Norrish reactions

Figure 5.4 GALDI mass spectra of ursonic acid (7, m/z 477) fresh (a), photoaged with (b) and without (a) glass filter. Spectrum (b) depicts the normal ageing pattern (cf. Figure 5.3), but ageing without glass filter and therefore with much more UV light results in a mass shift of (M+2) in spectrum (c) relative to (b), which is explained by Norrish reactions...

Figure 5.5 Norrish reaction of a triterpenoid (e.g. dammarane, oleanane or ursane type) leading to ring opening and subsequent oxidation of the ketone to the corresponding acid...

Barrier reactions Electron transfer Vander Waals reactions Rydberg reactions Exchange reactions Cleavage/addition Norrish reactions... 

Norrish reaction Pericyclic elimination Tandem cationic cyclization Bergmann cycloaromatization... 

At the same time photoreaction of acetyl alkyl radicals (reaction 3 in scheme 3), leading to chain process of acetic acid formation, indicates the proceeding of Norrish reaction of the 2" type. 

Norrish reaction of the 2" type is well known for ketones and esters. Acetyl group in radical R has greater degree of freedom than in valency-saturated molecule RH, and this facilitates the formation of intermediate six-member complex, through which Norrish reaction proceeds. 

Not excluding the possibility of more complex mechanism of PETP photodestmction and photooxidation it is supposed that photodestmction of the given polymer takes place according to the Norrish reaction of I and II type [2]. 

According to the Norrish reaction of the II type there occurs intramolecular separation of the hydrogen atom and formation of the radical with its following decay [238]. 

Norrish reactions, type I and type II, are the photochemical reactions of the excited carbonyl group, and in the case of polymers these give rise to degradation of the macromolecule. Type I is the primary process in which the bond between the carbonyl carbon and an a carbon is homolytically cleaved it is also commonly called the a cleavage ... 

Quantum yields of the Norrish reactions of both types depends on the structure of the ketone polymer in the following ways. 

The UV radiation affects aldehyde groups in polymers in the same way as ketone groups that is, Norrish reactions of type I and II are involved in the photochemical process [116]. These reactions are not very important in the photo-oxidative degradation of polymers, because the aldehyde groups are exclusively found at chain ends namely,... 

Norbomanone-type compounds can be fragmented by a Norrish reaction giving with high stereoselectivity iridomyrmecin-like compounds. 

Note that the Norrish reactions are not chain reactions Because two radicals are generated by the Norrish reactions in close proximity, there is a high probability that they will encounter each other before diffusing away into the rest of the reaction mixture. 

ESIPTmechanism (see Section 93.2.2) can no longer be repeated. Regarding hindered amine stabilizers, depletion is caused by the reaction of acyl radicals, stemming from Norrish reactions, with nitroxyl radicals. Nitroso and nitro compounds (see Chart 9.16) are formed when nitroxyl radicals are photolyzed (117]. 

Today, many commercially prepared photoinitiators are available. Most are aromatic ketones that cleave by the Norrish reaction or are photoreduced to form free radicals Cfor further details see Chapter 8). 

We photolysed 2 and 3-pentanone. We found ethylhydroperoxide in the experiment with 3-pentanone and methylhydroperoxide and propylhydroperoxide in the oxidation of 2-pentanone as product of the Norrish reaction. Other products were diketones and organic acids. Some main products at late retention time could not be identified. Tests with HPLC/fluorescence detection indicate that they are hydroperoxides. 

Today, many commercially prepared photoinitiators are available. Some consists of aromatic ketones that cleave by the Norrish reaction or are photoreduced to form free radicals. There are also numerous other two and three component photoinitiating systems. There are also those that decompose by irradiation with visible fight and make it possible to initiate the reactions with longer wavelength fight (see Chap. 10) Some examples of various photoinitiators are given in Chap. 10. Many others can be fotmd in the literature. 

The same reaction takes place in peroxides. Ketones and aldehydes cleave by the mechanism of the Norrish reaction. 

Photodegradation of ethylene-carbon monoxide copolymers by Norrish reactions. 

Polyester (Polyethylene terephthalate) a) Product yield Photooxidative reactions and direct cleavage via Norrish reactions Quantum yields, ., for CO, CO2, -OH and -COOH products fored on photolysis of PE. [110,154] [166]... 

Keywords photooxidation, UV absorbers (UVA), outdoor performance, weathering, accelerated weathering, kinetic chain length, photoinitiator, Norrish reactions, peroxide decomposer, hindered amine light stabilizers (HALS), photosensitizer, transition metal complex, UV stabilizer, time-controlled stabilization, reactive antioxidants, polymeric antioxidants. 

There have been two reports of using light to promote this reaction. These studies support the Cook intermediate 6 and also the oxidative byproducts previously mentioned. The reaction was proposed to initiate via a type II Norrish reaction forming 62, which rearranges into a mixture of two dienols. Dienol 64 proved analogous to the Cook intermediate and the believed productive compound to undergo Elbs cyclization to 65. Further... 

The Norrish I and II reactions may occur from the excited singlet (S ) or triplet (T ) states however the triplet state is much more favoured because of its longer lifetime (Table 1.1). Both Norrish reactions are responsible for the photodegradation of polymeric ketones (cf. section 3.2.1) and polymers containing main chain carbonyl groups. 

The photolysis of ketone or ester products, formed via p-scission reactions, occurs through Norrish reactions which may be of type I, when free radicals are... 

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