperoxide [reaction (16)]. The geminal chlorohydrine substructure in chlorohydroxymethylhydroperoxide is very unstable and rapidly loses HCl [on a low microsecond timescale (Köster & Asmus, 1971; Mertenset al., 1994)] giving rise to formic peracid [reaction (20)]. Formic peracid is generated in the reaction of 1,2-dichloroethene (see below), and this is a convenient method for its formation [for its reactions see (Flyuntet al., 2003a)].
Formyl bromide, the product of the analogous reaction of vinyl bromide with ozone also decomposes preferentially into CO plus HBr. With 3.6%, the formic acid yield is lower than the corresponding yield from formyl chloride (6%). Whether this is due to a faster decomposition or a slower hydrolysis is not yet known.
1,2-Dichloroethene is symmetrical, and the primary products are chlorohydroxymethylhydroperoxide and formyl chloride [reactions (32) and (33)]. HCl, CO and formic peracid are the main final products (see above).
Table 6.3 Products and their yields [with respect to ozone consumption (mol/mol)] in the ozonolysis of ethene and some of its methyl- and chlorine-substituted derivatives in aqueous solution according to Dowideit & von Sonntag (1998). Empty fields indicate that a given product is not expected to be formed and has not been looked for
Product Ethene Propene Me4 -Ethene
Cl-Ethene
1,1-Cl2 -Ethene
1,2-Cl2 -Ethene
Cl3 -Ethene
1,1-Cl2 -Propene
HCl 1.05 1.95 2.02 2.87 2.05
HC(O)OH 0.06 0.03 1.01 0.82
HC(O)OOH 0.02(a) 0.98(a) 0.88(a)
HOCH2COOH 0.07(c)
CO 1.01 1.08 0.04
CO2 0.90 0.02 0.95 1.01
CH2O 2.04 1.03 0.96
CH3C(O)H 0.97 1.03
(CH3)2CO 1.74
HOCH2OOH 1.08(b) 1.06(b) 0.96(b)
CH3CH(OH)OOH 0.99(b) 0.98(b)
(CH3)2C(OH)OOH (d)
Cl2CHC(O)H ,0.01(c)
ClCH2C(O)OH 0.08(c)
Cl2CHC(O)OH 0.04(c)
[(CH3)2C(OH)]2 ∼0.1
(a)Precursor of formic acid.(b)Precursor of aldehydes.(c)Product of partial oxidation reaction.(d)The reaction with molybdate-activated iodide is too slow for its determination.
C C
The reactions of 1,2-dibromoethene with ozone are analogous (Leitzkeet al., 2003).
6.6.2 Acrylonitrile, vinyl acetate, diethyl vinylphosphonate, vinyl phenyl sulfonate, vinylsulfonic acid and vinylene carbonate
Cyano, acetyl and diethyl phosphonate groups are electron-withdrawing substituents, and the ozonolysis of acrylonitrile, vinyl acetate and diethyl vinylphosphonate follow the same regioselectivity as observed for the ozone reaction with vinyl chloride discussed above, that is, the primary products are hydroxymethylhydroperoxide and the corresponding formyl derivatives, formyl cyanide, formyl acetate and diethyl formylphosphonate, respectively (Leitzkeet al., 2003). These mixed acid anhydrides are not stable in water. Stopped-flow with conductometric detection allowed the determination of the rate constant for their hydrolysis: formyl cyanide [k=3 s−1; k(OH−)=3.8×105M−1s−1]; formyl acetate (k=0.25 s−1); diethyl formylphosphonate [k=7×10−3s−1; k(OH−)=3.2×104M−1s−1] (Leitzke et al., 2003).
The ozonolysis of vinylphosphonic acid is somewhat more complex as the ozonide decays in two directions in a 1:3 ratio [reactions (35) and (36)] (Leitzkeet al., 2003). In the presence of excess H2O2, formic and phosphoric acids are the final products [reactions (37)–(39)]. In the presence of catalase, which destroys H2O2 but not α-hydroxyalkylhydroperoxides, the rate of formic acid formation is markedly slower and phosphonic acid is observed instead of phosphoric acid. Apparently, formylphosphonate hydrolyses only slowly [k(OH−)≈5 M−1s−1]. The slowness of this reaction when compared to the formyl compounds discussed above may be due to the fact that here OH−has to react with a doubly negatively charged species. At pH 10.2 and in the presence of borate buffer, H2O2reacts with formylphosphonate with an observed rate constant of kobs=260 M−1s−1 (phosphonate is not oxidised by H2O2 at any pH). There seems to be also a pH dependence between reactions (–37) and (38). While at pH 7 H2O2 elimination is observed, decomposition is favoured at pH 10.2, where the hydroxyperoxy(hydroxy)methyl phosphonic acid is fully deprotonated.
