Ozonolysis mechanism

An example of an allyl-type dipole is Ozonolysis Mechanism... 

In the trans case, however, it is clear from Figure 4 that something entirely different is happening. Here addition of the butyraldehyde has actually increased the total ozonide yield. The explanation for this probably lies in the suggestion made earlier that cis and trans oleflns can have different ozonolysis mechanisms. It is interesting to recall that while there have been several reports (i, 3, 6) of the formation of a primary ozonide, which is stable at low temperatures, for trans oleflns, cis olefins apparently give no such primary ozonide or one of completely different nature and stability. 

We are still not able to discuss an over-all ozonolysis mechanism in any specific, detailed way. Instead we still must settle for broader generalizations and postulated intermediates which are given properties to rationalize the experimental results. Still as a result of continued experimentation there are clear differences appearing, such as the over-all differences between cis and trans olefins. Further work and new approaches may shed further light on this apparently complex process. 

The conversion of the initial ozonide to a carbonyl compound and a carbonyl oxide can be considered a retro-l,3-cycloaddition, and the combination of the carbonyl oxide and the carbonyl compound is another 1,3-cycloaddition (Figure 11.76). Nonconcerted mechanisms are possible for each of the three steps in the Criegee ozonolysis mechanism, but Kuczkowski has summarized evidence suggesting that the reactions are concerted. 

Aims and Background. The aims of this chapter is to study the difference between two reaction ozonolysis mechanisms with respect to the number and nature of the double bond C=C substitutors. 

The first step of the ozonolysis mechanism is the initial electrophilic addition of ozone to the C=C double bond to form the molozonide intermediate. Its instability leads to a further reaction, producing a carbonyl and carbonyl oxide molecule (Scheme 2.10, II). The carbonyl and carbonyl oxide rearrange to create the stable ozonide intermediate (Scheme 2.10, III). A reductive workup is then undertaken to convert the ozonide specie into carbonyl products (Scheme 2.10,1) [19]. 

The development of ideas about ozonolysis and its mechanism has been reviewed. It is not an extensive review about ozone chemistry or ozonolysis reactions, but it highlights how the rational process was developed and how conclusions were drawn at a time when limited instrumental facilities were available. Several ideas and assumptions that were made to understand the chemical properties of the ozone molecule and the ozonolysis mechanism have been discussed. ... 

Compound A (C7Hi3Br) is a tertiary bromide On treatment with sodium ethoxide in ethanol A IS converted into B (C7H12) Ozonolysis of B gives C as the only product Deduce the struc tures of A and B What is the symbol for the reaction mechanism by which A is converted to B under the reaction conditions ... 

Low-temperature spectroscopy is indispensable for the studies of processes on the ice surface, illustrated by ozone adsorption and ethylene ozonolysis. Such results are important to clarify the mechanism of atmospheric pollutant elimination and air purification in the nature. 

The ozonolysis of ethylene in the liquid phase (without a solvent) was shown to take place by the Criegee mechanism.This reaction has been used to study the structure of the intermediate 16 or 17. The compound dioxirane (21) was identified in the reaetion mixture at low temperatures and is probably in equilibrium with the biradical 17 (R = H). Dioxirane has been produced in solution but it oxidatively cleaves dialky] ethers (such as Et—O—Et) via a chain radical process, so the choice of solvent is important. 

Ozonolysis in the gas phase is not generally carried out in the laboratory. However, the reaction is important because it takes place in the atmosphere and contributes to air pollution. There is much evidence that the Criegee mechanism operates in the gas phase too, though the products are more complex beeause of other reactions that also take plaee. ... 

The reduction of Co(III) by Ag(I) in perchlorate solutions has been studied by Sutcliffe et al. Since the initial product of reaction is the very reactive Ag(Il) species, all solutions were subject to preliminary ozonolysis to remove traces of reducible impurities. The final products of reaction are Co(II) and Ag(l). Kinetic data were obtained spectrophotometrically by following the disappearance of Co(III) at 605 m/i, a small correction being applied for the absorbance of Co(ll). With Ag(I) in excess, the disappearance of Co(III) is second order, i.e., plots of the reciprocal of the corrected absorbance versus time are linear. The rate is directly proportional to the concentration of Ag(I), and inversely proportional to the square of the concentration of Co(II). These results can be understood in terms of the mechanism... 

The following questions pertain to the details of mechanism of ozonolysis under modified conditions. 

Bieber reported that the reaction of bromoacetates is greatly enhanced by catalytic amounts of benzoyl peroxide or peracids and gives satisfactory yields with aromatic aldehydes. A radical chain mechanism, initiated by electron abstraction from the organometallic Reformatsky reagent, is proposed (Scheme 8.27).233 However, an alternative process of reacting aldehydes with 2,3-dichloro-l-propene and indium in water followed by ozonolysis provided the Reformatsky product in practical yields.234 An electrochemical Reformatsky reaction in an aqueous medium and in the absence of metal mediator has also been reported.235... 

These are only the suggested mechanisms. Less is known about how the ozonides are decomposed, at least in case of triple bonds. The knowledge about ozonolysis at present available is largely due to the work of R.C. Criegee (in Peroxide Reaction Mechanisms, interscience, New York, 1962, p. 29.). 

FIGURE 8-2 Mechanism of ozonolysis. A, Criegee zwitterion B, aldehyde C, ozonide. 

Ozonolysis of styrene and ethylidenecyclohexane in the presence of [ 0]benzal-dehyde yields stable secondary ozonides incorporating 0. O NMR showed that labelled oxygen appeared as the ether oxygen, not the peroxo bridge, thus confirming the Criegee mechanism as opposed to the so-called unified concept. ... 

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