Ozone and aldehydes

Figure 4. Sketch of the diurnal variation in the concentrations of nitrogen oxides, hydrocarbons, ozone and aldehydes under conditions of photosmog (Manahan, 1994).

This region of the spectrum around 300 nm is a crucial one for tropospheric photochemistiy in both clean and polluted atmospheres. As we have indicated earlier, it is here that species such as ozone and aldehydes photolyze to produce atoms and free radicals critical to the chemistry of the troposphere. 

Aldehydes are easily oxidized to carboxylic acids under conditions of ozonide hydroly SIS When one wishes to isolate the aldehyde itself a reducing agent such as zinc is included during the hydrolysis step Zinc reduces the ozonide and reacts with any oxi dants present (excess ozone and hydrogen peroxide) to prevent them from oxidizing any aldehyde formed An alternative more modem technique follows ozone treatment of the alkene m methanol with reduction by dimethyl sulfide (CH3SCH3)... 

Polymeric OC-Oxygen-Substituted Peroxides. Polymeric peroxides (3) are formed from the following reactions ketone and aldehydes with hydrogen peroxide, ozonization of unsaturated compounds, and dehydration of a-hydroxyalkyl hydroperoxides consequendy, a variety of polymeric peroxides of this type exist. Polymeric peroxides are generally viscous Hquids or amorphous soHds, are difficult to characterize, and are prone to explosive decomp o sition. 

Organic hydrotrioxides (formed by the low temperature ozonization of aldehydes, ethers and alcohols71,72), in 3-5 molar excess, have been used to convert dialkyl sulphoxides into the corresponding sulphones in good yield at — 78 to — 50 °C73. The yield of sulphone decreases with increasing temperature. 

Due to the retractive forces in stretched mbber, the aldehyde and zwitterion fragments are separated at the molecular-relaxation rate. Therefore, the ozonides and peroxides form at sites remote from the initial cleavage, and underlying mbber chains are exposed to ozone. These unstable ozonides and polymeric peroxides cleave to a variety of oxygenated products, such as acids, esters, ketones, and aldehydes, and also expose new mbber chains to the effects of ozone. The net result is that when mbber chains are cleaved, they retract in the direction of the stress and expose underlying unsaturation. Continuation of this process results in the formation of the characteristic ozone cracks. It should be noted that in the case of butadiene mbbers a small amount of cross-linking occurs during ozonation. This is considered to be due to the reaction between the biradical of the carbonyl oxide and the double bonds of the butadiene mbber [47]. 

The study of the detailed mechanism of free radical initiation (rate constant k ) and ozone decay (rate constant d) by the reaction with cyclohexane, cumene, and aldehydes gave the following results (7 = 298 K) ... 

Fujitani [6] separated the insecticidally active syrupy ester from pyrethrum flowers in 1909 and named the ester pyrethron. Yamamoto [7, 8] subjected the hydrolysis product of this pyrethron to ozone oxidation, and isolated Iram-caronic acid and aldehyde (1 and 2, respectively, Fig. 3). Although Yamamoto did not determine the structure of this acid, he presumed it to be pyrethron acid (Fig. 3). Eventually, the presence of a cyclopropane ring in the molecule of natural pyrethrins became clear for the first time in 1923. 

Physical characteristics of photosmog include a yellow-brown haze, which reduces visibility, and the presence of substances which irritate the respiratory tract and cause eye-watering. The yellowish color is owed to NO2, whilst the irritant substances include ozone, aliphatic aldehydes, and organic nitrates. The four conditions necessary before photosmog can develop are ... 

Harries showed that the degradation of rubber by ozone yielded chiefly levulinic acid and aldehyde (38). This fact, he concluded, indicated that rubber was made up of the repeating unit ... 

It is known that free radicals are formed when ozone reacts with carbon-carbon double bonds. Recently, it has been suggested that PAN probably forms free radicals when it reacts with aldehydes. Because hydroperoxy radicals are free radicals, they may have biologic effects similar to those of ozone and PAN. Certainly, for experiments in which the observed biologic damage cannot be attributed to the measured concentrations of ozone and PAN, free radicals or unstable compounds should be considered. 

Besides ozone, the main indicator of photochemical pollution, other important concomitant products are peroxyacetylnitrate (PAN), hydrogen peroxide, nitrogen dioxide, hydroxyl radicals and various aldehydes that are both products and primary pollutants, particles, sulfates, nitrates, ammonium, chloride, water, and various types of oxygenated organic compounds. The most important precursors of photochemical pollution are nitric oxide and hydrocarbons. The measurement procedures for the hydrocarbons are not as highly developed as those for ozone and the nitrogen oxides. 

Many deleterious effects have been associated with photochemically polluted air ozone is deflnitely associated with respiratory problems, plant damage, and material damage PAN has deflnitely been associated with plant damage, and some other members of this class of chemical compounds have been associated with eye irritation the hydroxyl radical is considered to be an important factor in the conversion of gas-phase intermediates to end products, such as sulfur dioxide to particulate sulfate the particulate complex is responsible for haze formation and has also been associated with eye irritation and respiratory effects. The aldehydes have been associated with eye irritation. Ozone and PAN themselves do not cause eye irritation. For purposes of control, much more research is needed, in order to relate the laboratory data about the concentrations of these various materials that have significant effects to their formation in the atmosphere from emission and their atmospheric distribution. The lack of convenient measurement methods has hindered progress in gaining this understanding. 

Although ozone and PAN are considered the two primary phytotoxic oxidants in the photochemical complex, the specific response of plants to many simulated atmospheres suggests the existence of other phytotoxic oxidants. The symptoms associated with many of these reactant mixtures are closely related to those caused by ozone and PAN. In some tests, the mixtures used would not have produced either ozone or PAN. In other cases, leaf age or the pattern of injury on sensitive test plants suggested one or more pollutants other than ozone or PAN. Field injury symptoms often resemble those reported for ozone or PAN, but the response pattern is sufficiently different that accurate diagnosis is difficult. Brennan et al. correlated development of oxidant symptoms with aldehyde concentrations in New Jersey and suggested that aldehyde may be a major phytotoxic component of the photochemical-oxidant complex. The symptoms were probably not responses to the aldehyde, but rather to some compound or group of compounds present under the same conditions as the aldehyde. ... 

If both hydrocarbons and aldehydes are eliminated, carbon monoxide and NOx alone can generate significant concentrations of ozone. 

Chemical/Physical. Anticipated products from the reaction of chloroprene with ozone or OH radicals in the atmosphere are formaldehyde, 2-chloroacrolein, OHCCHO, CICOCHO, H2CCHCCIO, chlorohydroxy acids, and aldehydes (Cupitt, 1980). 

A method for synthesis of ozonides that involves no ozone has been reported. It consists of photosensitized oxidation of solutions of diazo compounds and aldehydes. Suggest a mechanism. 

Rodler, D. R., L. Nondek, and J. W. Birks, Evaluation of Ozone and Water Vapor Interferences in the Derivatization of Atmospheric Aldehydes with Dansylhydrazine, Environ. Sci, Technol., 27, 2814-2820 (1993). 

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