C
Ozonolysis of vinyl phenylsulfonate is complex as some of the products, notably SO2and phenol, react rapidly with ozone (Leitzkeet al., 2003). Hydroxymethylhydroperoxide, formic acid and sulfuric acid are products, but the material balance is incomplete and reactions (41)–(45) must remain a tentative suggestion.
Similar uncertainties prevail in the ozonolysis of vinylsulfonic acid (Leitzkeet al., 2003).
Products
The ozonolysis of vinylene carbonate leads to the formation of formic acid, formic peracid, H2O2and CO2(Leitzkeet al., 2003). Potential decay reactions of this symmetrical ozonide are shown in reactions (46)–(52). The reaction that leads to H2O2[reaction (50)] must be minor (4%).
6.6.3 Acrylic, methacrylic, fumaric, maleic and muconic acids
For acrylic and methacrylic acids, the yields of the product based on ozone consumption are compiled in Table 6.4.
Mechanistically, acrylic and methacrylic acids follow the same pattern [reactions (53)–(59) and reactions
The second zwitterions are given in brackets as they may not be intermediates, but they indicate the directions the ozonides will take upon their decay. The absence of formic and acetic acids in the case of acrylic and methacrylic acids, respectively, shows that the other conceivable route, for example, reactions (60)–(62) is not taken in these acids.
C
Table 6.4 Product yields (mol per mol O3consumed) in the reaction of O3with acrylic and methacrylic acids according to Leitzke & von Sonntag (2009)
Product Acrylic acid
Anion (free acid)
Methacrylic acid Anion (free acid)
Formaldehyde(a) 0.52 (0.72) 0.43 (0.62)
Hydroxymethylhydroperoxide 0.43 (ND) see text (ND)
1-Hydroperoxypropan-2-one – 0.57 (ND)
Glyoxylic acid 0.54 (ND) –
2-Oxopropanoic acid – 0.41 (ND)
Gylcol aldehyde 0.48 (ND) –
Methylglyoxal – 0.54 (ND)
Formic acid absent (absent) –
Acetic acid – absent (absent)
Total Peroxide 1.01 (1.0) 1.04 (1.0)
H2O2 0.58 (ND) 0.46 (ND)
(a)Includes the yield of hydroxymethylhydroperoxide that decomposes into formaldehyde during the assay, ND=not determined,–product cannot be formed.
Moreover, cleavage along the O–O and C–C bonds of the envisaged dioxetane intermediate [cf. reaction (57)] does not take place.
Depending on the protonation state of the carboxylic group, there is some bias in the branching between reactions (55) and (56). As expected, the deprotonated carboxyl group decarboxylates more readily favouring reaction (56). Formaldehyde, which is produced from reaction (55), can be measured with some accuracy. A plot of the formaldehyde yield as a function of pH is shown in Figure 6.3.
The inflection point is at pH 3, markedly distant from the pKavalue of acrylic acid, 4.25 (dashed line). It may reflect the pKavalue of the ozonide, as the zwitterion is most likely not an intermediate, but if it were it would be too short-lived for the pKaequilibrium to become established during its lifetime.
The reactions of methacrylic acid ozonide are depicted in reactions (63)–(67).
base catalysed
Figure 6.3 The pH dependence of the formaldehyde yield in the ozonolysis of acrylic acid. The dashed line indicates how this dependence would look if it were governed by the pKavalue of acrylic acid. From Leitzke &
von Sonntag, 2009 with permission.
Interestingly, hydroxymethylhydroperoxide, formed in reaction (64), cannot be detected by its slow reaction with molybdate-activated iodide. It is too short-lived here, as the buffering properties of methacrylic acid that decompose hydroxymethylhydroperoxide into formaldehyde and H2O2 [reaction (60)] are much more pronounced than in the case of acrylic acid. This has been shown by production of hydroxymethylhydroperoxide in the reaction of buten-3-ol with ozone [reaction (31)] and subsequent addition of methacrylic acid. The latter rapidly catalyses reaction (66) and hydroxymethylhydroperoxide is no longer detected [note that the somewhat lower value of hydroxymethylhydroperoxide as compared to that of formaldehyde in the case of acrylic acid (Table 6.4) may be due to the, albeit much lower, buffering effects of acrylic acid. Due to its higher pKavalue, methacrylic acid is a better buffer, and thus a more efficient catalyst, than acrylic acid in close to neutral solutions].
