Chlorine atoms, from decomposition

Absorption of ultraviolet radiation by O3 causes it to decompose to O2. In the upper atmosphere, therefore, a steady-state concentration of ozone is achieved, a concentration ordinarily sufficient to provide significant ultraviolet protection of the Earth s surface. However, pollutants in the upper atmosphere such as nitrogen oxides (some of which occur in trace amounts naturally) from high-flying aircraft and chlorine atoms from photolytic decomposition of chlorofluorocarbons (from aerosols, refrigerants, and other sources) catalyze the decomposition of ozone. The overall processes governing the concentration of ozone in the atmosphere are extremely complex. The following reactions can be studied in the laboratory and are examples of the processes believed to be involved in the atmosphere ... 

Thus Pavllnec (26) detected grafting in a thermally degrading mixture of PP and poly(vinyl acetate) and Mlzutanl (19) found it In degrading mixtures of PP with PMMA, polystyrene and some related polymers. On the other hand McNeill and Nell (13) have shown that chlorine atoms from degrading poly(vlnyl chloride) are responsible for the accelerated decomposition of PMMA In mixtures of the two. 

Intramolecular chlorine isotope effects have been determined in metastable ion decompositions induced by El of carbon tetrachloride, silicon tetrachloride, hexachloroethane and hexachlorosilane [687]. In all cases, the isotope effects were normal, i.e. losses involving the lighter isotope Cl were favoured. The loss of a chlorine atom from (CCl3) and from (SiCl4) showed isotope effects greater than 1.50. Other... 

Chlorine atoms from the decomposition of chloro-paraffin fire retardants increase the rate of secondary hydrogen abstraction, accelerating degradation to volatile products but also favoring crosslinking and... 

The chlorine atoms that catalyze the decomposition of ozone come from chlorofluorocarbons (CFCs) used in many refrigerators and air conditioners. A major culprit is CF2CI2, Freon, which forms Cl atoms when exposed to ultraviolet radiation at 200 nm ... 

Much has been learned in recent years about the 00 dimer , O2O2, produced in reaction 17. It is actually dichlorine peroxide, OOOCl its geometry is now well established from submillimeter wave spectroscopy (15). Photolysis of OOOO around 310 nm the atmospherically important wavelengths -- yields chlorine atoms and ClOO radicals (16), as given in reaction 18, rather than two OO radicals, even though QO-OQ is the weakest bond (it has a strength of about 17 Kcal/mol (17)). Thermal decomposition of QOOQ (the reverse of reaction 17) occurs very fast at room temperature, but more slowly at polar stratospheric temperatures. Hence, photolysis is the predominant destruction path for CIOOQ in the polar stratosphere and two Q atoms are produced for each ultraviolet photon absorbed. 

This reaction is also an oxidation-reduction process whereby the oxygen atom is oxidized from the —2 oxidation state to the zero oxidation state as the chlorine atom is reduced from the +1 to —1 oxidation state. As diatomic oxygen is an effective disinfectant, pool owners should avoid the loss of O2 via the decomposition of the hypochlorite ion. Adding hypochlorite-containing disinfectant in the evening hours reduces the loss of the ion from photochemical decomposition. 

There is no published example of a cyclopropanation of the double bond in chlorocyclopropylideneacetate 1-Me with retention of the chlorine atom. Thus, attempted cyclopropanations under Simmons-Smith [37] or Corey [38] conditions failed [25]. The treatment of the highly reactive methylenecyclopropane derivative 1-Me with dimethoxycarbene generated by thermal decomposition of 2,2-dimethoxy-A -l,3,4-oxadiazoline 26 (1.5 equiv. of 26,PhH, 100 °C,24 h),gave a complex mixture of products (Scheme 7) [39], yet the normal cycloadduct 28 was not detected. The formation of compounds 29 - 33 was rationalized via the initially formed zwitterion 27, resulting from the Michael addition of the highly nucleophilic dimethoxycarbene to the C,C-double bond of 1-Me. The ring closure of 27 to the normal product 28 is probably reversible, and 27 can rearrange or add a second dimethoxycarbene moiety and a molecule of acetone to form 33. 

Pale-yellow (NSC1)3 may be recrystahised from carbon tetrachloride without decomposition when the temperature is kept below 50 °C. In the solid state, the six-membered ring adopts a chair conformation with ah three chlorine atoms in axial positions and equal S-N bond lengths ca 1.60 A). This arrangement is stabilised by the anomeric eifect (delocalisation of the nitrogen lone pair into an S-Cl c orbital). ... 

Sometimes the long-term effects are quite unexpected and difficult to predict. For example, millions of kilograms of CF2C12, which is used as a propellant, have been released into the atmosphere from aerosol cans. This compound appears to be wholly free of direct adverse physiological effects. However, as the substance diffuses into the upper atmosphere, it is slowly decomposed by sunlight to produce chlorine atoms. Serious danger then is possible because chlorine atoms are known to catalyze the decomposition of ozone, and it is the ozone layer in the upper atmosphere that absorbs most of the sun s ultraviolet radiation that is strongly harmful to life. 

Assuming all alkoxy radicals abstract hydrogen from cyclohexane at the same rate, and that there is no interference by chlorine atom chains in the hypochlorite decompositions. See Reference c. 

Decomposition of benzoyl peroxide in hexamethyldisilane at 80° C gives, as major products, benzene, benzoic acid, l,2-bis(pentamethyldisilanyl)-ethane and benzylpentamethyldisilane (151). The reaction of hexamethyldisilane in carbon tetrachloride with benzoyl peroxide (at reflux temperature) and with di-tert-butyl peroxide (in a sealed tube at 129° C) gives (chloro-methyl)pentamethyldisilane as the main product arising from the silane (150). In no case are rearrangement products formed. Therefore, in solution at relatively low temperature, the pentamethyldisilanylmethyl radical does not undergo rearrangement as in the thermolysis. The main fate of this free radical is dimerization in the absence of solvent or chlorine atom abstraction when carbon tetrachloride is present. 

Instability of the polymer is responsible for the primary step in decomposition and is attributed either to fragments of initiator or to branched chains or to terminal double bonds. The appearance of branching is the result of reactions of chain transfer through the polymer, while that of unsaturated terminal groups results from reaction of disproportionation and chain transfer through the monomer. During thermal and thermo-oxidative dehydrochlorination of PVC, allyl activation of the chlorine atoms next to the double bonds occurs. In this volume, Klemchuk describes the kinetics of PVC degradation based on experiments with allylic chloride as a model substance. He observed that thermal stabilizers replace the allylic chlorine at a faster ratio than the decomposition rate of the allylic chloride. 

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