Ozonization bonds

Sometimes there is more than one correct structural formula for a compound with double or triple bonds. Ozone, for example, can be correctly written as one of two forms. 

The rate constants of the reaction of 2,6-dimethyloct-7-en-2-ol separately with ozone and hydroxyl radical, in the gas phase, have been determined. The OH radical can either abstract hydrogen or add to the double bond. Ozone adds to the double bond. The formation of acetone, 2-methylpropanal, 2-methylbutanal, ethanedial, and 2-oxopropanal was discussed.191 The rate laws and activation parameters for the ozone oxidation of alcohols in aqueous solution have been determined and explained on the basis of formation of an ozone-alcohol complex.192 The reactivity of alkenes towards ozone, in aqueous solution, correlates well with Taft s equation.193... 

The arguments that were put forward for this are, however, not convincing. In the meantime it has appeared from accurate calculations (Coulson) that the character of a double bond has to be attributed to the common bond to a somewhat greater extent than to the neighbouring bonds. Ozonization here likewise furnishes the evidence (Wibaut and De Jong) for an appreciable preference of the attack by ozone on the common bond of 80 % against 20 % for the neighbouring bonds. 

IV. The methine CIV) when treated with oxidizing agents undergoes cleavage at the double bond, ozone yielding the two respective aldehydes... 

Sometimes surface polymer macromolecules have reactive double bonds. Ozone reacts with such bonds easily, producing ozonides, acids, and aldehydes [64]... 

It has been possible to show with the help of adsorption TLC, that isomers are formed when the methyl esters of unsaturated fatty acids are ozonised each of the isomers from methyl oleate has been isolated in a pure state [175]. Compounds which are not easily fractionated by argentation TLC, like as-methyl octadecenoate (methyl oleate) and mw5- mw5-methyl octadecadienoate (trans-trans methyl hnoleate ) or the corresponding triglycerides, have been separated as their ozonides [170]. This method is suitable, however, only for lipids with up to four double bonds. Ozone reacts with more highly unsaturated compounds to form short chain ozonides and undelfined side products [170]. 

Which molecule, O3 or O2, do you think has the stronger oxygen-oxygen bond If you deduced O2, you are correct. O2 has a stronger bond because it is a pure double bond. Ozone, on the other hand, has bonds that are intermediate between single and double, so O3 has weaker bonds. The effects of this are significant. As discussed in the Chemistry in the Environment box in Chapter 6, O3 shields us from... 

When writing a Lewis structure we restrict a molecule s electrons to certain well defined locations either linking two atoms by a covalent bond or as unshared electrons on a sm gle atom Sometimes more than one Lewis structure can be written for a molecule espe cially those that contain multiple bonds An example often cited m introductory chem istry courses is ozone (O3) Ozone occurs naturally m large quantities m the upper atmosphere where it screens the surface of the earth from much of the sun s ultraviolet rays Were it not for this ozone layer most forms of surface life on earth would be dam aged or even destroyed by the rays of the sun The following Lewis structure for ozone satisfies fhe ocfef rule all fhree oxygens have eighf elecfrons m fheir valence shell... 

To deal wifh circumslances such as fhe bonding m ozone fhe nolion of resonance befween Lewis sfrucfures was developed According fo fhe resonance concepf when more fhan one Lewis sfrucfure may be written for a molecule a single sfrucfure is msuf ficienl fo describe if Ralher fhe frue sfrucfure has an eleclron dislribulion fhaf is a hybrid of all fhe possible Lewis sfrucfures fhaf can be written for fhe molecule In fhe case of ozone fwo equivalenf Lewis sfrucfures may be wriffen We use a double headed arrow fo represenf resonance befween fhese fwo Lewis sfrucfures... 

Ozonolysis (Section 6 19) Ozone induced cleavage of a car bon-carbon double or triple bond... 