The hydroperoxide attributed to 1-hydroperoxypropan-2-one formed in reaction (65) reacts much faster with molybdate-activated iodide (k=0.15 M−1s−1) than hydroxymethylhydroperoxide (k=3.4×10−3 M−1s−1). Interestingly, the methyl substituent in this hydroperoxide has a marked effect on its decay.
Compared to aldehydes, ketones form hydrates (and thus also hydroxyalkylhydroperoxides) less efficiently [in water, formaldehyde is practically fully hydrated, CH2(OH)2, acetaldehyde ∼50%
hydrated, acetone,0.1% hydrated (Bell, 1966)]. Thus, the reaction analogous to reaction (57) will be inefficient, and water elimination occurs instead [reaction (67)]. The buffer properties of methacrylic acid may speed up this reaction. There are other α-hydroxyalkylhydroperoxides, where water elimination rather than H2O2 elimination is observed. A case in point is the muconic compound formed in the ozonolysis of phenol (Mvula & von Sonntag, 2003). Reasons for this are as yet not known, but one may recall that H2O2elimination is reversible while water elimination is not. A more detailed study of H2O2
with aldehydes would be required to elucidate this interesting aspect.
The reactions of O3with maleic and fumaric acids in aqueous solution has found some attention (Gilbert, 1977; Leitzke & von Sonntag, 2009; Ramseier & von Gunten, 2009), but mechanistic details were only understood recently (Leitzke & von Sonntag, 2009; Ramseier & von Gunten, 2009). These two isomeric acids are available in high purity. In aqueous solution, cis⇄trans isomerisation takes place. This may lead to an artefact, that is, the other isomer increases linearly with the ozone dose, when the work-up of the samples follows the sequence of ozone doses (Leitzke, 2003). It is now clear, that ozone does not inducecis⇄trans isomerisation. These two isomers give rise to the same products, but their yields differ remarkably (Table 6.5).
With fumaric acid, glyoxal is formed in 27% yield, while with maleic acid it is only 3%. This is a strong indication that these two isomers give rise to two ozonides that differ with respect to their preferred decay routes. This is in line with the concept that ozone addition to the double bond is a concerted reaction
Table 6.5 Product yields (mol per mol O3 consumed) in the reaction of O3with fumaric and maleic acids at pH 5.3 according to Leitzke & von Sonntag (2009)
Product Fumaric acid Maleic acid
Formic acid 0.77 0.92
Glyoxylic acid 0.85 1.05
Glyoxal 0.27 0.03
Total peroxide 0.31 ND
ND=not determined.
[reactions (68) and (69)]. In any case, a zwitterionic intermediate, if it is an intermediate, must be too short-lived to allow a rotation around the former C–C double bond.
Maleic acid
We are not yet in a position to present a mechanistic explanation for this difference, but the general mechanism suggested by reactions (70)–(74) accounts for the observed products.
(70)
The ozonolysis oftrans,trans-muconic acid in aqueous solution has been studied under the conditions of extensive ozonation (Gilbert, 1980). No distinction between primary and secondary products has been made. The primary products and their yields in the ozonolysis of cis,cis-muconic acid (Leitzke et al., 2003) are compiled in Table 6.6, and additional information on primary as well as secondary products is also available (Ramseier & von Gunten, 2009) [reactions (75)−(79)].
There is no organic hydroperoxide detected, and the total peroxide yield is due to H2O2. The other product is glyoxylic acid. This aldehyde reacts readily with H2O2, resulting in the subsequent formation of formic acid [reactions (80) and (81)]. For suppressing this secondary reaction by destroying H2O2, catalase has been added immediately after ozonolysis. Under such conditions, glyoxylic acid matches the H2O2yield, and formic acid is no longer formed. 4-Oxobut-3-enoic acid has been detected as a major product by GC-MS but was not quantified. Reactions (75)–(79) account for the observed products.
