Alkynes ozonation

R is hydrogen, alkenyl, or alkyne. In remote tropospheric air where NO concentrations ate sometimes quite low, HO2 radicals can react with ozone (HO2 + O3 — HO + 2 O2) and result in net ozone destmction rather than formation. The ambient ozone concentration depends on cloud cover, time of day and year, and geographical location. 

Alkynes, like alkenes, can be cleaved by reaction with powerful oxidizing agents such as ozone or KMnC, although the reaction is of little value and we mention it only for completeness. A triple bond is generally less reactive than a double bond and yields of cleavage products are sometimes low. The products obtained from cleavage of an internal alkyne are carboxylic acids from a terminal alkyne, CO2 is formed as one product. 

Treating alkynes with ozone or with basic potassium permanganate leads to... 

The two major characteristic oxidation processes of alkynes are their transformation to 1,2-dicarbonyl compounds and their cleavage reaction to carboxylic acids.710 The structure of the starting compounds has a decisive effect on the selectivity of oxidation. Since 1,2-dicarbonyl compounds proved to be intermediates in further oxidations, carefully controlled reaction conditions are often necessary to achieve selective synthesis. Certain oxidizing agents such as peroxyacids and ozone are nonselective oxidants. 

Compared to ozonation of alkenes, much less is known about the ozonation of alkynes,710 which yields 1,2-dicarbonyl compounds, carboxylic acids, and anhydrides. 1,2,3-Trioxolene (91), analogous to 74 in alkene ozonation mechanism (Scheme 9.14), and zwitterionic intermediates (92) were formulated on the basis of IR studies and trapping experiments ... 

Further chemistry of alkenes and alkynes is described in this chapter, with emphasis on addition reactions that lead to reduction and oxidation of carbon-carbon multiple bonds. First we explain what is meant by the terms reduction and oxidation as applied to carbon compounds. Then we emphasize hydrogenation, which is reduction through addition of hydrogen, and oxidative addition reactions with reagents such as ozone, peroxides, permanganate, and osmium tetroxide. We conclude with a section on the special nature of 1-alkynes— their acidic behavior and how the conjugate bases of alkynes can be used in synthesis to form carbon-carbon bonds. 

A number of useful reactions for the preparation of aldehydes and ketones, such as ozonization of alkenes and hydration of alkynes, have been considered in previous chapters. These and other methods of preparation are summarized in Tables 16-7 and 16-8 at the end of the chapter. Only a few rather general methods that we have not discussed will be taken up here. 

Ozonium Ion (H03+). Ozone is a resonance hybrid of canonical structures 50a-50d.135 Ozone does in fact act as a 1,3-dipole—that is, either as an electrophile or a nucleophile. The electrophilic nature of ozone has been recognized for a long time in its reactions with alkenes, alkynes, arenes, amines, phosphines,... 

B-5. Which alkyne yields butanoic acid (CH3CH2CH2C02H) as the only organic product on treatment with ozone followed by hydrolysis ... 

The mechanism and kinetics of the atmospheric oxidation of alkynes, initiated by OH radicals, have been studied particularly to determine the role of alkyne oxidation in tropospheric ozone formation. A general mechanism for OH-initiated oxidation of alkynes has been developed with the aid of thermodynamic calculations. In general, the significance of atmospheric alkynes to the formation of tropospheric ozone was found to be smaller than for alkanes and alkenes, due to the absence of the hydroxy peroxy-forming product channel in the OH-initiated atmospheric oxidation of alkynes.227... 

Dry ozonization of alkraes and alkynes has also been explraed. Sometimes these results resemble those of the homogeneous ozonization in an i rotic solvent ... 

Both internal and terminal alkynes are oxidatively cleaved with ozone to give carboxylic acids. 

Olefins with hindered double bonds may be transformed stereospecifically to oxiranes by treatment with ozone. The epoxidation of propylene has been achieved with alkoxyalkyl-hydroperoxides obtained by the ozonization of olefins in the presence of alcohol.The yield depends on whether the alcohol is a primary, secondary, or tertiary one. The low-temperature (—70°) epoxidation of olefins with a yield of about 30% has been performed with electrophilic intermediates produced in the course of the ozonization of alkynes these intermediates are probably five-membered cyclic trioxides. This epoxidation is almost totally stereospecific. 

I2 DMSO, " MeRe03/H202, " as well as by electrooxida-A ruthenium complex with a small amount of trifluoroacetic acid converts internal alkynes to the a-diketone. Ozone generally oxidizes triple-bond compounds to carboxyhc acids (19-9), but a-diketones are sometimes obtained instead. Selenium dioxide (Se02) with a small amount of H2SO4 oxidizes alkynes to a-diketones as well as arylacetylenes to a-keto acids (ArC=CH ArCOCOOH). " ... 

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