Most chlorofluorocarbons are hydrolytically stable, CCI2F2 being considerably more stable than either CCl F or CHCI2F. Chlorofluoromethanes and ethanes disproportionate in the presence of aluminum chloride. For example, CCl F and CCI2F2 give CCIF and CCl CHCIF2 disproportionates to CHF and CHCl. The carbon—chlorine bond in most chlorofluorocarbons can be homolyticaHy cleaved under photolytic conditions (185—225 nm) to give chlorine radicals. This photochemical decomposition is the basis of the prediction that chlorofluorocarbons that reach the upper atmosphere deplete the earth s ozone shield. 

Carbon—nitrogen double bonds in imines, hydrazones, oximes, nitrones, azines, and substituted diazomethanes can be cleaved, yielding mainly ketones, aldehydes and/or carboxyHc acids. Ozonation of acetylene gives primarily glyoxal. With substituted compounds, carboxyHc acids and dicarbonyl compounds are obtained for instance, stearoHc acid yields mainly azelaic acid, and a smaH amount of 9,10-diketostearic acid. 

Ozonation ofAlkenes. The most common ozone reaction involves the cleavage of olefinic carbon—carbon double bonds. Electrophilic attack by ozone on carbon—carbon double bonds is concerted and stereospecific (54). The modified three-step Criegee mechanism involves a 1,3-dipolar cycloaddition of ozone to an olefinic double bond via a transitory TT-complex (3) to form an initial unstable ozonide, a 1,2,3-trioxolane or molozonide (4), where R is hydrogen or alkyl. The molozonide rearranges via a 1,3-cycloreversion to a carbonyl fragment (5) and a peroxidic dipolar ion or zwitterion (6). 

Ozonation of Aromatics. Aromatic ring unsaturation is attacked much slower than olefinic double bonds, but behaves as if the double bonds in the classical Kekule stmctures really do exist. Thus, benzene yields three moles of glyoxal, which can be oxidized further to glyoxyUc acid and then to oxahc acid. Substituted aromatics give mixtures of aUphatic acids. Ring substituents such as amino, nitro, and sulfonate are cleaved during ozonation. 

Environmental Impact of Ambient Ozone. Ozone can be toxic to plants, animals, and fish. The lethal dose, LD q, for albino mice is 3.8 ppmv for a 4-h exposure (156) the 96-h LC q for striped bass, channel catfish, and rainbow trout is 80, 30, and 9.3 ppb, respectively. Small, natural, and anthropogenic atmospheric ozone concentrations can increase the weathering and aging of materials such as plastics, paint, textiles, and mbber. For example, mbber is degraded by reaction of ozone with carbon—carbon double bonds of the mbber polymer, requiring the addition of aromatic amines as ozone scavengers (see Antioxidants Antiozonants). An ozone decomposing polymer (noXon) has been developed that destroys ozone in air or water (157). 

Several theories have appeared in the Hterature regarding the mechanism of protection by -PDA antiozonants. The scavenger theory states that the antiozonant diffuses to the surface and preferentially reacts with ozone, with the result that the mbber is not attacked until the antiozonant is exhausted (25,28,29). The protective film theory is similar, except that the ozone—antiozonant reaction products form a film on the surface that prevents attack (28). The relinking theory states that the antiozonant prevents scission of the ozonized mbber or recombines severed double bonds (14). A fourth theory states that the antiozonant reacts with the ozonized mbber or carbonyl oxide (3) in Pig. 1) to give a low molecular weight, inert self-healing film on the surface (3). 

Ozonc-rcsjstant elastomers which have no unsaturation are an exceUent choice when their physical properties suit the appHcation, for example, polyacrylates, polysulfides, siHcones, polyesters, and chlorosulfonated polyethylene (38). Such polymers are also used where high ozone concentrations are encountered. Elastomers with pendant, but not backbone, unsaturation are likewise ozone-resistant. Elastomers of this type are the ethylene—propylene—diene (EPDM) mbbers, which possess a weathering resistance that is not dependent on environmentally sensitive stabilizers. Other elastomers, such as butyl mbber (HR) with low double-bond content, are fairly resistant to ozone. As unsaturation increases, ozone resistance decreases. Chloroprene mbber (CR) is also quite ozone-resistant. 

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