(75) (76)
Besides the conversion of glyoxylic acid by hydrogen peroxide into CO2and formic acid [reactions (80) and (81)] (for the kinetics of these reactions see (Leitzkeet al., 2001), 4-oxobut-3-enoic acid is likely to undergo the analogous reactions (78) and (79). In the absence of catalase, the ensuing products, formic acid and maleic acid, are indeed formed (Ramseier & von Gunten, 2009).
(80) OH (81)
There is the interesting aspect that upon ozonolysis of cis,cis-muconic acid, 4% singlet oxygen is formed (Muñozet al., 2001). This points to an additional pathway for the reaction of muconic acid with ozone. As the formation of the ozonide is exergonic by∼43 kJ mol−1(Naumov & von Sonntag, 2009, unpublished) and seems not to decarboxylate readily, an additional pathway in competition to the ozonide formation seems to be of (minor) importance. Approach of ozone to muconic acid may develop a zwitterionic intermediate with the positive charge delocalised over the two conjugated C–C double
Table 6.6 Product yields (mol per mol O3consumed) in the reaction of O3 withcis,cis-muconic acid according to Leitzkeet al.(2003)
Product pH 2 pH 8
H2O2 0.98 0.9
Glyoxylic acid 0.98(a) 0.99(a)
4-Oxobut-2-enoic acid not quantified(b) not quantified(b)
Formic acid absent absent
(a)Catalase had been added immediately after ozonolysis;(b)detected by GC-MS.
bonds. This is not a stable structure, possibly not even a transition state, but decarboxylation may already occur [reaction (82)] as it tends to develop. Singlet oxygen is subsequently released [reaction (83)].
(82) (83) (84)
The standard Gibbs free energy including CO2 elimination [reaction (82)] has been calculated at
−77 kJ mol−1. At the time when these measurements were made, the implications were not realised, and whether small amounts of 5-oxo-pent-2-enoic acid (or its decarboxylation products) are indeed formed has not been looked at. It seems worth mentioning that with aromatic compounds, where the positive charge of the zwitterionic ozone adduct can be distributed over one more double bond, such adducts are well-defined intermediates (Naumov & von Sonntag, 2010) (Chapter 7).
6.6.5 Cinnamic acids
Cinnamic acids contain two potential sites for ozone attack, the olefinic C–C double bond and the aromatic ring. The rate of reaction with the latter is much slower, and thus the site of ozone attack on cinnamic acids is the exocylic C–C double bond (Leitzkeet al., 2001). Electron-donating/electron-withdrawing substituents at the aromatic ring have a much smaller effect on the rate of reaction than one would expect if the site of attack were the aromatic ring (cf. Chapter 7). Although there is only a small effect of methoxy and nitro substituents on the rate of reaction (Table 6.1), there is a marked effect on the decay route of the ozonides [reactions (85)–(88)].
(85)
The ozonides of cinnamic acid and its 4-methoxy derivative decay only according to reaction (85), whereas the electron-withdrawing nitro substituent makes it possible that reaction (86) proceeds to 30%.
The hydroxyhydroperoxide only eliminates H2O2[reaction (87)], and the potentially competing water elimination that would lead to benzoic acid is not observed (,2%).
6.6.6 Dichloromaleic acid
With dichloromaleic acid only some preliminary experiments have been carried out (Leitzke & von Sonntag, 2009). These few data are still sufficiently interesting to be reported as they may serve as a basis for future investigations. Ozonation has been carried out at pH 3.1. Under such conditions, the Cl−yield was 3.9 with respect to ozone consumed, much higher than the maximum value of 2.0 for a non-chain reaction, and an acid is formed that has the same retention time as ketomalonic acid (not necessarily a correct assignment). When tertiary butanol (0.26 M), which scavenges reactive free radicals such as
†Cl, is added in excess, the Cl−yield drops to 1.7, and the unknown acid is no longer formed. This points to a free-radical-induced short chain reaction [reactions (26)–(29)] that runs in parallel to the normal non-radical reaction. The latter may be tentatively described by reactions (89) and (90) and ensuing HCl releasing reactions.
At this point, further suggestions would be mere speculation, as a detailed product study has not yet been carried out, but it is worth mentioning that short chain reactions are common in the peroxyl radical chemistry of chlorinated compounds (von Sonntag & Schuchmann, 1997) (cf. Chapter 14).
