Ozone polarity

Ozone (O3) IS the triatomic form of oxygen It is a neutral but polar molecule that can be represented as a hybrid of its two most stable Lewis structures... 

Although httie or no reduction in total organic carbon occurs during ozonation, partially oxidized polar compounds (containing, COOH,... 

Heterogeneous chemistry occurring on polar stratospheric cloud particles of ice and nitric acid trihydrate has been estabUshed as a dorninant factor in the aggravated seasonal depletion of o2one observed to occur over Antarctica. Preliminary attempts have been made to parameterize this chemistry and incorporate it in models to study ozone depletion over the poles (91) as well as the potential role of sulfate particles throughout the stratosphere (92). 

The process yields a random, completely soluble polymer that shows no evidence of crystallinity of the polyethylene type down to —60°C. The polymer backbone is fully saturated, making it highly resistant to ozone attack even in the absence of antiozonant additives. The fluid resistance and low temperature properties of ethylene—acryUc elastomers are largely a function of the methyl acrylate to ethylene ratio. At higher methyl acrylate levels, the increased polarity augments resistance to hydrocarbon oils. However, the decreased chain mobiUty associated with this change results in less fiexibihty at low temperatures. 

In the last decade, the refrigerant issue is extensively discussed due to the accepted hypothesis that the chlorine and bromine atoms from halocarbons released to the environment were using up ozone in the stratosphere, depleting it specially above the polar regions. Montreal Protocol and later agreements ban use of certain CFCs and halon compounds. It seems that all CFCs and most of the HCFCs will be out of produc tion by the time this text will be pubhshed. 

The discovery of ozone holes over Antarctica in the mid-1980s was strong observational evidence to support the Rowland and Molina hypothesis. The atmosphere over the south pole is complex because of the long periods of total darkness and sunlight and the presence of a polar vortex and polar stratospheric clouds. However, researchers have found evidence to support the role of CIO in the rapid depletion of stratospheric ozone over the south pole. Figure 11-3 shows the profile of ozone and CIO measured at an altitude of 18 km on an aircraft flight from southern Chile toward the south pole on September 21, 1987. One month earlier the ozone levels were fairly uniform around 2 ppm (vol). 

Amorphous non-polar polymers and crystalline solvents This situation is identical to the previous one and occurs, for example, when paraffin wax is mbced into rubber above its melting point. On cooling the paraffin wax tends to crystallise, some of it on the surface of the rubber. Such a bloom is one way of protecting a diene rubber from ozone attack. 

Predict whether (a) a boron trifluoride molecule, BF3, and (b) an ozone molecule, O, are polar. 

Susan Solomon and James Anderson showed that CFCs produce chlorine atoms and chlorine oxide under the conditions of the ozone layer and identified the CFCs emanating from everyday objects, such as cans of hair spray, refrigerators, and air conditioners, as the primary culprits in the destruction of stratospheric ozone. The CFC molecules are not very polar, and so they do not dissolve in rain or the oceans. Instead, they rise to the stratosphere, where they are exposed to ultraviolet radiation from the Sun. They readily dissociate in the presence of this radiation and form chlorine atoms, which destroy ozone by various mechanisms, one of which is... 

The catalytic cycle described earlier (reactions 8 and 9) cannot explain the rapid depletion of ozone over the South Pole, because reaction 9 requires free oxygen atoms, which are too scarce in the polar stratosphere to react at any appreciable rate with QO. Several catalytic cycles that do not require oxygen atoms have been suggested as being at work over Antarctica. 

A detailed analysis of the atmospheric measurements over Antarctica by Anderson et al. (19) indicates that the cycle comprising reactions 17 -19 (the chlorine peroxide cycle) accounts for about 75% of the observed ozone depletion, and reactions 21 - 23 account for the rest. While a clear overall picture of polar ozone depletion is emerging, much remains to be learned. For example, the physical chemistry of the acid ices that constitute polar stratospheric clouds needs to be better understood before reliable predictions can be made of future ozone depletion, particularly at northern latitudes, where the chemical changes are more subtle and occur over a larger geographical area. 

Toon, O. and Turco, R. (1991). Polar stratospheric clouds and ozone depletion. Scient. Am. 264, 68. 

It now appears that both the extreme magnitude and geographic limitations of the Antarctic ozone depletion are due to meteorologic patterns peculiar to the South Polar regions. The large decrease beyond the small reduction in the rest of the stratosphere apparently involves the circulation of the polar vortex, a complex interaction of Cl with oxides of nitrogen, their physical trapping in extremely cold (T < — 80°C) clouds and preferential removal of some species by precipitation. 

Blends of carboxylated nitrile rubber (XNBR) with EPDM are likely to provide an attractive combination of properties including oil resistance, heat and ozone resistance, high tensile strength, modulus, and hardness. However, the polar curing ingredients often diffuse from the nonpolar to polar component, thereby producing cure rate mismatch and inferior properties. Three different measures have been used to overcome the cure rate mismatch [29] ... 

This mechanism is based on the fact that the ozone uptake of elongated rubber containing a substituted p-phenylene diamine type of antiozonant is very fast initially and then decreases rather rapidly with time and eventually stops almost completely. The film has been studied spectroscopically and shown to consist of unreacted antiozonant and its ozonized products, but no ozonized rubber is involved [64], Since these ozonized products are polar, they have poor solubility in the rubber and accumulate on the surface. 

M. J. Molina, Polar Ozone Depletion. 1995 Nobel Lecture. Angewandte Chemie. International English Edition, 35 (Sept. 6, 1996) 1786-1785. 

Nitriles have good resistance to oil, aliphatic and aromatic hydrocarbons and vegetable oils, but are swollen by polar solvents such as ketones. The unsaturated main chain means that protection against oxygen, ozone and UV light is required. 

As with other polar polymers, these materials will resist oil, and the absence of a double bond in the main chain confers excellent stability to the deleterious effects of oxygen, ozone and light. 

---