Ozone substituent effects

Dias RMB, Vieira AJSC (1997) Substituent effect on superoxide elimination from peroxyl radicals of adenine and methylated derivatives. J Photochem Photobiol AChem 117 217-222 Didenko YT, Nastish DN, Pugach SP, Polovinka YA, Kvochka VI (1994) The effect of bulk solution temperature on the intensity and spectra of water sonoluminescence. Ultrasonics 32 71-76 Dillon D, Combes R, McConville M, Zeiger E (1992) Ozone is mutagenic in Salmonella. Environ Mol Mutagen 19 331-337... 

The trends in the ozonolysis rates of simple olefins require further examination. The object of the present work was to obtain kinetic data on the reaction of ozone with simple alkyl-substituted ethylenes. The results will allow a discussion of the substituent effects on the reaction rates and of the mode of ozone attack on the carbon-carbon double bond. 

From this superficial consideration of substituent effects it would thus appear that the ozone attack is electrophilic in nature and that important steric effects contribute in determining the rate constants. The influence of steric effects could be adequately illustrated by the... 

Reaction Mechanism. The proposed mechanism for the ozone-hydrosilane reaction (7) shown in Equation 2, as deduced by analyzing and correlating data on relative rates, substituent effects, deuterium isotope effects, low temperature NMR, and ultraviolet spectroscopy for a range of hydrosilanes, is a multistep one as follows ... 

In the HPPD ozonation, the free radical chemistry seems to be more important than in DOPPD, due to the stabilizing effect provided by the diarylamine moiety (40). The nitroxide pathway is therefore of relevant importance in HPPH ozonation to amine oxide and side chain oxidation pathways. Because of the influence of N-substituent effects the ozonation of HPPH occurs only on the aliphatic side of the molecule and a nitrone is the most abundant ozonation product. 

Substituent Effects in the Oxidative Cleavage of Organosilanes by Ozone... 

Figure 1 shows a plot of krei against Taft s o- constants (13) the slope (p ) is —0.40. Such a low value might indicate that the reactions were run near the isokinetic temperature. However, krei for EtaSiH -(CF3CH2CH2)3SiH at —60°C. was 100 55 compared with 100 63 at — 10°C., which would indicate that genuine substituent effects are being observed and that the reaction involves electrophilic attack by ozone. 

The reaction of ozone with olefinic compounds is very rapid. Substiments on the double bond, which donate electrons, increase the rate of reaction, while electron-withdrawing substituents slow the reaction down. Thus, the rate of reaction with ozone decreases as follows polyisoprene > polybutadiene > polychloroprene [48]. The effect of substiments on the double bond is clearly demonstrated in Tables 15.2 and 15.3. Rubbers that contain only pendant double bonds such as EPDM do not cleave since the double bond is not in the polymer backbone. 

The kinetics and product distributions of ozonolysis of vinylcyclohexane and methylene cyclohexane have been investigated.162 Steric hindrance of the cyclic substituent largely offsets electronic effects hi determining the rate of reaction. The main products of ozonation of catechols were quinones, while catechol acetals gave rise to compounds with an opened benzene ring.163 The ozonolysis of azoles such as pyrroles, oxazoles, and imidazoles has been reviewed.164... 

Oxidation with ozone, under physiological conditions, follows the rate order uric acid ascorbic acid > glutathione. The amounts of ozone absorbed and antioxidant consumed have been simulated with a mathematical model and reaction rate constants of the oxidations have been evaluated.194 Various facets of transition metal-catalysed oxidation of benzylic compounds with ozone have been reported. The correlation of the effect of substituents with Hammett constants and steric factors has been discussed. The reaction seemed to proceed via a radical mechanism.195... 

The effects contributed by alkyl groups to the relative rate constants, kreh for the reaction of ozone with cis- and trans-1,2-disubstituted ethylenes are adequately described by Taft s equation = k °reX -f pSo-, where So- is the sum of Taft s polar substituent constants. The positive p values (3.75 for trans- and 2.60 for cis-l,2-disubstituted ethylenes) indicate that for these olefins the rate-determining step is a nucleophilic process. The results are interpreted by assuming that the electrophilic attack of ozone on the carbon-carbon double bond can result either in a 1,3-dipolar cycloaddition (in which case the over-all process appears to be electrophilic) or in the reversible formation of a complex (for which the ring closure to give the 1,2,3-trioxolane is the nucleophilic rate-determining step). 

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