6.6.7 Pyrimidine nucleobases
Among the nucleic acid constituents, the pyrimidines react quite readily with ozone (Table 6.1 and Chapter 4). Among these, detailed product studies are only available on thymine and thymidine (Flyuntet al., 2002).
Their reactions are governed by the addition of ozone to the C(5)–C(6) double bond. Some key reactions of thymine are shown in reactions (91)–(97). For further details and the reactions of thymidine that are similar but yet markedly different in several aspects the interested reader is referred to the original paper (Flyunt et al., 2002).
The ozonide formed in reaction (91) predominantly (75%) opens the ring in such a way that the positive charge in the virtual zwitterion resides at N(1) giving rise to the acidic [cf. equilibrium (94)] hydroperoxide 1-hydroperoxymethylen-3-(2-oxo-propanoyl)-urea [reaction (92), (34%)] and to its hydrate 1-(hydroperoxy (hydroxy)methyl)-3-(2-oxopropanoyl)urea [reaction (93), (41%)]. The potential intermediate involved in these reactions is not shown.
N
Besides the 75% organic hydroperoxides discussed above, there are 25% H2O2formed in reactions (95)– (97). These hydroperoxides are short-lived and decay largely into 5-hydroperoxy-5-methylhydantoin, which can be reduced by a sulfide into 5-hydroxy-5-methylhydantoin and formic acid [reactions (98)–(102)], while the remaining 1-hydroperoxymethylen-3-(2-oxo-propanoyl)-urea is reduced to N-formyl-5-hydroxy-5-methylhydantoin (reaction not shown). Final treatment with base yields 5-hydroxy-N-formyl-5-hydroxy-5-methylhydantoin and formic acid in 100% yields.
(98) (99) interesting process. No 1O2 is detected with the nucleoside, Thyd (Muñoz et al., 2001). This is an
indication that the hydrogen at N1 must be involved. Singlet oxygen formation is much more prominent (45%) with 5-chlorouracil (Table 6.7). For this nucleobase derivative some preliminary data are available (Muñozet al., 2001) and shed some light on the mechanism. The hydrogen at N1 in the ozonide is most likely acidic [equilibrium (103)].
Subsequent cleavage of the ozonide yielding an isopyrimidine hydrotrioxide [reaction (104)] will be followed by the release of 1O2 [reaction (105); for a compilation of 1O2 yields in nucleic acid constituents see Table 6.7], Cl− release [reaction (106)] and water addition [reaction (107); for the formation and reactions of isopyrimidines see Al-Sheikhlyet al.(1984); Schuchmannet al.(1984)]. At high pH isodialuric acid and HCl are indeed major products.
Table 6.7 Singlet oxygen yields (in % of mol O3consumed) in the reaction of ozone with nucleic acid constituents (Muñozet al., 2001)
Substrate pH 1O2yield Molar ratio
substrate:ozone
Uracil (Ura) 3.5 No signal 9:1
7 6 9:1
11 7 9:1
1,3-Dimethyluracil 3.5 No signal 4:1
11 No signal 4:1
6-Methyluracil 3.5 No signal 10:1
7 12 10:1
10 15 10:1
5-Chlorouracil 3.5 No signal 4:1
7 45 4:1
11 43 4:1
Thymine (Thy) 3.5 No signal 4:1
7 4 4:1
10 8 4:1
Thymidine (Thyd) 7 No signal 10:1
10 No signal 4:1
(Continued)
6.7 MICROPOLLUTANTS WITH OLEFINIC FUNCTIONS
Chlorinated olefins are abundant micropollutants. Trichloroethene and tetrachloroethene, react too slowly with ozone for an efficient elimination upon ozone treatment (Table 6.1). Their degradation is hence only achieved in DOM containing water by the†OH route (Chapter 3). Mechanistic details are discussed in Chapter 14. Bacterial degradation of the primary chlorinated olefins leads to cis-1,2-dichloroethene and to vinyl chloride. In certain ground waters, these secondary pollutants may be of major importance.
Chlorinated olefins are abundant micropollutants. Trichloroethene and tetrachloroethene, react too slowly with ozone for an efficient elimination upon ozone treatment (Table 6.1). Their degradation is hence only achieved in DOM containing water by the†OH route (Chapter 3). Mechanistic details are discussed in Chapter 14. Bacterial degradation of the primary chlorinated olefins leads to cis-1,2-dichloroethene and to vinyl chloride. In certain ground waters, these secondary pollutants may be of major importance.